What is the value of testing soil for both nitrate and ammonium nitrogen when many of the interpretations, like PSNT for manured fields, only utilize nitrate values? Why test for ammonium when we can assume that all of the available nitrogen in the soil has converted to nitrate? The answer depends on the way the data is being utilized.
Nitrogen in the soil can take several forms, but plants can only utilize ammonium and nitrate. Ammonium is stable in the soil until it converts to nitrate which is mobile in the soil, and subject to loss via leaching and denitrification.
Nitrogen in manure starts as organic nitrogen, that material mineralizes to become ammonium, and then degrades to nitrate. Manures at application time will be a mix of organic nitrogen and ammonium depending on species and manure handling. Poultry and beef manure tend to contain more organic nitrogen, while dairy and swine liquid manures tend to have a higher ammonium content. While this can be measured, we cannot accurately predict the rate of conversion from organic nitrogen to ammonium, then to nitrate in the soil. When testing soil after a manure application, nitrate can be used for traditional PSNT interpretation assuming continued future release of nitrate from ammonium. By measuring the ammonium in the soil that assumption can validate or disprove this assumption. Natural soil ammonium levels are below 4 to 6 ppm. Soil test ammonium results at sidedress near, or below these levels, indicate that future release nitrogen release from the manure maybe limited and increased nitrogen rates may be warranted.
Inorganic fertilizers convert from urea to ammonium and then to nitrate. Commercial fertilizers can start at any three of these levels, UAN starts at all three. Testing for nitrate only can miss a large fraction of the available nitrogen in the soil. There are commercially available nitrogen conversion inhibitors to reduce the rate of conversion from urea to ammonium and ammonium to nitrate in the soil. Without inhibitors the conversion of urea to ammonium and ammonium to nitrate occurs quickly depending on soil moisture and temperature. Over a matter of few days to weeks. More information on using nitrate and ammonium soil tests to manage in season nitrogen applications can found on in the ALGL Newsletter. (link to article)
The conversion of ammonium to nitrate can also be inhibited by soil conditions. As soil moisture increases in warm soils, the rate of conversion from ammonium to nitrate increases. However, at soil saturation the conversion of ammonium to nitrate can stop while nitrate continues to leach and denitrify. Extremely dry or cold soil conditions can prevent both the conversion of ammonium to nitrate and the loss of nitrate.
As an example, following a recent wet spring, the ALGL agronomy staff worked with a grower that tested for nitrate only. The grower was trying to determine how much of the anhydrous ammonium they had applied spring preplant had been lost. The resulting nitrate values were very low indicating that nearly all of the applied nitrogen was lost. When the ammonium test was performed on these same samples the data values were high, high enough to indicate that all of the nitrogen remined in the ammonium form. The saturated soils prevented the conversion of ammonium to nitrate for 10 weeks, the ammonium test data defied the assumptions made in this situation and avoided an unnecessary nitrogen application.
Rather than assuming how much ammonium has converted to nitrate or assuming that there is more ammonium to convert to nitrate in the future, the most accurate way to manage these situations is to test the soil for both ammonium and nitrate.