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What happens to nitrogen fertilizer?

Around the County

March 5, 2010
From Wayne Schoper, Extension Educator, Nicollet County

In this era of high fertilizer prices and environmental concerns, we have to know where our crop nutrients are coming from and where they are going. The "big three" of corn and soybean production are Nitrogen (N), Phosphorus (P) and Potassium (K). Of these, Nitrogen is of special concern because it can be transferred throughout the soil profile by water. This means that it can be readily moved into ground water and can cause some pollution problems as well as not being available for the growing crop. Both P and K become fixated in the soil upon application and generally are not considered a pollution threat unless the soil physically moves through erosion to bodies of water.

Nitrogen is present in the soil profile through a variety of soil reactions. These processes are nitrification, mineralization and immobilization. In the big picture, immobilization is the conversion of nitrate-nitrogen into organic nitrogen. It can be considered the opposite of mineralization. George Rehm, retired University of Minnesota soil scientist, says that these two processes work against each other as mineralization makes N available to the growing crop and immobilization, at least temporarily, makes N unavailable to the growing crop. It is controlled by soil bacteria and, therefore, is a biological process strongly affected by soil moisture and soil temperature. If nitrate-nitrogen removal from the soil via immobilization is more than the addition via mineralization, there will be a net loss of nitrate-nitrogen from the soil system. This shows up in the spring and summer in area corn fields as the growing corn begins to grow rapidly. The main symptom here is yellow corn. Since corn is a grass, it needs a lot of nitrogen to produce a good yield. If N is limited, yields can be adversely affected.

These competing processes are affected by the type of residue or organic matter that is added to the soil. Manure is an excellent source of N fertilizer. Liquid manure contains mostly N that is in a more readily available form for crop uptake. Solid manure that contains a lot of straw or cornstalks will take longer to break down to make the N component available. The issue here is the Carbon/Nitrogen or C/N ratio. There is a general rule of thumb that the C/N ratio in soil organic matter is 10:1. This means that there is a balance of available N in the soil profile. When we add livestock manure that has a lot of bedding, such as corn stalks, we see this ration climb as high as 200:1 and the nitrogen in the soil profile becomes unavailable, at least for awhile, to the growing crop. This is when we can see yellow corn out in the field.

Article Photos

Wayne Schoper

In a perfect world, it would be nice to be able to predict the amount of N immobilized or mineralized. This prediction would aid considerably in formulation of N fertilizer guidelines. However, it is not possible to make this prediction based on some measure of N in the soil. Several researchers have attempted to develop this technology and, so far, none have been successful.

Will the addition of N fertilizer decrease the amount of nitrate-N immobilized and subsequently increase nitrate-N produced by mineralization? This practice has worked in the laboratory studies, but has not been proven in field research. Thus, addition of fertilizer N to enhance residue decomposition should not be a suggested management practice in Minnesota.

In crop production today, we need every pound of soil nutrients to be available. The immobilization process becomes a more important consideration as corn yields increase. Another note here is that high yielding corn produces more crop residue that needs to be broken down for use by next years crop. This is especially important when corn follows corn in the crop rotation. The use of banded fertilizer placed close to the seed can help to overcome the temporary loss of N due to immobilization.



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