Nitrogen in the soil isn’t always immediately available to the plant as a nutrient. In fact, up to 98 percent of the nitrogen present in the soil is a component of decaying organic, or plant matter, is unavailable to plants in its existing form. Much of the work converting soil nitrogen to an inorganic form usable by plants is done by various soil bacteria. These busy microorganisms, mostly invisible without magnification, combine elemental nitrogen with hydrogen and oxygen in a process called mineralization that makes N accessible to plants. Generally speaking, the forms most readily available to plants as sources of N are ammonium (NH4+) or nitrate (NO3-) ions. However, another process, called immobilization of N, is often taking place at the same time as mineralization. During immobilization, inorganic (available) N is converted to organic (and therefore unavailable) N by microbial action. When crop residues high in carbon and low in N (such as wheat stubble, grass clippings or dead cornstalks) are incorporated into the soil, microbes begin to decompose the plant matter. These microbes, once thought to be plants but now classified as “prokaryotes,” need N to build protein in their tissues. The small amount of N in the crop residue doesn’t meet its nutrient needs, and may be organic and unavailable besides. So where do the microbes get their N? From available (inorganic or mineralized) N in the soil: they convert inorganic N to organic N within their own proteins. By definition this is organic N, which is not available for crop plant nutrition. The microbes eventually die and decompose, and when they do much of this N will be returned to the available form for plants to use. However, the process is very slow. And, at that same time, mineralization and immobilization continue. If nitrogen is immobilized faster than it is mineralized, it won’t be long before there is next to no soil nitrogen available for hungry crops in that field.
Posted on: Tue, 23 Dec 2014 22:51:49 +0000
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