Assessing available N from fall- and spring-applied N
Excessive rainfall and flooding in early to late spring can result in the loss of some fall- and spring-applied nitrogen (N). Both of these N forms are subject to leaching through the soil into tile drains or groundwater. In addition, the nitrate form of N can converted to several gaseous forms and lost to the atmosphere from deep within the soil by a bacterial process called denitrification. Unfortunately, no matter what form of N was added to the soil it will eventually become nitrate. Calendar time since N application and spring temperatures influence the extent to which both fall- and spring-applied N convert to the nitrate form. Many factors affect how much N is lost from soil, therefore it is difficult to accurately estimate the amount of N loss that may have occurred by any point in time. One of the viable options to estimate the amount of remaining soil N is to consider soil sampling and analysis for the nitrate and ammonium forms of N.
click image to zoomFigure 1. Recommended soil sampling pattern in relation to two corn rows when N fertilizer has been banded with the row. Always sample perpendicular to the direction fertilizer was applied. (Source of image: Brouder & Mengel, 2003) Soil sampling strategies
Collect soil cores to a depth of at least 1 foot. Where earlier-applied fertilizer N was broadcast rather than banded, collect 20 to 30 soil cores per sample. Where earlier-applied fertilizer N was banded (e.g., anhydrous ammonia), collect 15 to 20 soil cores using the sampling scheme illustrated in Figure 1. Consider collecting a separate deeper soil sample from between 1- and 2-foot deep for a more complete assessment of plant available N, especially in sandy soils where leaching through the soil profile is the predominant form of N loss.
TIP: One sample should represent no more than 10 acres.
Dry or refrigerate the soil samples as soon as possible to stop the soil microbes from altering the N levels. Spread the soil thinly on plastic to air dry and hasten drying with a fan if possible. If you choose to use an oven to dry the soil, keep the temperature below 250°F. Alternatively refrigerate the samples and keep them cold through shipping to the laboratory. A list of certified soil testing laboratories is available at http://www.ag.purdue.edu/agry/extension/Pages/soil-testing-labs.aspx. Most should offer soil N test analysis services, but contact them first to confirm.
Soil-test laboratory analyses
Ammonium N (NH4-N) is just as available to plants as is nitrate N (NO3-N), but typically little accumulates in the soil because it is readily converted to nitrate under most conditions. However, if N fertilizer was recently applied, there may well yet be some ammonium N available in the soil for plant use.
TIP: When you submit the soil samples to the soil-testing laboratory, request analyses for exchangeable ammonium as well as for nitrate, particularly if anhydrous ammonia was applied relatively recently or a nitrification inhibitor was used with the N fertilizer.
Significant levels of soil ammonium are most likely if anhydrous ammonia was the N source, a nitrification inhibitor such as nitrapyrin or dicyandiamide (DCD) was used, and/or soil pH was low (below 5.5). In these situations, low levels of soil nitrate may indicate little conversion of ammonium to nitrate, rather than simply loss of nitrate.
If soil test values for ammonium and nitrate are reported as ppm or mg/L nitrogen (NH4-N or NO3-N), then pounds per acre of available N are calculated by multiplying the test results by 4 when the sample depth was 1 foot. For other sample depths, divide the sample depth (in inches) by 3 and then multiply by the test results.
Example: Soil NO3-N in a 1-foot sample was 30 ppm.
Conversion from ppm to pounds per acre is (12 inches / 3) x 30 ppm = 120 pounds per acre.
If soil test values are reported directly as NH4 or NO3, then these values must be converted to an ‘N’ basis first. The calculations are: NH4-N = NH4 / 1.2 and NO3-N = NO3 / 4.5.
click image to zoom Example: Soil NO3 was reported to be 90 ppm.
Conversion from NO3 to NO3-N is 90 ppm NO3 / 4.5 = 20 ppm NO3-N.