Figure 1
Active growing season of common Missouri crops.
Managing Nitrogen to Protect Water Quality
John Lory and Steve Cromley
Division of Plant Sciences
Nitrogen is a highly mobile nutrient that can be lost to the air, in runoff
and through the soil. The high mobility of nitrogen creates unique nutrient
management requirements to ensure fertilizer remains in the soil long enough
to benefit your crop.
A significant proportion of a poorly timed nitrogen fertilizer application
can be lost before the target crop has a chance to use it. For example, if nitrogen
for a corn crop is applied in early fall losses from winter and spring rains
can deplete much of the nitrogen fertilizer from the soil before the plant
can use it during the growing season.
There are three water quality concerns associated with loss of nitrogen from
agricultural fields:
- Using high nitrate-nitrogen drinking water to make milk formula
can reduce the oxygen carrying capacity of blood in babies and young livestock.
- Nitrogen
in runoff can contribute to eutrophication in some freshwater streams and lakes.
- Nitrogen is the primary contributor to the hypoxic (low oxygen) "dead
zone" that forms in the Gulf of Mexico each summer. Nitrogen lost from
Missouri fields ultimately reaches the Mississippi River, contributing to
the hypoxia problem in the Gulf.
Improving nitrogen management improves both water quality and the effectiveness
of fertilizer nitrogen for meeting agronomic goals.
How to prevent nitrogen losses
Avoid overapplication of fertilizer nitrogen
Research has shown that nearly all nitrogen applied in excess of crop needs
can be lost from the root zone in humid regions of the United States, including
Missouri. To avoid overapplication of nitrogen
- Base nitrogen applications on a fertilizer nitrogen recommendation
using an accurate yield goal.
- Adjust fertilizer recommendations for the nitrogen value of previous
legume crops such as soybean and alfalfa.
- Adjust fertilizer nitrogen recommendations for the nitrogen value of
any manure applications.
- Consider using a preplant nitrogen test on corn and wheat fields with
a history of manure application or other situations where you expect large
but unknown amounts of available nitrogen in the soil.
- Soil nitrogen tests may be useful after fall or early spring applications
of manure or other nitrogen fertilizers when weather patterns may have promoted
significant losses of nitrogen.
Apply nitrogen during periods of active uptake
- There are significant differences among crops in the timing and the duration
of active nitrogen uptake and utilization (Figure 1). Applying fertilizer
nitrogen close to or during the period of active uptake reduces the possibility
of nitrogen losses.
- In cornfields nitrogen applications are equally effective at producing
high yields from planting to nearly tasseling time. The biggest obstacle
to side-dress nitrogen applications is access to the field when there is a standing
crop.
- In winter wheat fields, splitting nitrogen applications between fall
and spring will increase yields and reduce nitrogen losses.
- Cool-season forages and winter wheat are best suited for fall and early
spring manure applications because they continue active growth into cool
weather. Active growth uses nitrogen and water, reducing leaching potential.
Prolong the time nitrogen is held by the soil
There are a number of strategies for extending the window of opportunity
for applying nitrogen fertilizer.
- Inject anhydrous ammonia and manure into nearly frozen soils in late
fall.
- Injection into nearly frozen soil (below 40 degrees Fahrenheit) will hold nitrogen
in the ammonium form, preventing nitrate leaching until after soils warm.
- Use an inhibitor with your fertilizer nitrogen.
- When using urea, apply with a urease inhibitor such as Agrotain.
- When injecting anhydrous ammonia or manure with lots of ammonium nitrogen
(e.g., swine lagoon effluent), use a nitrification inhibitor such as N-Serve.
- These products inhibit the activity of soil microorganisms or enzymes
for two or more weeks, preventing conversion of the fertilizer into mobile
forms of nitrogen. The effectiveness of these products is longer in cold soils
that slow breakdown of the inhibitors.
- Decisions on adding inhibitors are larely driven by the cost of the product
relative to the potential value of the conserved nitrogen.
Minimizing nitrate leaching
- Do not overapply nitrogen fertilizers.
- Time applications close to crop nitrogen need.
- Avoid fertilization strategies that leave substantial amounts of nitrate
nitrogen in the soil over winter.
- Avoid summer applications of manure on wheat stubble for next year’s
corn crop.
- Avoid fall anhydrous or urea nitrogen applications into warm soil without
the appropriate inhibitor.
- Nitrate leaching is more likely on soils that have a high infiltration
rate such as sandy soils.
- Overwatering of irrigated soils also causes nitrate leaching.
- Nitrate leaching is less likely on cool-season forages and winter wheat
that extend their growing season into the late fall and early spring.
- Apply fall anhydrous ammonia after soil temperature is below 40 degrees Fahrenheit.
- Consider the use of inhibitors to slow the conversion of spring-applied
nitrogen to nitrate.
Minimizing ammonia volatilization
- Inject liquid manure into the soil. This is particularly effective for
manure with high ammonium-nitrogen content, such as unagitated lagoon
effluent.
- Avoid urea fertilizers in high residue conditions unless steps are taken
to ensure contact of the fertilizer with the soil.
- Significant rainfall will leach urea into the soil if it falls soon after
application.
- Make sure anhydrous ammonia injection slots fully close.
G9218, new January 2006
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