Fate of Nitrogen From Fall and Spring Applied MAP and DAP Depends on Soil Temperature and Rainfall

KIM POLIZOTTO, PH. D.

PotashCorp Chief Agronomist

The source of phosphorus in most fertilizer blends today is monoammonium phosphate (MAP 11-52-0) or diammonium phosphate (DAP 18-46-0). It is common in many crop production systems to apply phosphorus (P2O5) and potassium (K2O) fertilizer needs in the fall, prior to fall- and/or spring-planted crops. In the US Corn Belt, it is fairly common for farmers to apply their fertilizer requirements for the next corn crop as well as the soybean crop that will follow it. These applications are often made soon after soybean harvest and when soil temperatures are greater than 50 F.

Using slow- and controlled-release nitrogen fertilizers, adding nitrogen fertilizer additives and/or timing nitrogen applications will all improve nitrogen efficiency.

The top questions that many dealers and farmers have about this practice are: How much of this fall-applied N is available to my spring-seeded crop? How much should I credit my regular nitrogen program with N from DAP or MAP?

Nitrogen Soil Reactions

Regardless of the fertilizer source of nitrogen, it eventually gets converted to nitrate (NO3-) in the soil. Urea and anhydrous ammonia nitrogen is quickly converted to ammonium nitrogen, and then the ammonium nitrogen is converted to nitrate nitrogen. These conversions are carried out by soil bacteria and take place very quickly when soil moisture is just below field capacity and temperatures are above 60 F. Nitrification is the term used to describe the conversion of ammonium to nitrate nitrogen.

Based on this research, it appears that dealers or farmers might want to make adjustments to their nitrogen fertilizer programs based on when MAP, DAP or AS nitrogen is applied.

Unfortunately, nitrate is the nitrogen form most easily lost in soils. Nitrate N can be leached during periods of heavy rain and can be denitrified when soils are saturated. Denitrification is the term used when soil bacteria strip oxygen from the nitrate nitrogen (NO3-). When this occurs, nitrogen gases N2 or N20 are formed and can then escape into the air.

Because of these soil reactions, nitrogen management can be a challenge, but slowing nitrification and minimizing denitrification can improve nitrogen utilization greatly. Using slow- and controlled-release nitrogen fertilizers, adding nitrogen fertilizer additives and/or timing nitrogen applications will all improve nitrogen efficiency.

The question to consider is when a small amount of nitrogen is applied with fall P and K fertilizer programs before soils get cold, and nitrification inhibitors are not used, how much nitrogen is left for the next crop?

Recent Research

Recent research conducted by Dr. Robert Hoeft at the University of Illinois and Dr. Gyles Randall at the University of Minnesota has specifically addressed these questions.

For the years 2004 through 2006, they fall- and spring-applied 0, 40 and 80 lbs/acre of N using DAP, MAP and ammonium sulfate (AS) as the N sources. The trials were conducted at sites in Illinois and Minnesota each of the three years. A laboratory incubation study was also conducted in Illinois to determine the rate of nitrification and recovery of ammonium and nitrate N from DAP and MAP.

The laboratory incubation study showed that DAP nitrified just slightly faster than MAP, which agrees with previous work in this area. Also, at moisture levels below field capacity, the amount of nitrate in the soil remained pretty constant over 14 weeks, for both MAP and DAP, indicating no denitrification was occurring, where soils were not saturated. When field moisture exceeded field capacity (saturated soils), however, denitrification was rapid for both MAP and DAP, and as much as 50 percent of the nitrate was lost in two weeks at temperatures of 70 F.

In the field studies, Hoeft and Randall were able to quantify nitrogen losses during the fall, winter and/or spring, following fall and spring MAP, DAP and AS applications. As might be expected, N losses varied from year to year, depending on the soil temperatures and rainfall. Because total applied N rates were low, corn grain yields reflected soil N differences among the treatments.

Observations and Conclusions

After three years of research in two states, the following observations and conclusions were reached:

1. Source of N, including MAP, DAP or AS, had little influence on soil inorganic N concentration in Illinois or Minnesota field trials.
2. Source of N had no effect on corn grain yields at either location.
3. Most years, plots with spring-applied N had significantly higher yields than those with fall-applied N. In 2006 in Minnesota, fall applications outyielded the spring treatments.
4. As much as 70 percent to 80 percent of the fall-applied N from MAP, DAP or AS might be lost in years where nitrification is complete and soils are warm and heavy spring rains contribute to spring leaching and/or denitrification.
5. In years where soils cool quickly in the fall and/or where soils are not saturated in the spring, there is little N loss from fall- or spring-applied N.

Based on this research, it appears that dealers or farmers might want to make adjustments to their nitrogen fertilizer programs based on when MAP, DAP or AS nitrogen is applied. For fall-applied MAP, DAP or AS, don't assume that all of the N will be available for a spring planted crop. If it is applied on cold soils and soils do not become saturated during spring rains, most of the N should be available. If applied in the fall when soils are warm and/or spring rains leave the fields saturated, N loss from any of these sources can be significant. If in doubt, it is probably safe to assume that there will be a benefit from about 50 percent of the N from these sources when fall applied. When spring applied, assume the same availability as your primary N fertilizer source.Although some nitrogen from fall-applied MAP or DAP fertilization programs is normally lost before spring planting, we shouldn't assume that this always translates to reduced yields. Because nitrogen from these types of applications is just a small part of the total nitrogen applied to most crops, it isn't likely that this N loss will affect yields. Nitrogen from the primary N fertilizer source plus N from in season mineralization of soil N will normally be adequate. Yield only will be affected if there is significant N loss from all of your fertilizer N sources.

Hoeft and Randall are presently preparing their research results for publication. This work will be very valuable in assessing, adjusting and explaining nitrogen additions in nutrient management plans. Agriviews and the PotashCorp website www.potashcorp.com will post an announcement when the results are summarized and reported.