If you want fall/early winter small-grain forage production and have not applied the nitrogen preplant, you can choose the nitrogen source for topdressing fall small-grain pasture primarily by cost, availability, and convenience. Base secondary considerations on the possibility of losing the applied nitrogen, which can happen by denitrification, leaching, and surface volatilization.
Nitrate (NO3) denitrifies when oxygen levels are low and bacteria and microorganisms strip the oxygen from the nitrate nitrogen molecule. This process forms nitrogen gas or nitrous oxide, which volatilizes from the soil. Conditions that promote denitrification are waterlogged or compacted soils and warm temperatures. Many times denitrification is manifested as pale green to yellow plants where water fails to drain in terrace channels and depressions.
Nitrogen leaches from soils that have more incoming water (rain) than they can hold. As the water moves through the soil, the nitrate (NO3) form of nitrogen in the soil solution is picked up and moved with the water. Seldom does the ammonium form (NH4) of soil nitrogen leach in significant amounts, although it would be possible in very coarse sands with little clay.
Nitrogen volatilizes when urea sources (46 percent nitrogen; most nitrogen solutions contain urea) of nitrogen break down on the soil surface and form ammoniacal gases that are then lost to the atmosphere. The rate of surface volatilization depends primarily on moisture and secondarily on temperature and surface pH of the soil. To minimize volatilization losses when using urea as a nitrogen source for topdressing small grains, apply the urea fertilizer when the soil surface is dry and at least 0.25 inch of rain is expected. Twenty to thirty percent of applied nitrogen can be lost if the urea becomes wet and then dries out on the soil surface or on plant residues. To reduce losses, eliminate applications to moist, warm, high-pH soil surfaces. To completely eliminate volatilization losses from urea, apply urea fertilizers preplant and immediately incorporate. There is little risk of losing nitrogen from urea when it is topdressed to small grains in February.
For additional discussion of urea fertilizers, see Urea: A Risky Alternative in the June 2000 issue of the News and Views.