• Nitrogen loss involves minerology, biology and soil chemistry and is driven by soil temperature and soil moisture on a time continuum.
• Leaching and denitrification are two nitrogen loss mechanisms to be concerned about.
• In addition to the use of nitrification inhibitors, the chemical form of nitrogen applied has some effect on nitrogen loss.
• Soil testing to a soil depth of 24 inches, in 12-inch increments is the best way to measure plant available nitrogen (PAN).

The amount of rain this spring and resulting saturated soils have many wondering how soil moisture has impacted nitrogen loss. The amount of nitrogen loss incurred depends on the soil moisture content, soil temperature, the fluxes of these two factors across time, the chemical makeup of the nitrogen source applied (% NH₄⁺ or NO₃^-) and whether a nitrification inhibitor was used.  The two possible loss mechanisms involved are leaching and denitrification.  Both of these factors require nitrogen to be in the nitrate (NO₃^-) form.  As shown in Table 1 below, the percentage of NH₄⁺ and NO₃^- applied varies with the source of nitrogen used.  

Table 1 below: Percent of Ammonium (NH₄⁺) and Nitrate (NO₃^-) in various nitrogen sources

The soil chemistry behind the conversion of the ammonium form to nitrate is an aerobic process that occurs in soils that are not frozen. Aerobic means that soil oxygen needs to be present for the process to take place. Temperature also plays a big role in this conversion as nitrification does not begin to occur at a meaningful rate until soil temperatures reach 50⁰F (Graph 1 below).

Graph 1 below: Nitrification of ammonium form nitrogen at various soil temperatures (Source: IPNI)

Soil moisture content that affects these loss mechanisms can differ from field to field based on soil drainage, soil texture, amount of rainfall, saturation continuum and soil temperature.Low soil temperatures will prolong the time period during which the NH₄⁺ form persists, and the period of time nitrification inhibitors will remain active. Saturated soils will stop all soil processes, except denitrification of the NO₃^- form which occurs in anaerobic conditions (oxygen absent).All these combined dynamics makes it impossible to estimate nitrogen loss within a field without testing the soil for plant available nitrogen (PAN). A PAN test is a way to quantify how much nitrogen is available to the crop during the growing season. PAN measures not only available nitrate nitrogen, but also ammonium nitrogen better representing total nitrogen at time of sampling available to the plant. Soil samples collected to determine the nitrogen status in a field should be representative of the field’s topography, drainage and soil types. The N-Watch website not only provides information on how to take a good sample but also outlines multiple methodologies designed for each type of application, whether banded or broadcasted.Sample number, area and number of samples are discussed at the N-Watch website and vary based on method of nitrogen application.

Interpretation of results vary from state to state. For example, if we use Iowa State University’s Late-Spring Soil Nitrate test (LSNT), samples are taken when corn is 6-12 inches in height and the top two foot of soil contains 25 ppm PAN, that indicates that sufficient nitrogen exists for the crop.Iowa State University, states that “In general, research has shown that the LSNT is most reliable at predicting a lack of corn response to additional N at values above the 25 ppm critical level. Reliability is lower for specifying the rate of N to apply when the test result indicates a potential deficiency (test results below 25 ppm). Antonio Mallarino & John Sawyer ISU ”.A portion of the reliability of the ISU’s LSNT test results at predicting nitrogen rates below 25 ppm is that it only tests nitrate-N where we are suggesting analyzing soils for PAN which includes ammonium-N plus nitrate-N and two separate 12 inch depth samples to a depth of 24 inches. Since 80% of the corn’s root system is found in the top 12 inches of soil this would account for the majority of the root zone.To compare ppm results to lbs/acre, multiply PAN ppm in the top 24 inches of soil by 4.This will give you the best estimate of measurable nitrogen in this zone.

At the time a soil sample is taken a portion of the nitrogen that will be available to the crop can be tied up within microorganisms mineralizing organic matter to multiple plant available nutrients. PAN soil testing does not account for mineralized nitrogen. If soil conditions are ideal for microorganisms, the potential for a higher amount of unmeasurable crop nitrogen exists. However, if conditions are not ideal for microorganisms, little nitrogen would be missed in the measurement. Since no reliable model exists to predict the amount of nitrogen tied up by microorganisms, the rule of thumb often used is for every % organic matter a soil contains expect 10-20 lbs. N/acre/year.This equates to 25-50 lbs. N/acre for a 2.5% organic matter soil for example. However, under ideal conditions amounts of nitrogen mineralized can be much higher than these amounts. Measurement and prediction of this process is very difficult due to the minerology, biology & chemistry of the soil and its interaction with weather.

In summary, the only method of measurement that exists for soil nitrogen is soil testing, with PAN being the most complete. By accounting for PAN in the top 24” of soil and estimating mineralization, a reasonable estimate of total nitrogen available to the crop can be made.For more information on soil testing of PAN contact your local FS Crop Specialist.