The key component of any efficient nitrogen (N) program is minimizing the risk of loss. Nitrogen losses can greatly reduce a grower’s profitability and can have environmental consequences. With a better understanding of the mechanisms that cause N loss and the conditions that increase the risk of loss, we can better decide when and how much to apply.
Plants can only take up N in two forms, ammonium (NH4+) or nitrate (NO3-). Regardless of your source of N, whether it be synthetic fertilizer (anhydrous ammonia, ammonium sulfate, urea, etc.), manure, or compost, the product must first be converted or mineralized to the ammonium form of N. Ammonium can then be converted to nitrate by a microbial process called nitrification. While plants can take up either form, they are generally able to access more N in the nitrate form because it moves freely with the soil water that is taken up during evapotranspiration. On the other hand ammonium is relatively immobile in the soil because it is held by the soil’s cation exchange complex. The difference in the mobility of these two forms of N is the main reason we may lose N before a plant is able to utilize it. In this case, the mechanism for N loss is leaching. In a well-drained soil, a heavy rainfall can move water downward through the soil profile, nitrate can move with the water and may move too deep for our crops to access or may be completely lost from the field if the nitrate makes it to a drainage tile. Leaching losses can be best minimized by applying N as close as possible to the time when the crop can utilize the N. In corn, the greatest N uptake occurs from V8 until R1. Losses can be further reduced by splitting N applications throughout the vegetative growing season.
Another potential mechanism for N loss to occur is denitrification. Denitrification is a microbial process that occurs poorly drained, saturated soils. When soils are saturated, there is not enough oxygen for microbial activity to thrive. Under these conditions, certain microbes are able to scavenge the oxygen atoms from a nitrate molecule in order to survive. The remaining N is then lost to the atmosphere in a gaseous form. Extended periods of saturation, especially when soil temperatures are warm, can lead to a significant loss of N. It is estimated that 4-5% of plant available N can be lost for every day of saturation. Denitrification can be minimized by ensuring you have adequate drainage in poorly drained areas and avoiding applications prior to heavy rainfall.
Volatilization is another potential loss of N to the atmosphere. Volatilization is the conversion of ammonium to ammonia gas. This is most likely to occur when ammonium forming fertilizers such as urea, or manures are applied to the surface of a warm soil with a high pH. Volatilization can account for losses as high as 80% when topdressing wheat in excessively warm conditions. Volatile losses can best be avoided by maintaining soil pH between 6.5 and 7.0, using a urease inhibitor with urea-based materials, and applying when soil conditions are moist and cool.