4R Nutrient Management Study Guide

Previous Legumes
When sod containing perennial legumes such as alfalfa, birdsfoot trefoil and clover are plowed under,
they supply an appreciable amount of N to the following crop. Table 2.3 below shows the reductions
that should be made in N fertilizer applications to crops following sod containing legumes.

Table 2.3. Adjustment of Nitrogen Requirement Where Crops Containing Legumes are Plowed Down

Type For All Crops, Deduct From N Requirement

Type of Crop kg/ha lb/acre

Less than one-third legume 0 0

One-third to half legume 55 49

Half or more legume 110 100

Perennial legumes seeded and 451 401
plowed in the same year

Soybean and field bean residue 02 02

1. Applies where the legume stand is thick and over 40 cm (16 in.) high.
2. F or all crops other than corn. For adjustments to corn fertilizer requirements, see Corn Nitrogen

Rate Worksheet, on page 21 of the Agronomy Guide for Field Crops.

Chart source: Agronomy Guide for Field Crops, Publication 811, 2009, p.162.

Performance Objective 8
Discuss the use of technologies to make ongoing adjustments to the nutrient rates
that may have been identified during the 4R nutrient management planning
process such as:

a. crop canopy sensors;
b. normalized difference vegetative index (NDVI);
c. post-season stalk nitrate;
d. soil nitrate test;
e. plant analysis.

Crop Canopy Sensors
Advances in precision agriculture technology have led to the development of ground-based active
remote sensors that can determine Normalized Difference Vegetation Index (NDVI). In simple
language, this means variation in vegetation such as the crop canopy. For example, studies have
shown that NDVI is highly related to leaf N content in corn (Zea maysL.). Remotely sensed NDVI is
used to detect variability in the crop canopy to determine N status during the growing season for in-
season site-specific N management.

Different crops and even varieties within a crop can differ in their basic reflective signatures as well as
show different changes in reflectance in response to various stresses. Attempting variable application
of N on say corn at sidedress can be very challenging. One approach is to supply a single row in each
planter pass with N fertilizer at planting and this would presumably represent an N sufficient status. At
sidedress time, the greenness of the unfertilized rows would then be compared to the fertilized row to

PROFICIENCY AREA II - Nitrogen 51

determine the degree of N limitation in that location and vary the amount of N fertilizer applied based
on the difference in greenness. The relationship between differences in greenness and crop fertilizer
N needs has to be known and programmed into the variable rate applicator. One problem with
this approach is the need to forgo application of starter N with the planter as this tends to mask the
variations in soil N supply, making it difficult to distinguish large differences in greenness. No starter
N can cause yield reductions in corn. Ontario research has also shown that crop yield response to
applied N was poorly correlated to differences in corn crop greenness at sidedress.

Regardless of the system or method used, relationships between crop greenness and fertilizer N
requirements are needed in order to predict fertilizer N application. These are crop and potentially
variety specific and can be affected by other factors such as growing season conditions.

Normalized Difference Vegetative Index (NDVI)
The Normalized Difference Vegetation Index (NDVI) is an index of plant “greenness” or photosynthetic
activity, and is one of the most commonly used vegetation indices. Vegetation indices are based on
the observation that different surfaces reflect different types of light differently. Photosynthetically active
vegetation, in particular, absorbs most of the red light that hits it while reflecting much of the near
infrared light. Vegetation that is dead or stressed reflects more red light and less near infrared light.
Likewise, non-vegetated surfaces have a much more even reflectance across the light spectrum.

Post-season Stalk Nitrate
The post-season stalk nitrate test allows growers to conduct a “post-mortem” evaluation of the adequacy
of their N program for the current growing season. The test is described as “post-mortem” because stalk
samples are taken after the grain is physiologically mature. Given that this is a very late season test, the
interpretation of the results offers no assistance in fine-tuning N management for the current year, but
rather provides insight into N management options for coming years (i.e. if stalk nitrate-N levels were
deemed excessive, and growing season conditions were normal, then one should consider lowering
N inputs in that field the next time corn is planted assuming all other factors remain constant). One
needs to consider other environmental factors that may have impacted the availability of N to the corn
plant, the corn plant N requirement and concentrations of nitrate remaining in the stalk at the end of the
season.

The basis for the test lies in the fact that corn plants deficient in N will usually remobilize stored N
from the lower portions of the stalk and leaves to the developing grain, resulting in lower stalk N
concentrations at the end of the season. Plants that take up excessive amounts of N (more than is
needed for maximum yields) will contain excessive amounts of nitrate-N in the lower stalk sections by
the end of the growing season, resulting in higher stalk nitrate-N concentrations.

The stalk nitrate test is probably best suited for identifying fields/situations where N supply was
excessive (exceeded economical rate) and, thus, costly to the grower and possibly the environment.
Typical situations where N uptake may be excessive include: manured fields or fields following alfalfa
that received additional (and possibly unnecessary) N fertilizer applications for the subsequent corn
crop, or growing seasons where yields were reduced due to lower rainfall, or possibly early fall frost.

52 PROFICIENCY AREA II - Nitrogen

Pre-plant Soil Nitrate Test (PPNT)
The soil N test is a measure of the nitrate-N in the soil. Referring back to the N cycle diagram, you
see that the nitrate in the soil comes from the nitrification of ammonium, which in turn comes from the
mineralization of soil organic N. The soil N test thus uses the amount of N mineralized and nitrified up
to the time of sampling to predict the amount of organic N that will be mineralized and supplied to the
crop during the growing season. The difference between the estimated crop requirement for N and the
predicted soil supply of N is the amount of fertilizer or available manure N that needs to be applied.

The pre-plant test measures the amount of mineral N (nitrate) in the root zone before planting and, in
Ontario, the soil N test is calibrated only for corn and barley. The pre-plant test allows farmers to adjust
N applications to meet the needs of each specific field. Cropping sequence significantly affects the
amount of N in the soil available to corn. The pre-plant test is most useful in continuous corn, second-
year fields, and fields with a long history of manure applications but not recent manure applications
or legume plow down. The soil N test is not recommended for these latter two scenarios due to the
uncertainty of the amount of manure or legume N that has been converted to nitrate and measured by
the soil N test, versus the mineralization of soil organic N.

The pre-plant test is most useful on medium- or heavy-textured soils and during years when precipitation
is normal or below normal. Below normal precipitation in autumn and winter can lead to higher spring
pre-plant soil nitrate levels. Exceptionally cool, wet spring conditions can lower early season soil nitrate
levels due to limited microbial activity in the soil.

Pre-Sidedress Soil Nitrate Test (PSNT)
Sampling when the corn is 15-30 cm (6-12 in.) tall, before the application of sidedress N, has
increased in popularity and is often the most reliable time for assessing fertilizer N requirements
using the soil N test. This is referred to as the pre-sidedress N test (PSNT). By delaying sampling
past the busy planting season, the PSNT allows more time for sampling and receiving results from the
laboratory. More importantly, considerable evidence indicates that N recommendations based on this
later sampling time are superior to those based on a planting time sample. This in large part is due
to the fact that yearly differences in weather conditions affecting mineralization and nitrification of
soil organic N tend to diminish as we move from early spring to early summer. Research is underway
to improve the soil N test’s prediction for fertilizer N requirement by considering growing season
conditions (e.g. precipitation and crop heat units).

Sometimes the fertilizer recommendations based on the nitrate-N soil test need to be modified. The N in
manure or legumes applied or plowed down just before sampling will not have converted into nitrates
and will not be detected by the soil test. Information will be provided with the test results on how to
make appropriate adjustments based on the N credits for these sources of N.

The nitrate-N soil test has not been adequately evaluated for:
• legumes or manure plowed down in the late summer or fall;
• legumes in a no-till system; and,
• soil samples taken prior to planting before the soil has warmed up significantly
(i.e. in mid- to late April)

In these circumstances, use the nitrate-N soil test with caution.

PROFICIENCY AREA II - Nitrogen 53

Plant Analysis
Tissue, leaf or plant analysis can be used to:

• determine the nutrient needs of established perennial crops such as tree fruit and grapes, and
• confirm the diagnosis of visual symptoms of unusual plant growth or deficiencies.

For perennial crops, tissue sampling is often preferred over soil sampling because it is difficult to take
soil samples in the root zone of perennial crops. Tissue analysis also helps show what nutrients are
being taken up by the crop as opposed to what is available in the soil. This may mean that the nutrient
limitation is not due to an inadequate amount of nutrient in the soil, but rather to soil conditions that
are limiting the plant roots’ ability to absorb that nutrient from the soil. This, however, is not usually the
case for N unless soils are waterlogged.

Plant analysis identifies a nutrient as being deficient when its concentration falls below a critical level
for a given plant part, in the specific crop at a particular stage of plant development. In order to
interpret the tissue analysis, the timing or stage of plant growth and the plant part being sampled is
very important.

Used along with a soil test, tissue analysis can identify possible nutrient limitations or deficiencies.
Although a tissue analysis may indicate a nutrient could be deficient or limiting, it is not easy to make
a fertilizer recommendation rate from a tissue analysis. As well, tissue analysis may not provide
information in time for correction for annual crops in the current growing season.

Competency Area 3
Determining the Right Timing of Nitrogen Application

Performance Objective 9
Discuss how the timing of soil nitrogen tests can impact test levels.

Given the precipitation patterns in Ontario, we typically expect the soil profile to leach completely over
the fall to spring period, thus removing any residual nitrate N from the root zone. In finer textured
soils, where leaching is less prominent, there is the risk of nitrate loses through denitrification. Thus, the
amount of nitrate observed in the soil in the spring is likely due to the mineralization and subsequent
nitrification of organic N in the soil. Since these processes are microbially mediated, earlier season
measurements (i.e. pre-plant) will result in lower soil N test values than samples taken later in the
season at pre-sidedress. As a result, for a given soil N test level, one would usually see a lower
recommendation for N on corn if the sample was taken pre-plant versus at pre-sidedress.

Sampling shortly after manure application or legume plow down is likely only measuring a fraction of
the N in these materials as there may have been insufficient time for mineralization and nitrification. For
example, mineralization and nitrification of the applied manure N will depend upon the time interval
between application and soil sampling as well as the weather conditions (precipitation, temperature)
and method of application. Information will be provided with the test results on how to make
appropriate adjustments. Similarly, sampling after application of a readily decomposable C source with
a wide C:N ratio (> 25:1) is likely to reduce the amount of nitrate-N in the soil, and thus lower the soil
nitrogen test values.

54 PROFICIENCY AREA II - Nitrogen

The nitrate-N soil test has not been adequately evaluated for:
• legumes or manure plowed down in the late summer or fall;
• legumes in a no-till system; and
• for very early pre-plant soil sampling done before the soil has warmed up significantly
(i.e. in mid- to late April).

In these circumstances, use the nitrate-N soil test with caution.

Performance Objective 10
Estimate the environmental risks in the timing of applying nitrogen based on:

a. climate;
b. soil type;
c. runoff;
d. irrigation;
e. leaching potential.

Although P is the main factor causing eutrophication (often associated with algal blooms, nuisance
aquatic plant growth, low oxygen levels in water, and death of aquatic organisms), N lost from
fertilizer and manure can make the problem worse. High ammonium-N concentrations in surface
water can also be toxic to aquatic organisms and impair water for livestock or human consumption.
Excessive nitrate-N in surface and drinking water also spoils water quality. While we typically view
nitrogen contamination of water to be a groundwater issue, tile drains divert leaching nitrates to surface
water bodies and surface applied ammonium N sources can be lost with surface runoff if runoff occurs
shortly after N application.

Climate
Application of fertilizer or manure on frozen and/or snow-covered soil is not recommended because
rapid snow melt or rainfall can move nutrients and other constituents from the nutrient sources to surface
waters as meltwater movement into the soil is restricted. For materials applied at other times of the
year, volatilization and leaching would be the likely pathway of N loss. Cool, wet and calm conditions
reduce the risk of ammonia volatilization, especially of materials left on the soil surface with either a
high ammonium or ammonium forming N content (e.g. urea). Soaking rains that are sufficient to move
the N into the soil before runoff occurs will also lower volatilization losses. Light rainfalls following
surface applied urea can result in higher ammonia volatilization losses. This occurs because there was
sufficient water to dissolve the fertilizer and allow urea hydrolysis, but not enough to move the fertilizer
into the soil. If the form of N applied was nitrate, or sufficient time after N application has allowed
nitrification to occur, then heavy rains at this time would increase the risks of nitrate leaching and
denitrification.

PROFICIENCY AREA II - Nitrogen 55

As mentioned in P.O. 9, we experience a relatively even pattern of precipitation in Ontario. Given
the lower temperatures and lack of significant plant growth in late-fall to spring period, the greatest
risk of water movement and thus nitrate leaching through the soil would be in this period. For fall
applied manures, the earlier in the fall the material is applied the greater the opportunity for N to be
mineralized and nitrified before soil freezing. This increases the risk of nitrate leaching. As the soil
moisture content increases, the risk of anaerobic conditions also increase and thus we would expect
greater denitrification and greenhouse gas production.

While gentle rains after N application are typically desirable in most cases, especially if N materials
are left on the soil surface, applications just prior to excessive rains that cause substantial leaching or
runoff should be avoided.

One would expect better N utilization by the crop if timing of N application coincides with crop
demand, which obviously is impacted by climate. Utilization of cover crops may be one method of
immobilizing excess soil mineral N in the fall, although this depends upon cover crop plant species and
cover crop biomass produced. Fall application of readily decomposable, high C:N ratio materials can
also help reduce soil mineral N losses in the non-growing season.

Soil Type
Coarse-textured soils have a lower water holding capacity and, therefore, a greater potential to lose
nitrate from leaching when compared with fine-textured soils. Some sandy soils, for instance, may
retain only 1/2 inch of water per foot of soil while some silt loam or clay loam soils may retain up
to two inches of water per foot. Nitrate-N can be leached from any soil if rainfall or irrigation moves
water through the root zone. Sandy soils typically have a lower CEC as well and thus leaching of
ammonium-N can also be a problem in some circumstances.

Finer textured soils or soils that are poorly drained result in a greater risk of denitrification and the
production of greenhouse gases.

Runoff
To reduce the potential for nutrient runoff into surface water, fertilizers and manure should be injected
or incorporated into the soil or be applied under conditions when runoff events are unlikely or at least
unexpected. Fall and early winter applications are typically better for fertilizer and manure application
than when the soil is frozen and/or snow-covered, although these times are associated with increased
risk for leaching of the mineral N forms applied. Late-spring and summer applications typically
represent the lowest risk of runoff and leaching N losses, provided application rates are appropriate in
terms of plant uptake/requirement.

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Irrigation
Nitrogen leaching during irrigation is directly related to the drainage volume. By increasing irrigation
efficiencies, both the drainage volume and amount of N leaching are reduced. Both water management
and N management are important in controlling N leaching. By using improved water management
practices that control the amount of water applied and using the proper time to apply water (when
the crop most needs and can utilize it), the irrigation efficiency is increased and N leaching is
reduced. Applying only enough N fertilizer to meet crop requirements for a realistic yield goal, time of
application and use of slow-release fertilizers are N management practices that will reduce N leaching.
The type of irrigation system utilized is also a factor. Drip and sub-surface drip irrigation systems offer
better water application uniformity and accuracy and reduced soil surface wetting which minimizes
nitrate leaching loss potential to groundwater.

Leaching Potential
The ability of a soil to retain water depends upon the inherent water holding capacity of the soil and
the current proportion of the water holding capacity that is already filled with water. Finer textured
soils typically hold more water at field capacity than coarser textured soils and thus have a lower
leaching potential. Wetter soils have more of the water holding capacity already filled and thus it takes
less additional water to cause leaching to occur. If the profile is dry, normal fine-textured soils can hold
up to two inches of available water and a gentle four-inch rain can completely infiltrate without any
leaching. However, if the soil profile is at or near field capacity, there might be as much drainage
as rain.

On an annual basis, leaching potential in Ontario is typically greatest in the fall to late spring period
when soils are wettest due to cooler temperatures and limited plant growth. Mineral N applied or
found in the soil during this period would be at the greatest risk of leaching and thus management
practices should be adopted that attempt to minimize mineral N soil content at this time.

Performance Objective 11
Estimate the risks of applying nitrogen on saturated, frozen, or snow covered soils.

Applications to frozen ground are at higher risk for runoff and loss of those nutrients. When the soil
is frozen, water is not able to infiltrate into the soil profile and the water and nutrients can runoff
to adjoining properties or waterways. Applications on snow will also likely see soil conditions that
are either frozen or saturated, again leading to increased risks of runoff or leaching. Ammonia
volatilization can also occur when ammonium based fertilizers or materials with high ammonium
contents are left on the surface of frozen soils. Applications of nitrate-containing materials to saturated
soils increase both the risk of nitrate leaching as well as denitrification.

PROFICIENCY AREA II - Nitrogen 57

Performance Objective 12
Discuss how the timing of nitrogen application is
dependent upon the nutrient source.

Drilled with the Seed
This method consists of placing the fertilizer with the seed in the seed row. Drilling fertilizer with seed
in excess of recommended rates can cause seedling damage and reduce yields. Depending upon
the equipment used, there can be a large variation in the concentration of fertilizer adjacent to the
seed. Greater spreading of the fertilizer and seed and lower rates of fertilizer, reduce the likelihood
of seedling damage. A double disc press drill places the seed and fertilizer close together in a narrow
furrow. A disc, air seeder or hoe drill can scatter the seed and fertilizer, depending on the opener used.
Wider spacings between rows increase the concentration of fertilizer in each seed row thus reducing
the safe rate of fertilizer that can be applied on an area basis.

Placing fertilizer with seed optimizes efficiency; however rates of N fertilizer need to be kept within safe
limits to prevent reduced germination and seedling damage due to ammonia toxicity and/or salt burn.
Since sources of K fertilizer are also quite soluble and have a relatively high salt index, one needs to
consider both the amount of N and K applied with or near the seed. Aside from the high ammonium
formation and increase in soil pH, some sources of urea may also contain sufficient quantities of biuret
which would decrease seed germination and seedling health. Urea application with the seed is usually
restricted to 10 kg N ha-1 for spring oats and barley, and is not recommended for other field crops.
Diammonium phosphate (18-46-0) is likely the next most damaging N source to place with the seed
due to its high, free ammonium content, with recommended safe rates of 20 or 30 kg N ha-1 for spring
oats and barley on coarse soils (sands, sandy loam) and finer textured soils (loams, silts, clay loams),
respectively.

For most field crops, it is unlikely that the entire crop requirement for N can be applied with the seed,
and thus the bulk of the N application should be applied either before or after seeding, or banded
away from the seed.

Aside from N source, several other factors affect the safe N rates applied with the seed such as
crop type (sweet corn is very sensitive to seed placed fertilizer and seed placed fertilizers are not
recommended on soybean, peas, beans and flax), row spacing (wider rows mean low safe rates), seed
and fertilizer spread, soil texture (higher rates on finer texture soils), and soil moisture.

Side Band Placement
This method consists of placing the fertilizer in a narrow band 2” to 3” to the side and/or 2” to 3”
below the seed during seeding. The efficiency of side banding is equivalent to placement with seed
and higher rates can be used safely. Nitrogen requirements of most crops can be met without causing
seedling damage when solution or dry fertilizer is placed at least 2” from the seed row. Similar to
seed placement restrictions, urea or ammonium based fertilizers pose the greatest risk to plant damage
and so safe application rates of N with these sources is lower than other N sources. Again, there is a
need to limit the amount of salt applied in the band so safe rates should consider both the amount of N
and K being applied. Higher rates can be applied with narrower row spacings and moving the band
further from the seed can increase the rate of fertilizer that can be applied in the band, but may limit
the effectiveness of other fertilizer nutrients such as P.

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Anhydrous ammonia cannot be placed in or near the seed row. However, equipment has been
modified to allow anhydrous ammonia to be applied at seeding time in a band or other arrangement
that is separated from the seed. The anhydrous ammonia should be separated from the seed by at
least 2” to 3” and placed below and to the side of the seed or to the side of the seed. It should not be
applied directly below or above the seed. The anhydrous ammonia tends to follow the furrow upward,
so attempts at placing it below the seed will likely lead to seed damage. Again, the safe application
rate will depend upon soil texture and crop type.

Mid-row Banding
This method places fertilizer between every seed row or every second seed row as part of the seeding
operation. The fertilizer is banded with knives, discs or coulters to a depth of 3” to 4”. This system is an
efficient method of N placement, which allows the application of high rates without risk of damage to
germinating seedlings. It is generally suited for liquid fertilizer sources (e.g. 28% UAN) or anhydrous
ammonia and as the band is a greater distance from the crop row, the rate of N application is of lesser
concern in terms of crop damage.

Banding into Soil Prior to Seeding
This method places the fertilizer below the soil surface in a band behind a shank at a depth of 3’” to
6”. It is often referred to as “deep banding”. Band spacings should not exceed 18” when applying
N fertilizer. The efficiency of this method of N placement in spring is equal to side banding or seed
placing fertilizer. Fall banding of fertilizer N sources would likely lead to greater N losses and reduced
nitrogen use efficiency by the crop, regardless of fertilizer material used. Deep banding of nitrate N
sources could result in increased risk of leaching or denitrification, especially if applied long before
planting.

Anhydrous ammonia should be applied only when soil conditions permit a good seal behind the
applicator shanks. Seeding can be done immediately after anhydrous ammonia application, provided
there is at least a 4” vertical separation of the injection point and the seed. Crop emergence may be
slightly reduced directly over the anhydrous bands, particularly for small seeded crops and if soils are
sandy or dry. However, plants will tiller or branch and yield will not be affected. The ammonia bands
should be perpendicular to the direction of seeding.

Surface Banding
This application method places a band or stream of liquid fertilizer on the soil surface. The equipment
used includes fertilizer floaters and field sprayers outfitted with dribble nozzles or streamer bars.
Surface banding improves N efficiency as compared with broadcast methods because volatilization
and contact with residues and possible immobilization, are reduced. The liquid stream also penetrates
a crop canopy better than a broadcast application and, as a result, more fertilizer reaches the soil
surface. Fertilizer materials that are high in ammonium, or ammonium forming, and increase soil pH
near the fertilizer (e.g. urea) run the risk of greater ammonium volatilization losses.

Nesting
This method uses a spoke wheel injector to place regularly spaced pockets or nests of liquid fertilizer
into the soil. N losses by volatilization are minimized as fertilizer N is placed within the soil.
Disturbance of soil and crop residue is minimal and post-seeding applications may be made into the
growing crop.

PROFICIENCY AREA II - Nitrogen 59

Broadcast and Incorporated
Granular or solution fertilizer is broadcast on the soil surface and incorporated into the soil with a
tillage implement. Nitrogen fertilizers, especially urea and liquid or dry fertilizers containing urea,
should be incorporated as soon as possible to minimize gaseous losses by volatilization. As the
fertilizer material is spread relatively uniformly over the soil surface, application of agronomic rates are
of little to no concern in terms of salt or ammonia damage.
Broadcast without Incorporation
This method usually results in the least efficient use of spring-applied fertilizer N. Fertilizer left on the soil
surface increases the risk of loss by runoff, erosion, ammonia volatilization (especially with fertilizers
containing urea) and immobilization by crop residue. This is the most commonly used method to
fertilize established pasture or hay land and is frequently used in zero tillage production.
If urea is surface applied and not incorporated (either by rain or tillage), N losses to the air (as
ammonia) can approach 40% of the applied N. In addition, a rapid pH increase after application
caused by hydrolysis of urea can result in ammonia release that can damage seedlings if the urea is
applied too close to the seed.
Ammonium nitrate (34-0-0) is a better N source than urea (46-0-0) for broadcast applications without
incorporation. Losses of urea are higher than losses of ammonium nitrate under conditions favouring
volatilization (e.g. high temperatures and high soil pH). Loss of urea can be minimized by applying
during periods of low temperature or just before it rains. Treating urea with a urease inhibitor will delay
volatilization losses for up to 14 days.

Nitrogen - Granular

Courtesy Fertilizer Canada

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Performance Objective 13
Discuss the opportunities that split application offers for 4R nitrogen management.

Dividing total N application into two or more treatments can more specifically match N supply with
a plant’s ability to utilize nutrients. Split application can be an important component of 4R nutrient
planning by addressing right rate and right time.

Depending on soil type, climate, agronomic practices and other factors, N fertilizer can be vulnerable
to loss. Denitrification, leaching and volatilization impose costs that include lost productivity and
negative environmental impact. Split-applying N fertilizer is one way to confront these challenges.
When a crop’s total N requirement is supplied with a single pre-plant or at-planting application, most
of the N must “wait” for the target crop’s future needs and that means the window for potential loss
remains open longer.

By postponing a portion of the N treatment until the crop is better able to utilize the nutrient, plants
take up the N more quickly and efficiently. That means growers get more from their fertilizer investment
and fertilizer losses that can contribute to environmental concerns are lessened. When all of the
N is supplied ahead of crop growth, more of that N is susceptible to denitrification, leaching or
volatilization.

Split application offers efficacy benefits on a wide range of crops and forages but its management must
be considered on a crop-by-crop basis. The timing of post-planting N applications is especially critical.
The target species must be immature and growing to provide time for the N to be absorbed and
metabolized in order to have the most efficient yield or quality impact. In the case of corn, for instance,
all of the N should be delivered to the plant before ears are set. In wheat, the second application of N
generally is best made 10 days to two weeks prior to the jointing stage when leaf tissue elongates to
form a stem and the plant’s N requirement increases as it begins its reproductive phase. Because of a
need for continuous, in-season production, forages especially benefit from split-applying N. All crops,
however, have different nutrient requirements.

Split application should not exceed total test-based N recommendations. While split-applying N
can enhance efficiency, it does not change what the plant needs and should not be used to exceed
recommendations. Those recommendations should always be based on reasonable yield goals derived
and developed from research applicable to a given growing region.

The downside for split applications is that wet conditions may prevent timely treatment. Also, dry
conditions can prevent fertilizer from reaching crop roots and extra fuel costs from additional trip
through the field must be considered.

PROFICIENCY AREA II - Nitrogen 61

Performance Objective 14
Discuss how cover crops can affect nitrogen availability in follow-up
cash crops and supplemental nitrogen application timing.

Good stands of actively growing cover crops, including legumes such as red clover, will sequester
significant amounts of soil mineral N. Cover crops following winter wheat have reduced the level
of nitrate left in the soil in October and November by 50% compared to where no cover crop was
planted. This results in less nitrate-N available to be lost over winter.

Under optimal growing conditions, non-legume cover crops (e.g. ryegrass, cereal grains) can take up
substantial amounts of N. Oilseed radish has been reported to contain up to 100 kg/ha of N in above-
ground growth under optimal growing conditions.

Although non-legume cover crops can sequester a significant amount of N, subsequent corn yields may
not be increased to the same extent as following legume cover crops. To date, it has been difficult to
show a consistent reduction in N fertilizer requirement for crops grown following a non-legume cover
crop.

Table 2.4 Adjustment of Nitrogen Requirement Where Legumes Ploughed Down

Type of Crop kg/ha lb/ac
Established forages
0 0
• less than 1/3 legume 55 50
• 1/3 to 1/2 legume 110 100
• 1/2 or more legume 78 for field corn 70 for field corn
Perennial legumes seeded and ploughed 45 all other crops 40 all other crops
in same year 30 for field corn 27 for field corn
0 all other crops 0 all other crops
Soybean and field bean residue

Chart source: Soil Fertility Handbook, OMAFRA Publication 611, p. 122.

Cover crops vary widely in the timing of N mineralization. Oilseed radish and spring cereals tend to
start to release N early in the spring when it may be subject to losses. Some cover crops, like ryegrass,
are extremely resistant to breakdown. Although they absorb significant quantities of N, little is released
to the next crop during the growing season. In some instances, cover crop residues with high C:N
ratios can, in the following year, lead to immobilization of soil mineral N thus lowering the N available
to the subsequent crop.

In addition, there are circumstances where cover crops can inhibit the growth of the following crop in
other ways. A heavy layer of crop residue can keep the soil cool and wet in the spring, slowing crop
germination and development as well as slowing nutrient mineralization. It may also physically impede
the operation of planting equipment, reducing the stand and harbor pests like slugs or nematodes
which can harm the crop. However, cover crops protect the soil from wind and water erosion and some
species help to reduce the number of pests including nematodes.

The above content was adapted from the Soil Fertility Handbook, OMAFRA Publication 611, p. 124.

62 PROFICIENCY AREA II - Nitrogen

Performance Objective 15
Evaluate the principles, appropriate use and impact to timing of nitrogen
applications for:

a. urease inhibitors;
b. nitrification inhibitors;
c. controlled-release nitrogen products;
d. slow-release nitrogen products.

Typically, N is not applied as the crop needs it but rather in larger applications at set periods in crop
growth. In some cases, applying all N at pre-plant does not result in optimal use of N. As well, N is
subject to environmental losses through volatilization, denitrification, leaching and runoff. Consider
the particular soil and cropping system and evaluate which N losses may be occurring and hindering
fertilizer use efficiency.

Biological and chemical inhibitors are sometimes added to fertilizer to temporarily enhance or
disrupt very specific soil reactions. Nitrification inhibitors are additives which slow the conversion
of ammonium to nitrate in soil, which may reduce the possibility of nitrate leaching or denitrification.
Urease inhibitors, another class of additives, can be used with urea fertilizer to temporarily delay its
transformation to ammonium by inactivating urease, a common soil enzyme. This delay can reduce
ammonia volatilization losses to the atmosphere, especially when urea is applied to the soil surface.
Since the formation of ammonium has been delayed, the subsequent formation of nitrate is also
delayed.

Urease inhibitors provide potential benefits in no-till or reduced tillage systems when surface applying
N and on soils that favour ammonia loss. Its use can provide some flexibility for application timing.
However, urease inhibitors have little value if the conditions for urea hydrolysis or volatilization are
not present.

Nitrification inhibitors can be valuable: on tile drained soils when leaching potential is high; on wet or
poorly drained soils; with fall fertilizer applications; when applying ammonium fertilizers; and, in no-till
systems. It is not necessary when applying sidedress and does not work well on coarse-textured soils.
With the low cation exchange capacity, ammonia can leach out of the zone containing the inhibitor.
Again the effectiveness of nitrification inhibitors depends upon conditions that would encourage both
the nitrification of ammonium and the loss of nitrate through either leaching or denitrification.

Slow-release and controlled-release fertilizers can be useful for improving nutrient use efficiency. There
are several mechanisms for controlling nutrient release from a fertilizer particle. The most common
is when a protective coating of polymer or sulphur is added to a fertilizer in order to control the
dissolution and release of nutrients. Typical release rates range from a few weeks to many months.
Other slow-release fertilizers may have low solubility or a resistance to microbial decomposition to
control nutrient release. Each of these products may be well suited to a specific set of conditions but
that does not mean they are well suited to all conditions. Specific products must be matched with
the proper soil, crop, and environmental conditions in order to get maximum benefit. Nitrogen is the
nutrient generally targeted for controlled-release, but there are circumstances when sustained release of
other nutrients is also desirable.

PROFICIENCY AREA II - Nitrogen 63

Competency Area 4
Determining the Right Placement/Method of Application for Nitrogen

Performance Objective 16
Discuss how the source of the nitrogen will determine the
best placement or method of application.

Urea (46-0-0)
A dry material in granular or prilled form urea rapidly hydrolyzes to NH4+. Losses increase with higher
soil pH, more crop residues and with higher temperatures.

Urea can be used as a starter, broadcast or top-dress application and can be used in fertilizer mixes
(dry or liquid). If urea is surface applied and not incorporated (either by rain or tillage), N losses to the
air (as ammonia) can approach 40% of the applied N. Within one day after application, about 66%
of urea-N is hydrolyzed to ammonia-N; all within one week. When not incorporated, significant N loss
by volatilization can occur until approximately 1/2 inch of rain has fallen.

In addition, a rapid pH increase after application caused by hydrolysis of urea can result in ammonia
release that can damage seedlings if the urea is applied too close to the seed. Conversion of
ammonium to nitrate results in the formation of hydrogen ions (H+), so, like most N fertilizers, repeated
urea applications will cause a reduction in soil pH over time.

Ammonium nitrate (34-0-0)
A dry material in granular or prilled form, in which half of the N is as nitrate and half is as ammonium.
Used for direct application and in the production of N solutions (see below); broadcast or sidedress. It
can be left on surface or incorporated into soil. When applied to the soil, ammonium nitrate dissolves
in the soil water and separates into ammonium and nitrate, both of which can be absorbed by plants.
It is slightly more quickly available to plants at low temperatures than urea but, under normal growing
conditions, there is no practical difference.

Calcium ammonium nitrate (27-0-0)
Calcium ammonium nitrate is applied the same as ammonium nitrate however it acidifies soil only half
as much as ammonium nitrate.

Urea-ammonium nitrate solution (UAN) (28-0-0 to 32-0-0)
A mixture of ammonium nitrate, urea, and water. Urea supplies about half of the N that may be subject
to volatilization loss. The other half of N is supplied by ammonium nitrate. Once applied, N solution
behaves exactly like dry urea and ammonium nitrate.

UAN can be broadcast or placed in the starter band. If broadcast, UAN should be incorporated into
the soil as the urea portion is subject to volatilization. However, because of its lower percent of N in
urea and ammonium form, volatilization losses per pound of N from UAN will be lower than for urea.
Banding with drop nozzles has been found to minimize volatilization losses.

Avoid applying UAN onto crop foliage as severe burning will result. To minimize injury, do not spray
on vegetation. For post-emergence application, use a directed spray or dribble between the rows.

64 PROFICIENCY AREA II - Nitrogen

Nitrogen – Prills

Courtesy Fertilizer Canada

The benefits of this product are its uniformity, ease of storage, handling and application. Like urea,
UAN will lower the pH because of conversion of ammonium to nitrate and subsequent release of H+.

Anhydrous ammonia (82-0-0)
A high-pressure liquid that turns into a gas when released. Anhydrous ammonia is applied by injecting
it into the soil where it vapourizes and dissolves in the soil moisture. It must be injected six to eight
inches deep on friable, moist soil. N loss by volatilization can occur if not properly injected or if soil
is too wet or too dry at application. To avoid vapour losses to the air, the anhydrous band must be
placed deep enough in the soil that the injection slot closes over.

Ammonium sulphate (21-0-0)
A dry crystalline material in which the N is all in the ammonium form. It is used for direct application
and blended complete fertilizers. It can be broadcast or sidedressed, left on surface or incorporated
into soil. It is useful for surface broadcast applications as there is less risk of ammonia volatilization.
It is a good starter N source and, where S is needed, ammonium sulfate is a good source of S.

Calcium nitrate (15-0-0)
It is highly hygroscopic and may liquefy completely when exposed to air with a relative humidity
above 47%. One method of application is dissolved in irrigation water.

Potassium nitrate (12-0-44)
A specialty fertilizer used for direct application and in blended fertilizers.

PROFICIENCY AREA II - Nitrogen 65

Performance Objective 17
Discuss how the time of the year, climate, tillage practices, and residue
management will impact the proper placement or method of application.

Time of Year
If N fertilizer sits on or in the soil long before plant uptake, there is potential for N loss to air,
groundwater, or immobilized by bacteria and fungi. These losses are a direct financial loss to
the producer, may decrease yields, and may have negative environmental and health effects.
Immobilization of the fertilizer N is not necessarily a loss as this N will eventually mineralize due to
microbial biomass turnover. However, the timing of this mineralization may be too late to provide
any benefit for the crop it was intended to fertilize. The closer fertilizer application can be timed to
maximum plant uptake, the smaller these potential losses.

Split fertilizer applications allow nutrient management to be adjusted for the current year yield
potential and can reduce losses. Timing the application so nutrients are available prior to maximum
nutrient demands, which come before plants reach their maximum size, is critical. The second of split
applications on small grains should be applied prior to early tillering, though actual timing will depend
on a variety of factors including soil nutrient levels and starter fertilizer amounts. In oilseed crops, the
optimal time for supplemental in-season fertilization would be before or during branching. Because
timing of spring fertilizer applications should ideally be based on plant growth stage, rather than
calendar date, the optimal date of a second application, or top-dressing, will vary with the crop and
year. In crops started later in the spring, top-dressing would occur later in the season. A cooler spring
will also delay timing of a second application due to delayed growth and nutrient uptake.

Regardless of the crop and year, adequate nutrients are necessary early in growth for maximum
production and to ensure that N is available for good grain or seed fill. Wheat plants take up
approximately 70% of their necessary N and phosphorus by early heading. Over 50% of the N and
phosphorus used for grain fill comes from the stem, leaves, and head of the plant, rather than directly
from the soil. If the nutrients are not available for early plant growth, then yield may be compromised,
and efficiency of fertilizer use is reduced. Both are financial losses to producers. However, applying
N fertilizer too early, for example in early fall when the soil is still warm and active, can also cause
fertilizer loss. The use of slow or controlled-release N products may more closely match plant uptake
and reduce potential N loss. Fall application of N fertilizers to bare soils should be avoided.

Ammonium based fertilizer N applications in the summer under hot, dry conditions can also lead to
excessive ammonia volatilization if the fertilizer is not incorporated or injected into the soil.

Climate
Nitrogen losses due to leaching, gaseous loss, immobilization and weed growth are probably higher
for fall-applied than for spring-applied N. These losses may be greater if the N is applied too early in
the fall (prior to mid-September) or when soil temperatures at the 4” depth are greater than 5°C. Loss of
N accounts for much of the difference in efficiency between fall and spring applications. Under dry soil
conditions, the efficiency of ammonium or ammonium based fertilizers banded in late fall can approach
that of spring banded because potential losses due to leaching or denitrification are low. Efficiency
of fall-applied N can be substantially lower under excessive moisture conditions in spring or fall and/
or an early fall application before soils have cooled to 5°C as significant amounts of nitrification will
occur. Regardless, fall applied N fertilizers for next season’s crop are typically at much greater risk for
N losses than spring applied fertilizers and should be avoided.

66 PROFICIENCY AREA II - Nitrogen

Poorly drained soils or depressions have high potential for loss of nitrate-N. These losses can be
minimized through proper placement and timing of N.

Time and method of application should be based not only on the needs of the crop and potential
losses from the soil, but also on coordination of the soil fertility program with an efficient overall farm
management system. Select a time and method of N application that permits preparation of a good
seed bed, conserves soil moisture, aids in prevention of soil erosion, allows for timeliness of operations
and maximizes net returns.

Tillage Practices
Conventional Tillage
Nitrogen can be broadcast or banded in conventional tillage systems. Broadcast fertilizer is spread
over the whole soil surface usually with a truck or tractor-pulled spreader. Nitrogen is usually mixed
into the soil by tillage which helps improve efficiency rates. Nitrogen may be lost if not worked in
properly. Banded fertilizer is applied in a narrow strip below the soil surface. This may be done
at planting time or sidedressed after the crop is up. Sidedressed N is more efficient than broadcast
because it is placed close to where the roots are at a time when the crop can make the best use of it.

Mulch Tillage
Because soils in mulch till systems tend to be a little cooler and wetter in spring than in conventional
tillage systems, there may be a benefit to using starter fertilizer. The balance of the fertilizer can be
broadcast and incorporated with secondary tillage passes as in the moldboard system. Rates of
application depend on the soil test. Nitrogen fertilizer must be injected or incorporated into the soil.

No-Till and Ridge Till Systems
The absence of tillage allows nutrients to accumulate in the top layers of soil. Nutrient levels below this
layer tend to be lower than conventionally-tilled fields. Nitrogen applications on cereals are similar to
conventionally-tilled fields. Nitrogen for corn should be placed below the residue. Avoid broadcasting
urea-based products. Many no-till farmers believe approximately 30 kilograms per hectare (27 lbs
per acre) of N should be in the starter fertilizer. No yield benefits result, but early crop appearance is
improved. Coulters are usually added to equipment to improve fertilizer placement. In ridge till, N is
knifed into the ridge away from corn roots or in the valley. Liquid N can be dribbled behind the disc
hillers on the cultivator where it will be covered with soil thrown by the sweeps.

Residue Management
Broadcasting UAN solution (28-0-0 to 32-0-0) is not recommended when residue levels are high
because of the potential for the N in the droplets to be absorbed by the residue. Dribbling the solution
in a surface band will reduce this and knife or coulter injection will eliminate it. Limited research
suggests that the same conclusions probably apply for grass hay or pasture. Surface banding of N can
be useful mainly when UAN solution is the N source of choice on a field with substantial residue cover,
and the producer does not want to inject the solution.

The above content was adapted from: Scharf, P. C., Lory, J. A., Best Management Practices for
Nitrogen Fertilizer in Missouri, University of Missouri Extension and University of
Missouri-Columbia, 2006.

PROFICIENCY AREA II - Nitrogen 67

Performance Objective 18
Discuss how crop stage will determine the placement or method of application.

At or Near Time of Seeding
Nitrogen fertilizer applied at or near time of seeding is usually the most effective for increasing yields.
However, seeds are living organisms (even in the dry state) and being exposed to ammonia can reduce
viability. Both ammonia vapours in the soil air and ammonia dissolved in the soil water cause damage
from ammonium fertilizers. The damage can commence as soon as the ammonia contact takes place
and is intensified by the duration of exposure, the ammonia concentration, crop species’ tolerance and
seed metabolic activity (stage of germination).

The safest application method for high rates of high ammonium content fertilizers is to place them away
from the seed by physical separation (combined N, P products) or by pre- or post-plant application
(straight N products). For the lower ammonium content fertilizers, e.g. MAP, close adherence to the safe
rate limits set for the crop species and the soil type is advised.

After Seeding
Under moist conditions, applying N up to two weeks after emergence is a good alternative to applying
N in the fall. However, if N fertilizer is broadcast without incorporation on dry soils, N utilization may
be delayed. If urea (46-0-0) is used, gaseous N losses may occur. Ammonium nitrate (34-0-0), while not
readily available, is the preferred N source for broadcast application after seeding.

Leaf burn may occur if N solution is sprayed onto leaf surfaces. Canola, flax, corn and sunflowers
are particularly susceptible to damage. In trials, cereals at seedling stages have been sprayed with N
solution at 40 lb N/ac with minimal damage and no reduction in yield.1 Leaf burn is minimal under
cool, wet conditions. Rain or irrigation immediately following N application washes all leaf surfaces
free of fertilizer and results in little or no damage. Broadcasting granular fertilizers does not cause
damage unless the foliage is wet.

N fertilizers can be applied to row crops following crop emergence and is usually referred to as “side
dressing”. Fertilizers banded into the soil should be applied at least 6” to 8” from the row in order to
minimize root pruning. Use care so that plants are not damaged by equipment. Applying N fertilizer
between every second row (similar to mid-row banding) is referred to as “skip row application”.

Mid-season applications of N fertilizer can also be used to increase the protein content in grain.
Nitrogen application to the growing crop through irrigation water has greater efficiency than placing
all the N at the time of seeding.

Fall-applied Nitrogen on cereals does not usually give yield and/or protein increases as great as those
obtained when equal amounts are added in spring. However, in many cases, the differences in yield
between fall and spring applications are small, particularly under dry soil conditions. Losses due to
leaching, volatilization, denitrification, immobilization and weed growth are usually higher for fall-
applied N and account for differences in yield and protein content.

1. Loewen-Rudgers, L., K. McGill, P. Fehr, G, Racz, A. Ridley and R. Soper. 1977. Soil fertility and fertilizer practices. In Principles and Practices of Commercial Farming. pp. 61-89.
Faculty of Agriculture. University of Manitoba.

68 PROFICIENCY AREA II - Nitrogen

Performance Objective 19
Discuss the role of nitrogen technology products and the considerations for
nitrogen placement or method of application for:

a. urease inhibitors;
b. nitrification inhibitors;
c. controlled-release nitrogen;
d. slow-release nitrogen products.

Refer also to Performance Objective 15 above.

Slow-release N products may be considered when attempting to reduce environmental losses from
volatilization, denitrification, leaching, and runoff. There are three general categories: uncoated
controlled-release, coated controlled-release, and bio-inhibitors. The controlled-release products slowly
break down N into the soil. Bio-inhibitors are not really slow-release. They inhibit microbial processes
that convert N into plant available form which is more susceptible to environmental losses. The term
“fertilizer technologies” encompasses all types of products.

Uncoated slow-release products include:
• Urea-formaldehyde reaction products which decompose in the soil by chemical or biological
processes or a combination of both;
• I sobutylidene diurea which relies solely on soil chemical processes to break down product;
and,
• Inorganic salts such as magnesium ammonium phosphate.

Coated, slow-release N products include:
• Sulphur-coated urea which releases N through oxidation of the S coating (used on turf), and
• Polymer-coated or poly-coated urea. Water moves in through the coating to dissolve the urea. The

N diffuses out through the porous polymer membrane. The polymer coating is unique to each
manufacturer.

Coated, slow-release products can reduce N leaching on sandy soils and may provide an alternative to
split application of N.

Bio-inhibitors include urease and nitrification inhibitors.

Urease inhibitors help to control volatilization and can be added to urea or mixed with UAN. If rainfall
occurs within two to three days of N application, a urease inhibitor may prevent volatilization losses
which can be 15% to 20% of N applied. Urease inhibitors chemically inhibit the activity of the soil
enzyme urease. This slows the breakdown of the urea, providing time for rainfall to move urea into the
soil. The effect of these products can remain for two weeks or more depending on conditions. Warm
temperatures and wetter conditions cause urease to repopulate faster. As noted under P.O. 15, urease
inhibitors provide potential benefits in no-till or reduced tillage systems when surface applying N and
on soils that favour ammonia loss. Its use can provide some flexibility for application timing. However,
urease inhibitors have little value if the conditions for volatilization are not present.

PROFICIENCY AREA II - Nitrogen 69

Nitrification inhibitors delay the conversion of NH4+ to NO3- for two to four weeks depending on the
soil pH and temperatures. There may be value in using a nitrification inhibitor when NO3- losses are
high from leaching or denitrification. Nitrification inhibitors can be beneficial: on tile drained soils
when leaching potential is high; on wet or poorly drained soils; with fall fertilizer applications; when
applying ammonium fertilizers; and, in no-till systems. It is not necessary when applying sidedress and
does not work well on coarse-textured soils. With the low cation exchange capacity, ammonia can
leach out of the zone containing the inhibitor.

Performance Objective 20
Evaluate the role of fertigation in 4R nutrient management planning.

Fertigation is the application of fertilizer via the irrigation system. Relating to the 4R philosophy,
fertigation helps address the right rate, time and place of nutrient management planning. Fertigation
provides increased flexibility of fertilizer application as nutrients can be applied any time during the
growing season according to the crop’s needs and growth stage. Nutrients can be applied uniformly
over the field if the irrigation system distributes water uniformly. It also allows for small dosage
application. Leaching and water contamination is less likely with fertigation because less fertilizer
is applied at any given time and the application can correspond to the peak crop requirements.
Fertigation can also be very targeted to the root zone depending on the type of irrigation system used,
e.g. subsurface drip. The water goes into the root zone of the plants where it can be absorbed and
used quickly. This mitigates runoff and leaching.
There are disadvantages to using fertigation as well.

• F ertilizer distribution is only as uniform as the irrigation water distribution.
• L ower cost fertilizer materials such as anhydrous ammonia often cannot be used.
• With overhead irrigation systems, fertilizer placement cannot be localized as in banding.
• Many fertilizer solutions are corrosive and can damage the irrigation system.
• E xcessively wet conditions may limit the opportunity to apply fertilizer N when needed, or

enhance its risk of loss when applied.

70 PROFICIENCY AREA II - Nitrogen

Competency Area 5
Environmental Risk Analysis for Nitrogen

Performance Objective 21
Discuss how to use water quality vulnerability assessment tools (e.g. Source Water
Protection Plans) on a site specific basis for nitrogen nutrient planning.

Tools such as Source Water Protection Plans identify the water resources, their use and the potential
factors that would affect the quantity and quality of that water. Assessment reports consider aspects
such as watershed characteristics (e.g. land use and activities, area, soils, bedrock geology,
physiography, topography, etc.), water flow or water budgets (precipitation, runoff, usage, etc.), current
condition of water resources and the potential threats to the quality of the water based on the activities
and characteristics of the land within that watershed.

Vulnerable areas for surface water are areas such as those near drinking water intake pipes (also
known as Intake Protection Zones or IPZs). The IPZs can be mapped and given vulnerability scores.
IPZs with greatest vulnerability usually require greater diligence to avoid contamination.

Similarly, Groundwater Vulnerability Analysis looks at underground sources of drinking water and risks
to the quality of this water. In general there are three main areas that are vulnerable to contamination:
Wellhead Protection Areas, Highly Vulnerable Aquifers and Significant Recharge Areas. Again the
report would identify and map these vulnerable areas and assign vulnerability scores.

Activities within areas of high risk of N movement or vulnerability of contamination should require a
more judicial use of fertilizer N. A better understanding of the pathways and timing of N loss to the
water resources will enable one to adjust method, timing, source and rate of fertilizer application to
reduce losses and improve crop use efficiency. In extremely sensitive areas, it may mean a substantial
change in the rate of N applied or cropping system used to achieve this goal.

PROFICIENCY AREA II - Nitrogen 71

Performance Objective 22
Evaluate nitrogen management decisions using a water quality
vulnerability assessment (e.g. Nitrogen Index).

The Nitrogen Index is a tool for reducing the risk of nitrate contamination of groundwater. It evaluates
the vulnerability of nutrient management practices with respect to the movement of nitrates. The N
Index combines source and transport factors to assess the risk of nitrate movement to groundwater on a
field-by-field basis.

The N cycle is complex and factors contributing to both nitrate sources and transport often interact.
When manure N converts to the nitrate form, it will move through the soil with water rather than bind to
soil particles.

Source Risk Factor
The net amount of nitrate in the soil following harvest may have come from:

• N applied for growing the current year’s crop;
• nutrients applied after crop harvest;
• residual N in crop residues, especially legumes; and,
• mineralized N and nitrified materials from soil organic matter.

In the case of N applied for the current year’s crop, it is the amount of N applied in excess of crop
requirements that is of most concern. For nutrients applied after harvest, as with fall application of
manure, there is an increased risk of nitrate movement to groundwater. The timing and method of
application and the type of manure will influence risk.

Transport Risk Factor
The transport factor evaluates the opportunity for nitrate to move down, with water, through the soil to
groundwater. In Ontario, crops are normally removing more water from the soil during the growing
season than is being added as precipitation, so there is no leaching during the growing season except
under abnormally wet conditions.

The fall, winter and early spring usually bring more precipitation than evaporation so water can move
down through the soil profile. This is the reason we are concerned with the amount of nitrate in the soil
after the growing season when there is no crop to absorb the nitrate and the risk of loss is high. Cover
crops grown after crop harvest help reduce this risk of loss by taking up nutrients and holding them in
organic form until spring.

72 PROFICIENCY AREA II - Nitrogen

Performance Objective 23
Be able to evaluate how changing a specific nitrogen management
strategy will affect the outcome of a risk assessment.

It cannot be stressed enough how understanding the N cycle, seasonal hydrology and crop N
utilization is paramount to being able to identify how a change in a specific N management strategy
will affect the risk of N loss. Below is a risk-assessment table for N addressing the potential for nutrient
losses based on soil type, when fertilizer is being applied, the form of N, placement, and whether it
is being stabilized to prevent loss. It serves as an example of several scenarios, but is by no means a
complete picture of the possible scenarios that could occur in the field.

Table 2.5 Nitrogen Risk Assessment

Situation Risk Approaches
Sandy soils
Leaching Avoid fall application, make split applications
beginning in the spring using inhibitors,
Fall application to silt loam or clay loam Denitrification, leaching slow-release forms, fertigation

Pre-plant applications to silt loam or Denitrification, leaching, runoff, Ammonia application with an N stabilizer
clay loam volatilization although still less efficient than spring
applied fertilizers.
At planting surface application with Volatilization, runoff, denitrification
no-till Ammonia with a N stabilizer, PCU, methylene
urea, urea with stabilizer, incorporation or
Sidedress application or fertigation Wet weather preventing timely injection of fertilizer
application
Urea with stabilizer, PCU, methylene urea,
UAN and stabilizer

Pre-plant or at-planting split applications
using inhibitors or slow-release formulations

Source: University of Nebraska, accessed from www.notillfarmer.com

Best management practices for reducing the risk of N losses include continuous no-tilling, cover crops,
precision tillage as needed to remove compaction, installing vegetative buffers as protection against
extreme erosion events, and physically placing nutrients in the soil so they are less vulnerable to runoff.

Sandy soils have a higher risk of N leaching losses. Soil will move with heavy rainfall events, even
with no-till, but it can be minimized by utilizing cover crops with active roots to keep the soil intact and
adopting structures that minimize runoff. Many farmers have adopted conservation tillage, leaving
larger amounts of residue on the surface that reduces soil movement and runoff.

Physical structures like terraces, contour farming, grassed waterways and buffer strips reduce
runoff as well, but on large-acreage farm operations with big equipment, these practices can be an
inconvenience.

The benefits of cover crops are being increasingly promoted. Besides protecting the soil against
erosion, they scavenge and recycle excess nutrients, such as residual nitrates, out of the soil and
contribute carbon back into the soil, which improves soil health and soil and nutrient resiliency.

PROFICIENCY AREA II - Nitrogen 73

When managing N, growers have tools to reduce the risk of N loss while increasing N-use efficiency.
The first recommendation is to move fall applications to the spring, and spoon-feed N over several
applications, such as pre-plant, at-plant and sidedress.

Second is stabilizing N and manure applied to protect it from loss. Urease inhibitors limit urea
hydrolysis and thus control loss of ammonia as a gas from urea and UAN. Nitrification inhibitors
(nitrapyrin and dicyandamide) can be used to control conversions of ammonia forms of N to nitrate,
limiting nitrate loss through leaching or denitrification.

Slow-release forms of N are available which parse out nutrients into the soil environment to match
the uptake needs of the crop, reducing risk of loss. Slow-release products include sulfur-coated urea,
polymer-coated urea, sulfur and polymer-coated urea, methylene urea plus triazone, and others.

Performance Objective 24
Evaluate management strategies that will reduce nitrogen loss to surface water
and groundwater, ammonia volatilization, and nitrous oxide emissions.

The diagram below illustrates the various forms and pathways that N can take as it cycles through an
agricultural production system. Before N can be used by plants, it must be converted into forms that
are available to plants; this conversion is called mineralization. The plants take up these mineral forms
through their root systems and form plant proteins and other organic forms of N. Livestock eat crops
and produce manure, which is returned to the soil, adding organic and mineral forms of N to the soil,
which can be used again by the next crop. Ideally, it would be most economically and environmentally
beneficial to keep all the N in this tight cycle for food production. In reality, however, some leakage
occurs. Where there is too much N leakage, there can be environmental harm.

Source: International Plant Nutrition Institute

74 PROFICIENCY AREA II - Nitrogen

Nitrate
Nitrate (NO3-) is an extremely soluble form of N. It does not bind with the surfaces of clay minerals
nor does it form insoluble compounds with other elements that it encounters when moving through
the soil. Because nitrate is soluble, it can readily move with soil water toward plant roots to be taken
up by them. However, if there is a large amount of water entering and passing through the soil root
zone, NO3- can be carried by percolating water beyond the soil root zone. This downward and lateral
movement through the rooting zone and possibly towards agricultural tile drainage systems is driven
by water infiltrating from rainfall or a snow melt. This leaching occurs at times of the year or at points
in a field where the amount of rainfall or snow melt exceeds evapotranspiration and the soil is near its
saturation capacity. Under such conditions, soil water moving downwards recharges groundwater or
contributes to tile drain flow, carrying nitrate with it.

Nitrite
Nitrite (NO2-) is produced naturally as part of the process of converting ammonium into nitrate. It
seldom accumulates in the soil, since the conversion from nitrite to nitrate is generally much faster than
the conversion from ammonium to nitrite. Nitrite moves much like nitrate in the soil and groundwater
zones.

Ammonia
Ammonium (NH4+) bonds to negatively charged surfaces of soil particles, clay in particular. The
concentration of ammonium in the soil is generally quite low (<1 mg/kg) because it is quickly converted
to nitrate under conditions that are favourable for mineralization. The exception is where high rates of
an ammonium fertilizer (anhydrous ammonia, urea or ammonium sulphate) or high rates of manure are
applied. Occasionally, heavy rainfall washes this concentrated ammonium from the field into surface
water. A small part of this ammonium can be converted to dissolved un-ionized ammonia (NH3),
which can harm fish. The conditions that favour ammonia generation are alkaline pH and warm water
temperatures.

Volatilization and Denitrification
Natural losses of N, in addition to nitrate leaching, occur through ammonia volatilization and
denitrification. Ammonia volatilization occurs when manure or an ammonia-based fertilizer (particularly
urea) are applied to the surface of the soil without incorporation into the soil. Over half of the
ammonium N from manure can be lost to the air under warm, dry conditions, greatly reducing the
fertilizer value of the manure. However, the concentrations of ammonia released are not high enough
to cause direct environmental or human health harm outdoors, and most of the ammonia is re-deposited
within a few hundred metres of where it was released.

Denitrification is a natural process where microbes in the rooting zone use the oxygen in nitrate where
there is not enough air in the soil. This process converts the nitrate into gaseous forms of N, primarily
N2, but also into nitrous oxide (N2O) or nitric oxide (NO). Conditions that favour dentrification within
the rooting zone are soils with slow internal drainage (fine-textured soils), an ample carbon supply
and saturated soils from shallow groundwater or heavy rainfall. Denitrification can also occur in
the groundwater and surface water environments. In some aquifers, denitrification can result in the
complete conversion of nitrate to dissolved N gas, which is not harmful to aquatic ecosystems or human
health. However, denitrification cannot be counted on to eliminate all the N leaching to groundwater or
running off to surface water.

PROFICIENCY AREA II - Nitrogen 75

Management Strategies
• Match N applications with crop requirements; use the spring or pre-sidedress soil N test where
available (e.g. for corn and barley).
• When planning nutrient applications, account for N contributions from other sources, such as:
green manure crops, previous crop rotations, manure or biosolid application.
• Apply most of the N just before the time of maximum crop uptake (e.g. sidedress corn).
• S plit applications of N through techniques such as fertigation.
• Practice crop rotations to make efficient use of N and maintain healthy soils.
• E stablish cover crops as needed to “tie up” any excess N at the end of the season.
• Practice timely tillage to incorporate manure, balancing the risk of soil compaction with the
losses of N to the atmosphere if the manure is not incorporated quickly.
• Avoid applying manure near surface water or on steeply sloping land.
• K eep application rates low enough to prevent runoff.
• M ix manure into the soil as soon as possible after
applying it.
• On tile-drained land, keep application rates of liquid
manure below 40 m3/ha (3,600 gal/ac) or pre-till
the field before applying it. This will help prevent the
movement of manure directly to tile through cracks or
earthworm channels.
• U se buffer strips and erosion control structures to filter runoff
before it enters surface water. Buffer strips in riparian zones
have proven to reduce nutrient movement off the field into
nearby surface water sources. Buffer strips consume excess
nutrients before they flow into surface water and enhance
opportunities for groundwater denitrification.

Buffers include: riparian buffers, filter strips, grassed waterways,
shelterbelts, windbreaks, living snow fences, contour grass strips,
cross-wind trap strips, shallow water areas for wildlife, field borders,
alley cropping, herbaceous wind barriers, and vegetative barriers. Birdseye view of an agricultural landscape

with riparian forest buffers and other types of
conservation buffers. Photo source: USDA NRCS.

Performance Objective 25
Compare the differences in the geographic scale, soil, topography, and location
of watersheds (e.g. national, regional, local) on the environmental impacts of
nitrogen on surface and groundwater resources.

Generally, aquifer vulnerability is represented by soil-drainage characteristics, the ease with which
water and nutrients can seep to groundwater, and the extent to which woodlands and riparian areas
are interspersed with crop land. Whether nitrates actually enter groundwater, depends on underlying
soil and/or bedrock conditions, as well as the depth to groundwater. The risk of N leaching into
groundwater occurs particularly where the soil is sandy, gravelly, or shallow over porous limestone
bedrock. If depth to groundwater is shallow and the underlying soil is sandy, the potential for nitrates
to enter groundwater is relatively high. However, if depth to groundwater is deep and the underlying
soil is heavy clay, groundwater contamination from nitrates is not likely.

76 PROFICIENCY AREA II - Nitrogen

Performance Objective 26
Discuss the role of nitrogen in the eutrophication process and the potential
consequences of eutrophication.

Environmental concerns arise when N is leached into groundwater or delivered as runoff during rainfall
events to streams, rivers, and lakes. Excess nutrients in aquatic systems continue to act as fertilizer and
can stimulate the growth of plants and algae leading to oxygen deficient conditions. This is known
as eutrophication.

In extreme cases of eutrophication, microscopic algae present in the water grow to densities so high
that they reduce the light available to rooted plants living on the bottom. This shading effect may
prevent photosynthesis causing the plants to die, resulting in the loss of important habitat for fish and
other organisms. Greater production of algae may also lead to an increase in the frequency and
duration of periods of low dissolved-oxygen concentration known as hypoxic events, which can cause
further damage to the system as conditions do not support enough oxygen to sustain fish and other
aquatic creatures.

Nutrient-enriched aquatic systems sometimes become dominated by noxious species of algae that form
floating surface scums called blooms. Some of these algal species produce toxic substances that can
negatively impact other plants and animals, including humans.

For many years, eutrophic conditions in inland freshwater systems have been attributed to excessive
inputs of phosphorus rather than N. With higher concentrations of readily available P in surface waters,
N may also contribute to the growth of aquatic plants and algae. In salt water systems, both N and P
contribute to eutrophication.

Performance Objective 27
Discuss the role of nitrogen in drinking water standards.

The Ontario Drinking Water Standards (ODWS) set 10 mg/L (10 parts per million) nitrate as N
(NO3--N) as the maximum allowable level for drinking water in Ontario. Medical researchers concluded
that a concentration of 10 mg/L in drinking water is appropriate to avoid blue-baby syndrome in
human infants. Recent research suggests that consistently high levels of nitrate in surface waters can
harm some forms of aquatic life, particularly amphibians.

For nitrite as N (NO2-), the ODWS set 1 mg/L (1 part per million) as the maximum level for drinking
water in Ontario. Nitrite levels in drinking water should not exceed this value. The Canadian guideline
for aquatic water quality has an upper limit for nitrite of 0.06 mg/L (60 µg/L or parts per billion).
While nitrite is much more toxic to aquatic life than nitrate, nitrite tends to convert quickly to nitrate.

Unlike nitrate and nitrite, ammonia is not a human health concern in drinking water at the levels
typically observed in the environment. However, it is toxic to fish at high concentrations. The Provincial
Water Quality Objective (PWQO) for dissolved un-ionized ammonia is 20 µg/L.

PROFICIENCY AREA II - Nitrogen 77

PROFICIENCY AREA III

PHOSPHORUS

Competency Area 1
Determining the Right Source of Phosphorus

Performance Objective 1
Discuss the most common sources of phosphorus used in Ontario.

Ammonium Polyphosphate
The most common ammonium polyphosphate fertilizers have a grade of 10-34-0 or 11-37-0.
Polyphosphate fertilizers offer the advantage of a high nutrient content in a clear, crystal-free fluid that
is stable under a wide temperature range and has a long storage life. A variety of other nutrients mix
well with polyphosphate fertilizers, making them an excellent base for complete mixed fertilizers. They
can also be used as carriers for micronutrients that may be needed by plants, although these may need
to be in a chelated form to prevent formation of insoluble sludges.

In polyphosphate fertilizer, 50% to 75% of the P is present in chained polymers. The remaining P
(orthophosphate) is immediately available for plant uptake. The polymer phosphate chains are primarily
broken down to the simple phosphate molecules by enzymes produced by soil microorganisms.

Commercial fertilizer sources include monoammonium phosphate, diammonium phosphate, fluid
ammonium, triple superphosphate, and some registered materials manufactured from biosolids.
Manures from beef, dairy, swine, poultry and other livestock generally supply large quantities of
phosphorus when they are land-applied. Advisors need to know how to calculate the available and
total amount of phosphorus contained in manures. Biosolids may supply considerable P, and may be
applied as non-agricultural source material (NASM) under a NASM plan approved by the Ministry of
Agriculture, Food and Rural Affairs, or as fertilizers if the product is registered with the Canadian Food
Inspection Agency.

Monoammonium phosphate (MAP)
Monoammonium phosphate (MAP) is a widely used source of P and N. It has the highest P2O5 content
of any common solid fertilizer (48% to 61%). Its N content is 10% to 12%. Granular MAP is water
soluble and dissolves rapidly in soil if adequate moisture is present. Upon dissolution, the two basic
components of the fertilizer separate to release NH4+ and H2PO4-. Both of these nutrients are important
to sustain healthy plant growth. The pH of the solution surrounding the granule is moderately acidic,
making MAP an especially desirable fertilizer in neutral and high pH soils.

Agronomic studies show that there is no significant difference in P nutrition from various commercial
P fertilizers under most conditions. Granular MAP is applied in concentrated bands beneath the soil
surface in proximity of growing roots or in surface bands. It is also commonly applied by spreading
across the field and mixing into the surface soil with tillage. In powdered form, it is an important
component of suspension fertilizers.

78 PROFICIENCY AREA III - Phosphorus

There are no special precautions associated with the use of MAP. The slight acidity associated with
this fertilizer reduces the potential for NH3 loss to the air. MAP can be placed in close proximity to
germinating seeds with minimal concern for NH3 damage. Band placement of MAP protects the P from
soil fixation and facilitates a synergism between ammonium and phosphate uptake by roots. When
MAP is used as a foliar spray or added to irrigation water, it should not be mixed with calcium or
magnesium fertilizers. A high purity source of MAP is used as a feed ingredient for animals.

Diammonium phosphate (DAP)
Diammonium phosphate (DAP) is the world’s most widely used P fertilizer. It is popular because of its
relatively high nutrient content and its excellent physical, handling and storage properties. The standard
grade of DAP is 18-46-0. Fertilizer products with a lower nutrient content may not be labeled as DAP.

The high nutrient content of DAP reduces handling, freight, and application costs. DAP fertilizer is an
excellent source of P and N for plant nutrition. It is highly soluble and thus dissolves quickly in soil to
release plant-available phosphate and ammonium. An alkaline pH develops around the dissolving
granule. As ammonium is released from dissolving DAP granules, volatile ammonia can be harmful to
seedlings and plant roots in immediate proximity. To prevent the possibility of seedling damage, care
should be taken to avoid placing high concentrations of DAP near germinating seeds. The ammonium
present in DAP is an excellent N source and will be gradually converted to nitrate by soil bacteria,
resulting in a subsequent drop in pH. Therefore, the rise in soil pH surrounding DAP granules is a
temporary effect. This initial rise in soil pH neighboring DAP can influence the micro-site reactions
of phosphate and soil organic matter. There are differences in the initial chemical reaction between
various commercial P fertilizers in soil, but these dissimilarities become minor within weeks or months,
and are minimal as far as plant nutrition is concerned. Most field comparisons between DAP and
monoammonium phosphate (MAP) show only minor or no differences in plant growth and yield.

Monoammonium Phosphate (MAP)

Courtesy Fertilizer Canada

PROFICIENCY AREA III - Phosphorus 79

Some of the polyphosphate will decompose without the enzymes. The enzyme activity is faster in moist,
warm soils. Typically, half of the polyphosphate compounds are converted to orthophosphate within
a week or two. Under cool and dry conditions, the conversion may take longer. Since polyphosphate
fertilizers contain a combination of both orthophosphate and polyphosphate, plants are able to use
this fertilizer source very effectively. Most P-containing fluid fertilizers have ammonium polyphosphate
in them. For most situations, the decision to use dry or fluid fertilizers is based on the price of nutrients,
fertilizer handling preferences, and field practices rather than significant agronomic differences.

Some liquid fertilizers have been formulated to contain 100% orthophosphate, rather than
polyphosphate. These are agronomically equal to ammonium polyphosphate.

Triple Superphosphate (TSP)
Triple superphosphate (TSP) was one of the first high analysis P fertilizers that became widely used
in the 20th century. Technically, it is known as calcium dihydrogen phosphate and as monocalcium
phosphate, [Ca(H2PO4)2.H2O]. It is an excellent P source, but its use has declined as other P fertilizers
have become more popular.

TSP has several agronomic advantages. It has the highest P content of dry fertilizers that do not contain
N. Over 90% of the total P in TSP is water soluble, so it becomes rapidly available for plant uptake.
As soil moisture dissolves the granule, the concentrated soil solution becomes acidic. TSP also contains
15% calcium (Ca), providing an additional plant nutrient. A major use of TSP is in situations where
several solid fertilizers are blended together for broadcasting on the soil surface or for application in a
concentrated band beneath the surface. It is also desirable for fertilization of leguminous crops, such as
alfalfa or soybeans, where no additional N fertilization is needed to supplement biological N fixation.
The popularity of TSP has declined partly because its total nutrient content (N + P2O5) is lower than in
ammonium phosphate fertilizers.

Performance Objective 2
Discuss considerations to determine the right source of phosphorus based on:

a. crop type and cropping system;
b. c limate (temperature, precipitation, leaching, and runoff patterns);
c. soil texture and the effect of soil pH;
d. e nvironmental concerns in the local area (surface and groundwater);
e. crop stage.

Crop Type
The primary consideration is ensuring food safety for crops grown for direct human consumption.
Manure and improperly managed compost can be a potential source of pathogen contamination. For
fresh market fruits and vegetables, allow at least 120 days between the application of manure and
harvest. Proper composting of manure results in sufficient heating to reduce the level of most pathogens
(OMAFRA Publication 363).

80 PROFICIENCY AREA III - Phosphorus

Cropping System
Consider the phosphorus needs and the application opportunities for the next crop in the rotation
as well. For example, corn and cereals consistently show greater response to starter fertilizer than
soybeans, so allowing soybeans to utilize the residual fertility from other crops in the rotation has
agronomic merit. The source needs to be economical for the amount required. Where risks of runoff are
high, the source needs to be one that can be mixed with the soil or placed below the surface in such a
manner as not to increase risks of erosion. When applying sources with a large volume of liquid, such
as liquid manures, risks of surface runoff and leaching to tile drains through macropores should be
controlled.

Soil Texture
Soils high in clay have the greatest risk of runoff and of macropore flow to tile drains, and thus require
materials that can be mixed into the soil or applied in subsurface bands.

Soil pH
Avoid applying diammonium phosphate in bands near seedlings. Monoammonium phosphate or triple
superphosphate are much more suitable for band application near the seed row in such soils. Soils with
pH below 5.5 or above 7.5 may tend to have high capacity to fix phosphate in unavailable forms.
Applying agricultural lime to raise the pH of acidic soils will improve P availability as well as provide
other benefits for crop growth. In such soils, banding should be the first option, if P fixation is the
issue, with over-application of organic P a second option. Organic sources of P may be preferred, if
available, because large amounts of P can be supplied at relatively lower cost, and the organic matter
may block fixation sites, increasing the availability of the applied P.

Local Environmental Concerns
In watersheds where phosphorus loading reduction is a high priority, soil test levels are high, and
manure P is in surplus to crop removal, manure P either needs to be exported out of the watershed,
or its production needs to be reduced through livestock feeding strategies or herd reductions. In
watersheds where P loading reduction is a high priority, and soils have high runoff potential, it is
particularly important to use P sources that can be injected, subsurface applied, or incorporated
immediately following broadcast application.

Crop Stage
As a nutrient, phosphorus is most often limiting the early growth of crop seedlings, and moves only
slowly through the soil matrix. Therefore, it is best applied before planting, mixed into the soil, or at
planting, banded near the seed row. For crops with high P requirements, like potatoes, some benefit
can be obtained by applying part of the P application just prior to hilling, allowing the hilling operation
to mix it with soil that is placed near the growing plants. Liquid manure application rates need to avoid
saturating the soil with water from just before planting through the growing season.

PROFICIENCY AREA III - Phosphorus 81

Competency Area 2
Determining the Right Rate of Phosphorus

Performance Objective 3
Interpret how soil test phosphorus levels relate to crop yield response
and potential environmental impacts.

The general relationship of soil test P to crop yield and P loss in runoff is shown in the figure to the left.
Information more specific to Ontario soils is shown on the right. The critical value of soil test, beyond
which response to phosphorus is less frequent, is commonly accepted as 20 ppm for corn in Ontario
soil test recommendations.

Crop YieldCritical
P loss in runoffvalue
for crop yield

Low Medium Optimum High

As soil P increases so does crop yield and the potential for P loss in Relative yield of corn versus soil test phosphorus from 18 site-years
runoff. The interval between the critical soil P value for yield and in an Ontario study conducted by Philom Bios and Dow Elanco
(1993-1994). Adapted from Publication 611, OMAFRA.
runoff P will be important for P management.

As soil test levels increase, the concentration of dissolved phosphorus in runoff and drainage water
increases. A study of Ontario soils (Wang et al., 2010) found that phosphate concentrations in runoff
water (arising from artificial rainfall applied at 75 mm per hour) rose linearly with soil test phosphorus,
whether by Olsen or Mehlich-3 methods, and that the best predictor of dissolved P concentrations in
runoff was the ratio of phosphorus to aluminum in a Mehlich-3 extract. The linear relation predicted that
dissolved P in runoff would increase by 0.00275 mg/L for each 1 ppm increase in Olsen soil test P.

Another study of the same Ontario soils (Wang et al., 2012) found that dissolved phosphorus in
drainage water leached through the soil (by adding 10 mm of artificial rain to soil cores previously
wetted to field capacity) began to increase sharply at soil test levels higher than 48 ppm Olsen or
112 ppm by Mehlich-3.

The potential for phosphorus loss is related to other factors as well. Proper interpretation of the soil test
as an indicator of risk of environmental impact requires the context of a phosphorus index accounting
for all sources of phosphorus, and transport factors governed by landscape, soil hydrology and
connectivity to water bodies.

82 PROFICIENCY AREA III - Phosphorus

Performance Objective 4
Evaluate how different soil test phosphorus extraction methods affect the
interpretation of crop yield response and potential environmental impacts.

Laboratories may report values for soil test phosphorus other than Olsen-P, the extractant recommended
for Ontario soils. These values may include Bray P1, Mehlich-3, and others. Recommendations for crop
nutrient need in Ontario are based on a calibration of crop response to the Olsen P level. The general
trend is that results from all of the extractants increase as soil fertility improves, but each extractant has
a unique relationship to crop response, and there is less or no public data relating values from other
extractants to crop response.

Regarding potential environmental impacts, the two references given above for Performance Objective
3 (Wang et al., 2010 and 2012) provide information relating values from several different extractants,
including Olsen-P, Bray P1 and Mehlich-3, to phosphorus concentrations in runoff and leaching water.
Again, each extractant has a unique relationship to impact, in this case, one component of risk of
phosphorus loss.

The important point for both crop response and environmental impact is that interpretation must be
based on published evidence, and is specific to each extractant. The accuracy of each extractant will
vary with soil conditions, and their interaction with the extractants. This is particularly true in alkaline
soils, where the acidic extractants (Mehlich-3 and Bray) can be neutralized by free carbonates and
produce erroneous results.

Performance Objective 5
Estimate the environmental risk of applying phosphorus above crop response
optimums.

Application of phosphorus at rates higher than needed for crop response can lead to increases,
or buildup, of soil test P. Higher soil test P levels are one factor considered in the Phosphorus Index
(OMAFRA, 2005), and generally will increase the risk rating, depending on the current soil test level
and the level of transport factors. Higher rates of phosphorus application also directly increase risk of
loss, particularly when the source is surface applied without incorporation.

PROFICIENCY AREA III - Phosphorus 83

Performance Objective 6
Justify the considerations for phosphorus application rate based on:

a. soil characteristics including leaching;
b. topography and runoff;
c. crop conditions, crop type, and growth stage.

Soil Characteristics
Recommended phosphorus application rates generally do not adjust for soil characteristics other than
soil test level.
Soils more prone to leaching (sandy soils) do not necessarily lose much phosphorus by leaching, since
phosphorus is adsorbed to soil particle surfaces or precipitated as calcium or magnesium phosphates in
soils with reasonable pH levels. Leaching of phosphorus can be an environmental impact of concern in
soils that have been built up in soil test P such that P sorption has been saturated.
Vertical movement of P to tile drains (sometimes referred to as leaching) has been documented in
medium to fine-textured soils where macropores (either cracks from soil drying or biopores such as
earthworm burrows) provide pathways for runoff diverted from the surface to the tile drains.
Topography and Runoff
In soils highly prone to surface runoff or to macropore flow to tile drains, loss of dissolved phosphorus
can be a concern when high rates of soluble forms of phosphorus are surface applied without mixing
into the soil. Topography and soil texture influence the potential for surface runoff. While losses tend
to decrease when rates are lowered, the influence of placement and timing of application can be even
more influential.
Steeply sloping fields can also be susceptible to soil erosion, leading to high losses of particulate
phosphorus if suitable practices are not put in place (reduced tillage, no-till, cover crops, soil erosion
structures).

A young windbreak that will one day offer protection to
the sandy soils of the Town of Erin, Wellington County.

Courtesy Credit Valley Conservation

84 PROFICIENCY AREA III - Phosphorus

Crop Conditions, Crop Type, and Growth Stage
Phosphorus application rates may be based on crop removal of phosphorus in order to maintain soil
test P at optimum levels. If so, recommendations for rate should consider a reasonable estimate of
crop yield potential, and use its estimated nutrient content to calculate anticipated crop removal of
phosphorus with the products harvested from the field. Phosphorus is normally applied at or before
planting, and thus growth stage is not a consideration for most crops. A few exceptions include
potatoes, which can benefit from a broadcast phosphorus application just before hilling, and perennial
forages, which often receive a maintenance application after each cut. Research at the University of
Guelph showed that there was no difference in crop response between multiple applications or annual
applications of P and K. From a risk of loss perspective, P should be applied to forages after first or
second cut to match the season with lowest risk of surface runoff and provide adequate time for applied
P to bind with the soil before runoff occurs.

Source: International Plant Nutrition Institute

PROFICIENCY AREA III - Phosphorus 85

Performance Objective 7
Calculate phosphorus credits from:

a. previous phosphorus application;
b. manure;
c. biosolids and other organic amendments;
d. wastewater.

Previous Phosphorus Application
Previous phosphorus applications are an important consideration when they have not matched crop
removal. Applications at rates in surplus of crop removal tend to increase soil test levels, while rates
in deficit of crop removal do the opposite. Soil testing is the recommended method for accurately
assessing change in soil test values. To calculate an estimate of current soil test level from one previous
(preferably from less than four years past), increase it by 1 ppm for each 25 to 37 kg/ha of phosphate
surplus or decrease it by the same amount in the case of phosphorus deficit. This calculation does not
impact the phosphorus rate recommended substantially unless the surplus or deficit of the phosphorus
balance is large.

Manure
Manure credits for phosphorus are determined from an analysis of the manure to be applied, or from
tables for the specific type of manure (OMAFRA Publication 811). Values reported as “P” (the elemental
form) can be converted to P2O5 (the oxide form) by multiplying by 2.29. In tables or laboratory reports,
“available phosphorus” may be reported, and is calculated as 40% of total phosphorus. In the long
term, 80% to 100% of the total P in manure contributes to soil test P.

Biosolids and Other Organic Amendments
Biosolids and other organic amendments are often applied at rates providing more phosphorus than
needed for either optimum crop yields, or to replace crop removal. Rates of application may be
governed by a NASM plan, based on current soil test P levels (OMAFRA Sewage Biosolids).

Wastewater
Wastewater from sources on or off the farm applied to cropland (most often by irrigation) should be
analyzed for phosphorus to determine its contribution to crop nutrition and to ensure that excessive
amounts are not applied. Several application options exist for greenhouse nutrient feedwater,
covered by Ontario Regulation 300/14 (OMAFRA, 2016). See P.O. 1, Proficiency Area 1, Nutrient
Management Planning.

86 PROFICIENCY AREA III - Phosphorus

Performance Objective 8
Justify the potential need to adjust the phosphorus application rate based on
legacy phosphorus and application method.

The term “legacy phosphorus” refers to that which has accumulated as a result of past human activity.
In cropland, the main source of legacy phosphorus is that which has been applied in the past from
fertilizers, manures, biosolids, or other sources, net of crop removal. Soil test phosphorus adequately
reflects historical phosphorus accumulation or depletion, so there is no need for separate consideration
of legacy phosphorus in the soil. Legacy phosphorus also includes that which has accumulated in
stream and river sediments. Managing its impact on water quality may involve protection of stream
banks from erosion, but is not affected by the choice of phosphorus application rate.

Application method or “right place” may in some cases influence the amount applied. When
phosphorus sources are band-placed in or near the seed row, maximum safe rates should not be
exceeded (OMAFRA Publication 611 Table 7-4, or Publication 811, Table 9-21). The maximum
safe rate for application of diammonium phosphate is lower than that for other fertilizer sources. For
replenishment of crop removal, however, application method requires no adjustment to phosphorus
application rate.

Competency Area 3
Determining the Right Timing of Phosphorus Application

Performance Objective 9
Discuss the importance of the following on phosphorus application timing:

a. intensity of precipitation;
b. type of precipitation;
c. duration of precipitation;
d. runoff.

Precipitation and runoff are risk factors for phosphorus loss in runoff and drainage water mainly when
phosphorus sources are broadcast applied and left on the soil surface. Broadcast applications of
phosphorus sources should be avoided when substantial rainfall is imminent or forecast to occur before
the material can be incorporated into the soil.

Broadcast applications in late summer or early fall (i.e. after wheat or small grain harvest) are less
likely to contribute to runoff phosphorus loss than applications in late fall or early spring, because soils
are drier and can absorb more rain before generating runoff, and the ratio of precipitation to potential
evaporation is smaller. Broadcast application of P on frozen or snow-covered soil in the winter is never
the right time, no matter what the source, because runoff risks are very high when rain falls on frozen
soil. For band-applied phosphorus, application timing is a consideration only if rain is likely to create
channel erosion of the bands.

PROFICIENCY AREA III - Phosphorus 87

Performance Objective 10
Discuss the mechanisms of phosphorus loss to surface water.

Phosphorus is lost in two forms, either particulate or dissolved. Particulate losses occur as a result of
soil erosion. In the particulate form, phosphorus is attached to suspended particles of sediment. These
particles are usually clay or silt. In cultivated fields, most of the P lost (70% or more) is in the particulate
form. Surface runoff from grass, forest, or noncultivated soils, however, carries little sediment and is
generally dominated by dissolved P (about 80% of total P loss).

Losses of dissolved phosphorus also occur when sources containing soluble phosphorus are left on the
soil surface without mixing with the soil. These sources include commercial fertilizer, manures, biosolids
and fresh plant residues.

Performance Objective 11
Discuss reduction strategies and management for particulate phosphorus loss.

The loss of particulate forms is managed by controlling soil erosion, and preventing buildup of soil
test P to unreasonably high levels. Soil erosion and soil test P are both accounted for in the Ontario
Phosphorus Index (OMAFRA, 2005). An implementation for RUSLE 2.0 has also been made available
(OMAFRA, 2014).

Control of soil erosion is managed by choice of crop rotation, cover crops, and tillage strategies. In
general, minimal or zero tillage helps reduce soil erosion by increasing soil cover by crop residue.
Contour tillage or tillage across rather than along the slope helps to minimize soil erosion. Timeliness
of tillage and other field operations to minimize soil compaction is also important. Maintaining soils in
a well-drained condition through use of grassed waterways and tile drains also helps prevent erosion.
Buffers help stabilize stream banks, protecting them from erosion and particulate P loss as well. Control
of wind erosion through use of windbreaks can also help prevent transfer of particulate P to rivers and
lakes.

Preventing buildup of soil test P is accomplished by applying at rates in accordance with
recommendations based on soil testing, ensuring that application rates do not exceed removals when
soil test P is at optimum levels, and applying less P than crop removal, or none, on soils with higher
soil test P than necessary. Buildup of soil P stratification should also be managed by ensuring sufficient
depth of P placement, by either injecting or incorporation.

88 PROFICIENCY AREA III - Phosphorus

Performance Objective 12
Discuss reduction strategies and management for dissolved phosphorus loss.

Losses of dissolved phosphorus are managed by ensuring that any applied nutrient forms containing
soluble phosphorus are mixed into the soil before any runoff-generating rainfall, or placed below the
soil surface. Both manures and fertilizers contain soluble P, and thus when left on the soil surface, may
dissolve into runoff water.

Runoff risks vary seasonally. Generally surface runoff occurs more frequently in late fall and early
spring. Any soluble nutrients applied in winter are therefore susceptible to loss in runoff water
during spring rains and snowmelt. Surface runoff is much less likely in late summer and early fall
(e.g. following harvest of wheat and small grains). Surface broadcast applications are much less
likely to increase losses of dissolved P when made in late summer or early fall. For situations where
incorporation is not practical, such as perennial forages, P applications that follow first or summer cuts
risk less harm to water quality than those made in early spring or late fall.

Cover crops, and careful management of tillage and field operations to prevent soil compaction can
help increase infiltration and soil water holding capacity, and reduce the proportion of rainfall received
that runs off.

The overall management strategy may include drawdown of soil test P, if soil test P levels are much
higher than the agronomic optimum, and field structures to control runoff. These practices are outlined
in greater detail in OMAFRA (2011). It may also include drainage water management if it is suited to
the farm landscape (see Performance Objective 19 for further detail).

Performance Objective 13
Discuss how phosphorus contamination of surface water can occur from tile
drainage due to timing of application.

In many tile-drained fields, macropores created by cracking in clay soils, or by earthworm channels in
any soil, cause preferential flow of surface water directly to the tile, with little time for interaction of this
water with the soil matrix. Because of the rapid nature of preferential flow, sorption of phosphorus to
the surfaces of soil particles is minimal. If phosphorus fertilizer or manure is broadcast on the surface
within a few days of a rainstorm large enough to generate macropore flow, this flow path can allow
water, highly enriched in dissolved phosphorus, to reach the tile drain. This is an important pathway
of loss for the western basin of Lake Erie (Fisher, 2014). Producers are advised to pay close attention
to the weather forecast, and avoid broadcasting P fertilizer when there is more than 50% chance of
intense rain within the next few days. Levels of dissolved P in runoff decline considerably if a runoff
event occurs more than three to five days after application.

PROFICIENCY AREA III - Phosphorus 89

Competency Area 4
Determining the Right Placement/Method of Application for Phosphorus

Performance Objective 14
Discuss the importance of the following in determining the optimal placement or
method of application of phosphorus:

a. intensity of precipitation;
b. type of precipitation;
c. duration of precipitation;
d. runoff.

Precipitation and runoff are risk factors for phosphorus loss in runoff and drainage water mainly when
phosphorus sources are broadcast applied and left on the soil surface. When applied phosphorus
is placed below the soil surface, or mixed with the bulk soil, risks of dissolved phosphorus loss are
minimized (IPNI, 2013). The process of application or soil mixing, however, may disturb the residue
cover of the soil and make it more prone to soil erosion. For this reason, on soils prone to soil erosion,
risks of intense rainstorms and runoff need to be considered when choosing the combination of
placement and timing for phosphorus application.

Performance Objective 15
Discuss the relationship between tillage practices/system on
phosphorus management.

In cropping systems where the soils are moldboard plowed every few years, soil test phosphorus levels
are fairly uniform within the plow depth. In no-till cropping systems, particularly where phosphorus
sources are surface applied, soil test phosphorus levels are stratified, with the top two to three cm of soil
around three times as high as the top 15 to 20 cm. When conservation tillage tools are used, soil test P
stratification is intermediate between these two extremes (Bruulsema, et al., 2012).

In no-till systems, the soil mycorrhizal network flourishes, providing more phosphorus to the crop for a
given soil condition. Soil conditions in no-till, however, are often colder and wetter in the spring than
in tilled soils, often resulting in higher crop phosphorus need for early season growth and greater
crop response to applied phosphorus. The two considerations often balance each other, so the
recommended amounts to apply do not change depending on tillage system.

90 PROFICIENCY AREA III - Phosphorus

Performance Objective 16
Discuss the considerations for phosphorus placement and method of application
based on the risk of phosphorus runoff.

Consideration of risks of phosphorus runoff may
lead to different choices for placement than
when crop response is the only consideration.
In many soils, little difference in crop response
is expected between broadcasting on the
soil surface as compared to broadcast and
incorporated, or band application. Placing
phosphorus below the soil surface can
dramatically reduce risks of loss in the dissolved
form in surface runoff or in tile drainage water
(Bruulsema et al., 2012; Culman et al., 2014).

Spatial variability within fields should be Monoammonium Phosphate (MAP). Courtesy Fertilizer Canada
managed to prevent accumulation of extremely
high soil test levels in parts of the field where
either nutrient removals are low, or where
historical buildup has occurred from large
manure applications or depositions.

Performance Objective 17
Plan the best placement or application method for phosphorus to
minimize the transport of phosphorus offsite.

Selection of the “right place” requires consideration of the following principles:
1. The choice of right place depends on source, rate and time of application.
2. Phosphorus needs to be available where plant roots are growing.
3. Concentrating phosphorus into enriched zones can improve availability in soils with high phosphorus

sorption capacity.
4. Crop residue cover on the soil surface should be managed to control soil erosion.
5. Spatial variability in phosphorus needs should be addressed.

The steps to determining any 4R nutrient stewardship plan are designed to be consistent with the
principles of adaptive management. These are described in IPNI’s 4R Plant Nutrition Manual,
Chapters 7 and 9. The five steps include setting sustainability goals, gathering production information,
formulating the plan, implementing the practices, and monitoring the effectiveness of those practices.

1. Setting sustainability goals.
For the whole farm or enterprise, these goals need to consider the interests of stakeholders who
may include neighbors, customers, local public interest groups, farm or business associations, or
other organizations active in voluntary promotion of sustainability improvement. When farmland
is leased, discussions should occur between the land owner and the farmer operator to determine
who is responsible for implementing sustainability practices and monitoring their effectiveness. Set

PROFICIENCY AREA III - Phosphorus 91

economic, environmental and social goals for the enterprise, with performance indicators chosen with
consideration of the concerns of the people listed above. For phosphorus, and its placement, important
concerns are likely to include cropland productivity, soil fertility, and water quality.

2. Gather needed production information – for each field:
a. Crop to be grown, and its target yield and quality (P influences crop maturity as well as yield).
b. Soil characteristics including texture, organic matter, pH, levels of available nutrients.
c. For decisions on application timing and placement, expected number of days of suitable soil

conditions for field operations based on soils and typical weather.
d. Water drainage, infiltration rates, potential for macropore flow to tile drains, pathways to surface

water, opportunity for buffers and biofilters to reduce loss of particulate P.
e. Location, dimensions and surface area (e.g., legal description, GPS coordinates, map).
f. Equipment available for applying nutrients; opportunity and potential for applying variable rates of

nutrients at a sub-field scale.
g. Reliable recommendations and decision support tools.

3. Formulate the Plan - for each field:
a. Review soil test P levels, and decide whether to build, maintain or draw down.
b. Estimate crop P removal based on target yield.
c. C onsider the supply of all available nutrients and choose the most feasible nutrient source and the

appropriate rate, time and place for its application.

4. Implement the chosen practices.
This can be done by the farm manager or in combination with advisors, fertilizer retailers or custom
applicators, buyers, and regulatory staff. Recording and tracking precisely what was done, and how
well it fit in with the logistics of field operations and timely crop planting, is an important part of the
adaptive management cycle.

5. Monitor the effectiveness of the practices employed.
The final step in the cycle of adaptive management assesses performance through the chosen indicators
to determine whether the practices selected achieved the intended results. This assessment then
influences the next cycle of planning decisions (i.e., from step 2). The impact of many practices cannot
easily be measured within a single growing season and will need to be assessed over multiple years to
document improvements.

Monitoring can include:
a. timeliness of field operations, particularly crop planting.
b. in-season and at-harvest crop nutrient concentrations;
c. yields achieved in relation to target;
d. calculation of P nutrient balances;
e. monitoring of water quantity and quality leaving the farm at drainage outlets (not economically

feasible for P losses for most farms);
f. measuring or assessing soil P fertility and soil health using appropriate indicators.

Establishing buffers, grassed waterways and biofilters can be effective practices for keeping particulate
P in the “right place” (out of drainage water leaving the field). Excluding livestock from streams can
also be considered a “right place” practice, since it keeps the manure from grazing cattle from being
directly deposited in the stream (OMAFRA, 2011; Zeckoski et al., 2012).

92 PROFICIENCY AREA III - Phosphorus

Performance Objective 18
Discuss how phosphorus contamination of surface water can occur from tile
drainage due to placement and method of application.

Phosphorus sources left on the soil surface have the first interaction with water from any rainstorm. If the
rainstorm is large enough to generate flow through macropores to the tile drains, this interaction results
in water with relatively high concentration of dissolved phosphorus moving out of the fields through tile
drains to contaminate surface water in ditches, streams and rivers. Placing phosphorus on the surface
of tile drained soils with macropores constitutes an acute risk for delivery of dissolved phosphorus to
drainage water, elevating concentrations of phosphorus in stream water and harming water quality in
streams, rivers, and lakes. For phosphorus sources applied with a large volume of water, e.g. liquid
manure, the risk is particularly acute, and can be managed by tilling the soil directly over the tile drains
prior to liquid manure application.

Conservation tillage seeks to maintain residue cover and control soil erosion, limiting loss of particulate
forms of phosphorus. When tillage is reduced or eliminated, available forms of phosphorus tend to
accumulate in the uppermost few centimeters of soil, particularly if the main phosphorus application
method is broadcasting without incorporation. Such stratification of available soil phosphorus is
a chronic risk to water quality, and can be measured by soil testing and comparing the top few
centimeters to the typical 15 to 20 centimeter sampling depth.

To manage both the acute and chronic risks in soils with tile drainage and macropore flow, it is
necessary to get the phosphorus incorporated into the soil in some way (Fisher, 2014). Injection without
tillage may be possible, particularly for fluid forms of fertilizer. Other possibilities include strip tillage
with injection of fertilizer or manure, or light tillage after broadcasting that leaves sufficient crop residue
cover to control soil erosion. In cases of high soil phosphorus stratification, where soil erosion risks are
low, occasional (e.g., once in 10 or more years) inversion tillage with a moldboard plow may reduce
phosphorus losses to water.

Performance Objective 19
Discuss how to use drainage water management to reduce
phosphorus nutrient losses to surface water.

Drainage water management involves controlling the level of the soil water table at specific times of
year, as opposed to allowing free drainage through the tiles. Since the total flow of water is reduced,
the total phosphorus load from the field may be decreased. The concentration of phosphorus in the
water, however, may also be influenced. It may either increase or decrease as a result of changes in
the oxidation state of the soil. Release of phosphate from iron phosphate complexes often occurs in
more reduced conditions. There is also more time for the water to interact with subsoil, which can in
some instances reduce the concentration of dissolved phosphorus.

When drainage water management is combined with subirrigation, by capturing drainage water in
a retention pond for use later in the summer by pumping it back, phosphorus losses can be reduced
dramatically. Adequate space for a retention pond is a major limitation for subirrigation, however.
Research in controlled drainage with subirrigation was shown to reduce particulate phosphorus loss by
10% (Frey, Hwang, Park et. al, 2016) to 15% (Tan and Zhang, 2011) in Ontario, relative to regular
free drainage. Drainage water management generally works best on flat fields with minimal slopes.

PROFICIENCY AREA III - Phosphorus 93

Competency Area 5
Environmental Risk Analysis for Phosphorus

Performance Objective 20
Discuss how to use water quality vulnerability assessment tools (e.g. Source Water
Protection Plans) on a site specific basis for phosphorus nutrient planning.

The Ontario Ministry of the Environment and Climate Change has approved 22 source water protection
plans within the province (MOECC, 2016). Any field falling within a risk management zone of a source
water protection plan must follow the specifications of that plan. Most plans do not address phosphorus
specifically, but in certain zones deemed vulnerable there may be restrictions affecting animal penning,
manure storage or fertilizer use. In addition, the use of nutrient management plans may be required or
encouraged (Conservation Ontario, 2013).

Performance Objective 21
Evaluate phosphorus management decisions using a water quality
vulnerability assessment (e.g. Phosphorus Index).

The Ontario Phosphorus Index requires input data for the following:

1. Universal Soil Loss Equation (USLE) rating for the field. A = R x K x LS x C x P (OMAFRA, 2012)
• A represents the potential long-term average annual soil loss in tonnes per hectare (tons per
acre) per year.
• R is the rainfall and runoff factor by geographic location. The greater the intensity and
duration of the rain storm, the higher the erosion potential.
• K is the soil erodibility factor. It is a measure of the susceptibility of soil particles to detachment
and transport by rainfall and runoff. Texture is the principal factor affecting K, but structure,
organic matter and permeability also contribute.
• L S is the slope length-gradient factor. The steeper and longer the slope, the higher the risk for
erosion.
• C is the crop/vegetation and management factor. It is used to determine the relative
effectiveness of soil and crop management systems in terms of preventing soil loss.
• P is the support practice factor. It reflects the effects of practices that will reduce the amount
and rate of the water runoff and thus reduce the amount of erosion. The most commonly used
supporting cropland practices are cross-slope cultivation, contour farming and strip cropping.

2. Water Runoff Class
a. Soil hydrological group
b. Maximum field slope within 150 m of top of bank of surface water

3. Soil test phosphorus.

4. Commercial fertilizer application rate and method.

5. Manure/biosolid application rate and method.

94 PROFICIENCY AREA III - Phosphorus

The P Index can impact a nutrient management plan in two separate ways:
• sets minimum separation distances for nutrient application close to surface water, and
• d etermines maximum phosphorus application rates in vicinity of surface water.

The following table recommends management actions according to the P Index value.

Table 3.1. Phosphorus Application Rates and Setback Distances for P Index Ranges

Minimum Setback1 from Minimum Setback from
SurafpacpelieWdaotveer rifcPro2Op5 is
Phosphorus Generalized Interpretation of Phosphorus Surface Water if cPr2oOp5 is
applied up to removal [ft (m)]
Index for Site Index for Site

removal2 [ft (m)]

Very low potential for P movement from the 10 (3) 100 (30)
site. If farming practices are maintained at
< 15 the current level there is a small chance that
P losses from this site will have an adverse
impact on surface waters.

15 - 29 Low potential for P movement from the site. 10 (3) 100 (30)
The chance for an adverse impact to surface
water exists. Some remedial action should
be taken to lessen the potential for P loss if
application is close to surface water.

30 - 50 Moderate potential for P movement from the 10 (3) 200 (60)
site and for an adverse impact on surface
waters to occur unless remedial action
is taken. In areas close to surface water,
soil and water conservation along with P
management practices are needed in order
to reduce the risk of P movement and water
quality degradation.

High potential for P movement from site and

for an adverse impact on surface waters.

Remedial action is required to reduce the

> 50 risk of P movement. All necessary soil 100 (30) Do not apply over
and water conservation practices plus a crop removal

P management plan must be put in place

to avoid the potential for water quality

degradation.

1. W ith manure application, it is recommended that the minimum separation distance be met in order to address direct surface runoff concerns. See Section Q, Table 15: Minimum
Separation Distance (with established buffer zone) in OMAFRA Publication 818, Nutrient Management Workbook, for more details.

2. The maximum allowable application rate is recommended to be the lowest rate calculated in the Table in Section S - Maximum Rates, OMAFRA Publication 818, Nutrient
Management Workbook.

Chart source: OMAFRA 2005, Determining the Phosphorus Index for a Field Factsheet, Agdex #531/743.

PROFICIENCY AREA III - Phosphorus 95

Performance Objective 22
Be able to evaluate how changing a specific phosphorus management strategy will
affect the outcome of a risk assessment.

A phosphorus management strategy includes source, rate, time and place of phosphorus application,
interacting with tillage and crop rotation choices and conservation practices and structures deployed at
edge of field to intercept runoff water. All of these strategy components are reflected in risk assessment
tools that are generally available from state or provincial agricultural extension and soil conservation
agencies. In Ontario, the recognized risk assessment tool is the Ontario Phosphorus Index (OMAFRA,
2011).

As described in Performance Objective 21, there are numerous management practices that affect the
outcome of the assessment conducted with the Ontario Phosphorus Index.

Table 3.2. Effect of Best Management Practices (BMPs) on P Index (adapted from OMAFRA, 2005).

Site Characteristic Management Practices that will Lower P Index Example of BMPs
Soil Erosion
Any practice to reduce soil erosion. Reduce slope length; tillage to increase surface
Water Runoff Class residue; plant cover crops; crop rotation, strip
In some instances, tile drainage installation may cropping, contour tillage.
Phosphorus Soil Test change the effective soil hydrologic group rating.
Tile drains may reduce runoff water volumes and
Commercial Fertilizer The management of fertilizer and manure thus lower risk of P loss in surface runoff. They also
Application Rate application methods/rates will control the rate at increase risk of P loss through tile drains by increasing
which the phosphorus level in the soil changes. connectivity. A new version of the P Index, in
Commercial Fertilizer preparation, may address these factors more explicitly.
Application Method Applying less fertilizer to a field will lower the
Manure /Biosolid level of phosphorus accordingly. The phosphorus level of a field can be lowered on a
long-term basis by reducing or eliminating application
Application Rate The use of an application method that rates of manure/fertilizer and/or using crops with
Manure /Biosolid incorporates the fertilizer quickly and efficiently higher P removal capabilities.
Application Method will result in a lower rating factor.
A reduction in the commercial fertilizer application rate
Applying less manure to a field will lower the from 60 lbs P2O5/acre to 30 lbs P2O5/acre will reduce
level of phosphorus accordingly. the P Index by 1 point.

The use of an application method that By changing the application method from
incorporates the manure quickly and efficiently non-incorporated to placed with planter, the
will result in a lower rating factor. P Index is reduced by 10.5 points.

A reduction in the manure/biosolid application

rate rferdomuce60thlebsPPIn2Od5e/xacbrye to 30 lbs P2O5/acre
will 3 points.

Changing the application method from
non-incorporated on bare soil to injected will cause the
P Index to be reduced by 10.5 points.

96 PROFICIENCY AREA III - Phosphorus

Performance Objective 23
Evaluate management strategies, including modifying phosphorus transport
processes, which will reduce phosphorus loss to surface water and groundwater.

Tile drainage can increase or decrease phosphorus loss from farm fields. It can reduce soil erosion,
because drained soils are less frequently saturated with water, and saturated soils generate more
runoff from a rainfall of a given size. Since tile drains provide a quicker connection for transport of
water, however, it can increase the transport of phosphorus from the soil surface to the edge of field.
Management strategies to control phosphorus loss need to account for the transport pathways.

Grassed field borders and grassed waterways can slow down runoff and remove some of the sediment
and soil-attached phosphorus at the edges of fields. They also help stabilize stream banks, preventing
transport of particulate P from bank scour. They are less effective, however, in controlling the transport
of dissolved phosphorus.

Diversion terraces, constructed across slopes, reduce erosion and runoff by intercepting, detaining and
safely conveying runoff to an outlet.

Water and sediment control basins (WASCoBs) are commonly installed to prevent bank and gully
erosion on farmland. Ponded water is slowly released to an underground drainpipe, either through a
riser pipe or a blind inlet constructed by filling in the bottom of the ponded area with sand and gravel.

Buffer strips planted to permanent vegetation such as grass, shrubs, trees or some combination of these
can help stabilize stream banks, prevent gullies from forming, and filter out some sediment-bound
phosphorus. They also help absorb nitrate, and provide wildlife habitat.

Cover crops can protect the soil surface from erosion. They are effective in reducing loss of sediment
bound P, but not very effective in reducing loss of dissolved P.

Furrow management and contour cultivation can help minimize soil erosion and surface runoff.
Wetlands can trap particulate P until they fill with sediment. They have low efficacy in removing
dissolved P.

Performance Objective 24
Discuss how tillage system (including no-till) affects environmental
losses of phosphorus.

Tillage influences soil erosion and is accounted for in the Universal Soil Loss Equation component of
the Ontario Phosphorus Index. Avoidance of tillage or conservation tillage, however, results in some
degree of soil phosphorus stratification, with higher levels accumulating in the top few centimeters of
soil. This stratification effect is not accounted for in the Phosphorus Index. The concentration of soluble
phosphorus in runoff water, or in water moving to tile drains by macropore flow, is influenced by
the soil test phosphorus level in the top few centimeters of soil. For this reason, conservation tillage
combined with surface applications of phosphorus may increase losses of soluble phosphorus while
decreasing losses of the particulate form.

PROFICIENCY AREA III - Phosphorus 97

Performance Objective 25
Compare the differences in the geographic scale, soil, topography, and location
of watersheds (e.g. national, regional, local) on the environmental impacts of
phosphorus on surface and groundwater resources.

Scale
Phosphorus losses may affect water quality in local streams, ponds, and reservoirs as well as in larger
rivers and lakes. Abundant algal growth can make the water in some of these water bodies unpleasing
or unsuitable for drinking or swimming, and the deposition of dead algal biomass can deplete bottom
waters of oxygen, limiting growth of fish and other aquatic animals.
Soil
Soils of finer texture are generally more prone to runoff and thus losses of phosphorus. Sandier soils
tend to lose less phosphorus, even when tile drained, since water flow to tiles is primarily by matrix
flow. One exception would be sandy soils with soil test P built up to extremely high levels; such
soils may transfer dissolved phosphorus to streams either through tile drains or by lateral flow of the
groundwater, where water tables are found at a shallow soil depth.
Topography
Soils with steeper and longer slopes are more prone to both surface runoff and soil erosion. Control of
soil erosion on such soils may be more important than control of losses of dissolved phosphorus.
Location of Watershed
Climate is an important controller of runoff. Seasonal charts comparing precipitation to potential
evapotranspiration can help guide decisions on application timing for fertilizers, manures and other
materials containing phosphorus. In Ontario, runoff risks in fall versus spring do not differ as much as in
the Western Corn Belt (e.g. Iowa) where runoff risks are generally lower in the fall than in the spring.

98 PROFICIENCY AREA III - Phosphorus

Performance Objective 26
Discuss the role of phosphorus, including legacy phosphorus, in the eutrophication
process and the potential consequences of eutrophication.

Phosphorus in aquatic systems can accelerate freshwater eutrophication. Eutrophication is one of
the most common impairments of water quality in North America. Eutrophication restricts water use
for fisheries, recreation, and industry due to the increased growth of undesirable algae and aquatic
weeds, and oxygen shortages caused by their death and decomposition. Also, an increasing number
of surface waters have experienced periodic and massive harmful algal blooms (for example,
cyanobacteria), which contribute to summer fish kills, and may restrict water use for public drinking
water supplies (Sharpley et al., 2006).

Legacy phosphorus refers to the accumulation of phosphorus in land or in aquatic systems that is
the result of past human activities, including management of both nutrients and land. It includes the
buildup of phosphorus in agricultural soils, to levels either optimal for or in excess of crop requirements,
and modifications of the transport pathways from land into streams, rivers and lakes. High levels
of phosphorus built up in the soil from past nutrient applications can result in elevated P losses from
these fields for many decades after P applications cease, delaying improvements in water quality.
Within rivers and lakes, a range of processes control the retention and release of phosphorus from
accumulated sediment, with resulting influences on eutrophication (Sharpley et al., 2013).

Permanently vegetated buffer
protecting the banks of the Credit River.

Courtesy Credit Valley Conservation

PROFICIENCY AREA III - Phosphorus 99

PROFICIENCY AREA IV

POTASSIUM, SECONDARY
MACRONUTRIENTS AND

MICRONUTRIENTS

Competency Area 1
Determining the Right Source of Potassium,
Secondary Macronutrients and Micronutrients

Performance Objective 1
Discuss the most common sources of potassium,
secondary macronutrients and micronutrients used in Ontario.

There are numerous products that contain potassium. The predominant products available for purchase
in Ontario are muriate of potash, sulphate of potash, sulphate of potash magnesia and potassium
nitrate. There are many more liquid mixed products that are commercially available as blends that
contain potash. These are mainly used as starter or pop-up fertilizers on the seed or as prescribed
by the manufacturer. Most livestock manure contains significant concentrations of potassium, and can
provide most or all of the potassium requirements for crops on land receiving manure.

Table 4.1. Granular Fertilizer Ingredients

Grade1 Other Salt equCiavCaOle3nt4 Bulk Relative cost/
(%) nutrients2 index3 density5 unit nutrient6

lb/lb lb/cu ft kg/L

Potassium nitrate 12-0-44 70 -1.9 (B) 75 1.20 2.54

Muriate of potash 0-0-60 45% Cl 115 neutral 70 1.10 1.00
(red)

Muriate of potash 0-0-62 46% Cl 116 neutral 75 1.20 1.00
(white)

Potassium sulphate 0-0-50 18% S 42.6 neutral 75 1.20 2.34

Sulphate of 0-0-22 20% S 43.4 neutral 94 1.50 3.71
potash-magnesia 11% Mg

1. Grade: guaranteed minimum percentage by weight of total N, available phosphoric acid (P2O5) and soluble potash (K2O) in each fertilizer material.
2. Other nutrients nutrients other than N, P or K.
3. S alt index: comparison of relative solubilities of fertilizer compounds with sodium nitrate (100) per weight of material. When applied too close to the seed or on the foliage the

higher salt index materials are more likely to cause injury.
4. C aCO3 equivalent: pounds of lime required to neutralize the acid formed by one pound of the N supplied by the fertilizer material. “B” following the lime index indicates a basic

(acid-neutralizing or alkaline) ingredient. Note: acid-forming effects can be up to twice as great as indicated, depending on plant uptake processes.
5. Bulk density: expressed as pounds per cubic foot or kg/L, important since fertilizers are metered by volume rather than weight in spreaders or planting equipment.
6. Relative cost/unit: based on 1998 prices of urea for N, triple superphosphate for P and muriate of potash for K.

Source: Soil Fertility Handbook, Publication 611, OMAFRA, 2006, p. 152.

100 PROFICIENCY AREA IV - Potassium, Secondary Macronutrients and Micronutrients