Nitrogen fertilizer management strategies


Nitrogen fertilizers are expensive and may become unavailable to your crop. Because of this, corn producers will need to manage nitrogen fertilizer carefully to remain profitable in 2006. Listed below are some strategies for getting the most from your investment in nitrogen fertilizer.

Understand what happens to the nitrogen fertilizer you apply
Studies have shown that only 40 to 60% of the nitrogen you apply is taken up by your crop. Another 20 to 30% remains in the soil after harvest and 10 to 20% becomes unavailable to plants during the growing season. The losses are due to three processes, volatilization, leaching and denitrification. Volatilization occurs when fertilizers containing urea undergo rapid hydrolysis in the soil. Significant losses of ammonia gas can occur. Nitrate-nitrogen is susceptible to losses from leaching and denitrification. Leaching is most likely to take place in coarse-textured soils. Denitrification of nitrate-nitrogen occurs under saturated conditions on fine-textured soils. Over 100 lbs. of nitrogen per acre can be lost from denitrification in five days under the proper conditions.

Select your nitrogen fertilizers and understand their management requirements
The three main nitrogen fertilizer sources are anhydrous ammonia, urea and urea ammonium nitrate solutions (UAN). Anhydrous ammonia is still the least expensive form of nitrogen. It must be injected 6 to 8 inches deep and the slot must be sealed to prevent losses. If applied properly, it is the most stable nitrogen source as it is the slowest fertilizer to be converted to nitrate. Urea is subject to volatilization losses. Volatilization occurs rapidly under warm air temperatures in fields having low CEC’s, heavy residues and adequate moisture. These losses can be reduced by incorporating the fertilizer with tillage at least 1 to 3/4 inches deep or by at least 3/4 of an inch of irrigation or rain fall. Urease inhibitors are available to mix with the fertilizer and prevent volatilization for 10 to 14 days following a surface application. The urea will not be available to plants during this time. UAN solutions are comprised of half urea and half ammonium nitrate. Therefore, half of the nitrogen is subject to volatilization losses. About 25% percent of the nitrogen is in the nitrate form at the time of application and is subject to losses from leaching or denitrification.

Identify the most economical nitrogen application rate
The first step is to determine your price per pound of actual nitrogen and the market price you expect to receive for your corn. Determine the corn to nitrogen price ratio from Table 1. Next you will need to set realistic yield goals for your fields. A realistic yield goal is the average yield you attained in the field for the last five production years for that crop. Find the point on Table 2 where your yield goal and your corn to nitrogen price ratio intersect. You may have to interpolate between rows and columns. This is the most economical nitrogen rate for this field.

Table 1. Corn: Nitrogen Price Ratio

N cost ($/lb) Corn Price ($/bu)
1.00 1.50 2.00 2.50 3.00 3.50 4.00
  —-corn:N price ratio—-
.10 10:1 15:1 20:1 25:1 30:1 35:1 40:1
.15 7:1 10:1 13:1 17:1 20:1 23:1 27:1
.20 5:1 8:1 10:1 12:1 15:1 18:1 20:1
.25 4:1 6:1 8:1 10:1 12:1 14:1 16:1
.30 3:1 5:1 7:1 8:1 10:1 12:1 13:1
.35 3:1 4:1 6:1 7:1 9:1 10:1 11:1
.40 2:1 4:1 5:1 6:1 8:1 8:1 10:1
.45 2:1 3:1 4:1 5:1 7:1 8:1 9:1
.50 2:1 3:1 4:1 5:1 6:1 7:1 8:1
Source: MSU Extension Bulletin E-802

Table 2. The most profitable nitrogen rate (lbs N/acre) at various yield goals and corn to nitrogen price ratios. Adjust the nitrogen application rate for all possible nitrogen credits.

Corn:N Price ratio Yield potential of soil (bu/A)
85 100 115 130 145 160 175 190
  - - -most profitable N rate (lb N/A)- - -
4:1 70 80 100 110 120 130 140 150
5:1 80 90 100 110 130 140 150 170
7.5:1 85 100 115 125 145 160 170 190
10:1 90 110 130 140 160 180 190 210
15:1 100 120 140 160 180 200 220 240

Corn growers can reduce their nitrogen fertilizer application rates by taking credit for the nitrogen contributions from legumes, manure applications and the soil. The credits should be subtracted from the most economical nitrogen rate determined above. Soybeans will contribute 30 pounds of actual nitrogen to the following corn crop. Established alfalfa and clovers will contribute between 40 and 90 pounds of actual nitrogen, depending on the plant population. Manure can be an excellent source of nitrogen. The pre-sidedress nitrate test is a proven method for determining the nitrogen contributions from manured and non-manured fields. Do not take nitrogen credits when growing wheat. Wheat’s peak demand for nitrogen occurs earlier in the growing season before organic nitrogen has been converted to plant available forms.

Table 3. Economic returns from pre-sidedress nitrate testing

PSNT Credit(lbs./acre) Cost Savings on Nitrogen ($/acre) Economic Gain($/acre)
30 8.70 5.70
60 17.40 14.40
90 26.10 23.10
120 34.80 31.80
Source: Dr. Carrie Laboski
Assumptions: Nitrogen cost is $0.29/lb of nitrogen
Sampling and analysis costs are $3/acre

Apply nitrogen fertilizers at the optimum time
Always apply at least 20 pounds of actual nitrogen per acre in a 2 x 2 band at planting time. Increase this amount to 30 to 40 pounds per acre when planting into heavy residues. Ideally, the rest of your nitrogen should be applied in early June as this coincides with the beginning of the crop’s peak demand for nitrogen. By applying most of your nitrogen in June, you will significantly reduce the potential for nitrogen losses due to leaching and denitrification. If you must apply your nitrogen prior to planting, consider using a nitrification inhibitor. These products can delay the conversion of ammonium to nitrate by 4 to 10 weeks. Nitrification inhibitors will work best if the nitrogen fertilizer rate is slightly deficient. Please see Table 4 to determine the probability of realizing an economic return from nitrification inhibitors.

Table 4. Probabilities of realizing an economic return from nitrification inhibitors

- - -Time of Nitrogen Application- - -
Soil Management Group Early Spring <50 F Late Spring >50 F   Side Dress
Clays (0, 1) Good-Fair Poor-Fair Poor
Clay loams (1.5) Fair-Good Poor-Fair Poor
Loams (2.5) Fair Fair Poor
Sandy Loams (3) Fair Fair Poor
Loamy Sand (4) Fair Fair Poor
Sands (5) Fair-Poor Fair Poor
Good=Economic response expected at least 60% of the time.
Fair=Economic response expected 40-50% of the time.
Poor=Economic response expected less than 30% of the time.
Source:Nutrient Management to Protect Water Quality Bulletin WQ-25, 01/1996.

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