Soil Nitrogen Content

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Soil Nitrogen Content

By Willi Evans Galloway



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You can manage the nitrogen content of your soil by adding organic matter and planting crops that fix nitrogen. Organic matter, such as compost, composted manure, and cover crops, not only increases nitrogen and other nutrient levels; it also improves soil structure and stimulates beneficial microorganisms.

Before your plants can put the nitrogen in organic matter to good use, it must be converted into a form that plants can metabolize. "The metabolism of a plant is set up to only take up the inorganic* forms of nitrogen," explains Dennis Keeney, Ph.D., professor emeritus and former director of the Leopold Center for Sustainable Agriculture at Iowa State University. Tiny soil microbes make nitrogen available to plants by breaking down organic matter and steadily releasing two inorganic forms of nitrogen—ammonium and nitrate.

Seasonal fluctuations of ammonium and nitrate levels are normal, because the soil microbes that break down organic matter are less active in cool soil. Also, soil has the capacity to retain ammonium until the plants need it, but soil can't hold on to nitrate. This means that your plants must either use nitrate or lose it, because at the end of the season, leftover nitrate moves out of the soil by leaching, through runoff, and by converting into gaseous atmospheric nitrogen. Nitrate that finds its way into watersheds causes serious, negative impacts on water quality. You can prevent excess nitrate from entering the environment by following recommended application rates for compost (1- to 2-inch layer), composted manure (100 pounds per 1,000 square feet), and fertilizers (yes, even organic fertilizers can cause excess nitrate problems if applied incorrectly).

Due to the high mobility of nitrate and seasonal fluctuations in nitrogen levels, nitrogen tests may not give an accurate assessment of the nitrogen availability in your soil. Dr. Keeney suggests testing the amount of organic matter in your soil instead. "Organic matter is about 50 percent carbon and 5 percent nitrogen on average. So it is easy to use your organic-matter figure to estimate total nitrogen," says Dr. Keeney. It's good to have soil that's composed of at least 5 percent organic matter. Most soil testing labs will perform an organic-matter test upon request. If a test shows that your soil has low organic matter content, begin adding organic matter and supplement your plant's nutrient needs with an organic fertilizer, such as fish emulsion.

*Inorganic nitrogen is the product of the breakdown of organic nitrogen by natural processes.

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