EAST LANSING, Mich. — The main forms of nitrogen used in modern farming systems include urea, nitrate, ammonium and organic nitrogen (contained in living, or formerly living, plant and animal materials). Applying and using nitrogen by plants on farms in Michigan and elsewhere involves a dynamic and complex system referred to as the nitrogen cycle.
The nitrogen cycle involves the fixation of atmospheric nitrogen by lightning, microbes (alone or in combination with leguminous plants) and the addition of synthetic fertilizers. The resulting nitrate, ammonia or ammonium, when included in a plant-growing system in the soil, then undergoes a variety of changes. Some of these changes are desired and encouraged by farming practices such as incorporating fertilizers to discourage volatilization of nitrogen-containing ammonia as gas. Some parts of the nitrogen cycle are undesirable to farmers, including denitrification by soil microbes under saturated soil conditions with resulting loss of plant-available nitrogen to the atmosphere as nitrogen gas.
Plants can take up two forms of nitrogen including nitrate (NO3-) and ammonium (NH4+). The nitrate form has a negative electric charge and doesn’t “stick” to clay and organic matter particles in the soil as well as the positively charged (cation) ammonium form. For this reason, nitrate nitrogen moves through soil with soil moisture and can be more easily lost to leaching. Also, nitrification in the soil—a soil microbial processes in aerobic and above-freezing conditions with favorable pH of 5.5-7.5—converts ammonium to nitrate.
In short, there is a lot to consider regarding nitrogen sources for crop production. Crop nutrient requirements, soil conditions, forms of nitrogen available, cost and available equipment all come into play. Your local Michigan State University Extensionoffice can direct you to a plant agriculture educator to assist with these decisions.
For more detail on the nitrogen cycle, visit the University of Minnesota’s “Understanding Nitrogen in Soils” publication.
“A quick look at the nitrogen cycle and nitrogen fertilizer sources – Part 2” will provide some highlights of the various nitrogen fertilizers used by Michigan farmers.