Released: March 08, 2007

Understanding Soil Nitrogen

MANHATTAN, Kan. – It’s understood that not all of the nitrogen (N) that crop producers pay for will actually be available for plant uptake. Some of it will be lost through volatilization, denitrification, leaching or surface runoff and some may even be unavailable due to immobilization, said David Whitney, Kansas State University Research and Extension Professor Emeritus of agronomy.

Producers have heard these terms and others numerous times, but many may still be wondering just what exactly do these words mean?

• Mineralization is the process in which organic N-containing compounds are broken down into nitrate by microbes and made available for plant uptake. Since mineralization is a microbial-mediated process, soil temperature, moisture, pH and aeration all affect the rate of process.

• Nitrogen fixation is a biological process in which elemental N is converted into organic N compounds through microorganisms in the soil. Organic N compounds are then converted into plant-available nitrate or ammonium through further microbial activity. Legumes are responsible for most biological N fixation however, some fixation does occur by free-living organisms in the soil. Fertilizer application rates should be adjusted when legumes are used in crop rotations.

• Immobilization can occur when organic matter with high carbon to nitrogen ratios, such as wheat straw, corn and grain sorghum decompose. Immobilization is generally a short term process in which the available nitrate-N in the soil is “tied-up” during decomposition. Crops planted during an immobilization period can show N deficiency if no supplemental N is supplied.

• Nitrate-N leaching can occur in areas where there are sandy soils. Leaching can happen when excess rainfall or irrigation water causes a rapid movement of the water below the root zone. Positively charged N that is in the form of the ammonium ion, however, doesn’t leach since it is held in the soil by negatively charged clay particles and organic matter.

• Denitrification is a process by which nitrate or nitrite ions (except for ammonium-N) are broken down into gaseous forms of N by anaerobic organisms under waterlogged soil conditions. These gaseous forms are then lost to the atmosphere. For substantial losses to occur, however, soils usually have to be waterlogged for several days.

• Volatilization refers to the loss of N in the form of ammonia from the soil to the atmosphere and can occur when anhydrous ammonia is applied improperly. It can also occur from manure or urea-containing fertilizers that are left on the surface of the soil. To minimize losses, producers can incorporate the manure or urea into the soil or apply it shortly before an expected rainfall. A rainfall of a half-inch or more will move urea into the soil.

“Producers should utilize agronomic practices that maximize mineralization and N fixation, and minimize volatilization, denitrification and leaching,” Whitney said.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Leah Bond
lbond@ksu.edu
K-State Research& Extension News