Recent persistent rains and wet soils in some areas of Kansas can contribute to some level of N loss this fall. Will these conditions affect N availability and wheat growth? And if so, what is the need for an N fertilization adjustment?
Wheat growth and biomass accumulation is very limited in the fall, and therefore the N demand during this time of the year is no more than 20-40 lbs N /acre, with the majority of the N uptake occurring in the spring after green-up. A topdress N fertilization program with N applications in late winter/early spring helps synchronize the time of high demand with the availability of N in the soil. However, a fall pre-plant N fertilization program is also a good alternative for most soils in Kansas, considering that the months with the lowest precipitation in Kansas are typically during fall and winter, resulting in low risk for N loss.
It is important to keep in mind that N must be in the nitrate-N (NO3-) form to be susceptible to leaching or denitrification loses. Factors such as N fertilizer source and conditions for the nitrification process this fall should be considered, in addition to the amount of precipitation and the soil type.
Since nitrification is a biological process, how quickly ammonium-N (NH4+) in soil converts to nitrate-N is a function of soil oxygen content, soil temperature, pH, N application method, and perhaps most importantly, how long the N has been in the soil.
Nitrification is an aerobic process and requires high levels of soil oxygen. Conditions that reduce oxygen supplies, such as wet soils, will inhibit nitrification and keep N in the ammonium form. Optimum soil temperatures for nitrification are in the range of 75-85 degrees F. However, nitrification occurs any time the soil temperature is above freezing, just at a slower rate. As a result, the timing of N application is critical for estimating the amount of N that may be present as nitrate.
Fertilizer application method is also a key factor impacting nitrification rate. When urea or UAN are broadcast, nitrification will occur more rapidly than when those materials are subsurface banded. Broadcast fertilizer is in contact with more soil containing the bacteria responsible for nitrification, so the nitrification process occurs more rapidly. Also, some sources such as anhydrous ammonia require several months to convert all the ammonia to nitrate, reducing the risk of N loses.
In summary, for wheat fields already fertilized with the full rate of N this fall, the risk of N loss will be higher for sandy soils, and early N application using sources such as UAN (25% in the nitrate form), or broadcast urea. However, for medium-textured soils, N fertilizer source such as anhydrous ammonia (or with the use of nitrification inhibitors) the risk for any N loses is minimum. One option to help producers assess their situation is to create some “reference strips” in the field by adding some additional N. Adding 1 pound of urea to an area 10 feet wide by 25 feet long would be equivalent to adding around 80 pounds of additional N. Observing the differences between the “reference strip” and the balance of the field can provide a good idea of the degree of N loss.
Dorivar Ruiz Diaz, Soil Fertility and Nutrient Management Specialist