Compare fertilizer options

Farming! There are always so many questions and few 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 areas that experienced excess rainfall in June and July. At midsummer, the spring and sidedress N applications looked much better vs. 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 are associated with each.

Fall-applied N, for the most part, saves time and offers ease of application. The fields are fit, usually, for spreading urea or knifing anhydrous, and it’s one less spring task on your list. Denitrification losses are minimized by applying N fertilizer with soil temperatures under 50 degrees F. Normally, the ground is not as wet (there have been multiple exceptions over the past couple of 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 well or long, such as sandy soils.

N cycles

N goes through many cycles from the time it is introduced into the ground through the time it is taken up in the plant. Commercially applied N is applied in two different forms, nitrate (NO3-) and ammonium (NH4+). Plants take up the nitrate form more readily than the ammonium form. Organic matter is another source of N and varies widely throughout our region. N is mineralized from the organic matter into NH4+ by way of ammonification.

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 N application in our region. Each has advantages and disadvantages depending on the year.

How loss occurs

Nitrogen loss occurs in three 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 it happens when water flushes the nitrate N below the root zone. Leaching is frequently a problem in sandy soils with low organic matter.

Denitrification occurs when soils become waterlogged. Oxygen 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 lost during heavy summer rainfall.

This was a short look at the N process. I hope it gives you an idea of how nitrogen works in the soil, allowing you to make better decisions for fertilizer management.

Spelhaug is an agronomist with Peterson Farms Seed, Harwood, N.D. For more information, contact him at 866-481-7333 or adam@peterson
, or visit

This article published in the October, 2011 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.