November 23, 2008
Added profit: $24/acrea
aBased on saving 36 pounds of nitrogen costing $0.67/pound
Credit Soil for Nitrate Nitrogen (Save $24/acre)
Gary W. Hergert, Nutrient Management Specialist
Aaron J. Nygren, Extension Educator
Because the climate across Nebraska ranges from semi-arid to semi-humid, there can be significant residual nitrate-nitrogen (NO3-N) in the root zone from the previous year. This nitrogen typically results from unused fertilizer and nitrogen mineralized from soil organic matter. It is a profitable and environmentally sound practice to routinely sample soil for residual N03-N and adjust the calculated nitrogen recommendation accordingly.
Soils should be sampled to a depth of 36 inches, and preferably to 48 inches for most Nebraska crops. Take at least eight cores per sample to determine residual NO3-N. (See NebGuide G1740, Guidelines for Soil Sampling, or Extension Circular EC 154, Soil Sampling for Precision Agriculture, for further information.)
|Table 1. Expected nitrogen savings by accounting for residual NO3-N from deept soil sampling.|
|Average residual soil NO3-N (lb/acre)|
|Average yield goal (bu/acre)|
|N recommendation based on 1.2 lbsN/bu (lbs N/acre)3|
|N recommendation based on 1.2 lb N/bu - residual N (lbs N/acre)|
|N savings compared to 1.2 factor|
|N savings above standard credit of 34 lb N/acre (lbs N/acre)4|
|1Based on 49,854 site years of data (Central Platte NRD).|
2Based on 150 site years of data (Mid-Nebraska Water Quality Demonstration Project); 85% of the 457 soil samples had more than 34 lb residual soil NO3-N/acre.
31.2 (lb N/bu x average yield goal (bu/acre).
4UNL credits 3 ppm when no soil test for nitrate-N is available; this equals 34 lb nitrogen/acre.
|Table 2. Economic savings by using residual nitrate-N in recommendation at various nitrogen prices ($/acre) |
and $1.00/acre cost of deep sampling.1
|1$/lb nitrogen x average residual soil NO3-N (lbs/acre-$1/acre (sample cost). |
2[N price ($/lb N x average residual nitrogen in Table 1 (82 lbs N/acre)]-sampling cost ($1/acre).
3[N price ($/lb N x average residual nitrogen in Table 1 (80 lb nitrogen/acre)]-sampling cost ($1/acre).
|Table 3. Economic savings by reducing nitrogen application by 48 lb and 46 lb nitrogen/acre based on typical residual nitrate-N at various prices of nitrogen and $1.00/acre cost of deep sampling|
|1$/lb N x average residual soil NO3-N after a default value of 34 lb NO3-N/acre is used from analytical laboratory - $1/acre (sample cost). |
2[N price ($/lb N x average residual nitrogen in Table 1 (82 lb N/acre) - default 3 ppm N from laboratory 34 lb N/acre)] - sampling cost ($1/acre).
3[N price ($/lb N x average residual N in Table 1 (80 lb N/acre) - default 3 ppm nitrogen from laboratory (34 lb N/acre)] - sampling cost ($1/acre).
If a residual NO3-N soil test is not used to make the nitrogen recommendations, some labs will suggest a default residual NO3-N concentration. A common default concentration is 3 ppm NO3-N (34 lbs/acre for a 36-inch depth). Most default values are conservative to avoid over-crediting soil nitrate.
How much extra would it cost if you determine your nitrogen recommendations without a proper residual NO3-N soil sample? The answer depends on
- the crop you will grow,
- the previous crop,
- your residual nitrate level, and
- the cost for the source of nitrogen you use.
More residual nitrogen is likely if the previous crop was corn, another non-legume or dry bean compared with soybean.
Long-Term Nebraska Data Shows Residual Nitrogen
Data from central Nebraska provides a snapshot of what we commonly see from many locations. The Central Platte Natural Resources District (CPNRD) requires producers to report soil residual NO3-N concentrations, yield goals, and other information regarding crop nitrogen management annually in their Phases II and III Groundwater Management Areas (GWMA). The average NO3-N from approximately 57,000 soil tests taken over 15 years (1980-2002) in the CPNRD GWMA was 82 lb nitrogen/acre in a three-foot depth. One hundred and fifty site years of data collected from the Mid-Nebraska Water Quality Demonstration Project (MNWQDP) (south central Nebraska, 1990-1997) had an average residual NO3-N content of 80 lb nitrogen/acre.
If producers ignore these NO3-N levels, they would buy another 80 lb per acre of nitrogen that they wouldn't need to because it was already in their fields (Table 1). At current nitrogen prices, the savings from not applying this nitrogen ranges from $48 to $75 depending on nitrogen source price (Table 2). If producers used the conservative default of 3 ppm for residual NO3-N, which is equivalent to 34 lb nitrogen/acre, they would have saved 46 lb to 48 lb of nitrogen valued at $25 to $45, depending on nitrogen source price (Table 3).
Web Site Aids in Estimating Nitrogen
A sample page from the Corn Nitrogen Calculator clearly shows the differences in nitrogen recommendations for corn following corn when factoring in residual NO3-N. This assumes a corn price of $4.50/bu; ammonia-N applied preplant dry plus fertigation, an ammonia-N price of $0.61/lb, and a cost for urea at $0.90/lb. These prices result in corn-price to nitrogen-price ratios of 6.6 to 1 and 5.1 to 1. These ratios cause a slight reduction in the amount of fertilizer applied compared to the longer term corn-to-nitrogen price ratio of 8 to 1 (unadjusted nitrogen rate).
In the example, an irrigated field with a 200-bushel yield goal would have a recommended nitrogen rate ranging from 140 to 162 lb N/acre with no accounting for residual NO3-N compared with 112 and 130 lbs N/acre when accounting for 10 ppm residual NO3-N. The N savings ranges from $22 to $24 per acre.
The Nebraska Soil Test Web site has fertilizer recommendations for all crops. The Corn Nitrogen Calculator is available on the Web at http://soilfertility.unl.edu, for those only interested in making corn nitrogen recommendations. Properly estimating residual NO3-N is essential to maintaining profitability in times of fluctuating grain and fertilizer nitrogen prices.