Farming! There are always so many questions – which seldom have black and white answers! That is why farmers know so much about so many subjects.
A big question this fall is: after a growing season like this past summer, do I fertilize this fall or not? Nitrogen loss was huge in the geographies which experienced excess rainfall in June and July. At mid-summer, the spring and side-dress N applications looked much better versus the fall applied N. But later in the season, some of those areas showed N deficiency as well. I’d like to compare the systems a bit along with the types of N-loss that is associated with each.
Fall applied Nitrogen is, for the most part, a time savings and ease of application event. The fields are fit, usually, for spreading urea or knifing anhydrous and it’s one less spring task on your list when the planting window can get extremely tight. Denitrification losses are minimized by applying N fertilizer with soil temps under 50F. Normally the ground is not as wet, (I know there have been multiple exceptions over the past couple years) and tillage work will leave fewer compaction zones. Spring application allows more flexibility to change your mind on which crop to plant as well as ensure that a fall application is not wasted on ground that is later planted to a legume crop. Spring applied N is also better for soils that do not hold nutrients as well or as long, such as sandy soils.
Nitrogen performs many cycles from the time it is introduced into the ground through the time it is taken up in the plant. Commercially applied in two different forms, nitrate (NO3–) and ammonium (NH4+), the plants take up the nitrate form most readily. Organic matter is another source of N for the plant and varies widely throughout our region. N is mineralized from the organic matter into NH4+ by way of aminization. Ammonification occurs by way of microorganisms. Mineralization is increased with warmer soil temperatures and adequate, not waterlogged, soil moisture.
Anhydrous ammonia, Urea, and Urea-ammonium nitrate (28%) are the three main sources for Nitrogen application in our region. Each has advantages and disadvantages and is used more or less depending on the year.
Nitrogen loss occurs in 3 main ways: Volatilization, Leaching, and Denitrification. Volatilization happens naturally in soils but for commercial purposes it is the loss that happens to surface applied fertilizer that is not incorporated in a timely manner. Leaching occurs normally with the nitrate (NO3–) form since that it is the most soluble in water. Leaching sometimes occurs with runoff on the soil surface, but normally in our area it happens when water flushes the nitrate N below the root zone in the soil profile. Leaching is frequently a problem in sandy soils with low organic matter presence. Denitrification occurs when soils become waterlogged, O2 is excluded and anaerobic conditions result. The nitrate changes to the N2 form and is then released into the air. Organisms and bacteria that work in anaerobic conditions are responsible for the conversion since they remove the oxygen from the nitrate. This is usually the culprit in heavier soils – N is held from fall or spring applied applications and is then is lost during heavy summer rainfall.
This was a short, scientific look at the N process. While I’m sure I did not do it justice, I hope it gives you an idea how nitrogen works in the soil, allowing you to make better decisions for fertilizer management for next year’s crop.