Assessing available N from fall and spring applications
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.
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.
click image to zoomBrouder & Mengel, 2003Figure 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. Sample handling
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 250F. Alternatively refrigerate the samples and keep them cold through shipping to the laboratory. A list of certified soil testing laboratories is available here. 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.