Soil Organic Matter
Carbon to Nitrogen Ratio
Plants depend on microbes to break down organic matter and make the nutrients available to them. To grow, microbes need carbon for energy such as sugars, carbohydrates, fats, and other substances for life processes and nitrogen to synthesize nitrogen-containing cellular components, such as amino acids, enzymes, and DNA. The ratio of organic carbon to total nitrogen is referred to as the carbon to nitrogen (C:N) ratio. There is always more carbon than nitrogen in organic matter. This ratio indicates the proportion of nitrogen and other nutrients relative to carbon in that material. For instance, a ratio of 10:1 means that there are 10 units of carbon for each unit of nitrogen in the soil.
Mineralization-Immobilization
As mentioned, the C:N ratio determines whether nitrogen that will be mineralized (released) as the material is decomposed or if nitrogen will be immobilized (tied up) by the decomposer community as it breaks down the material. On the average, soil microbes must incorporate into their cells about eight parts of carbon for every one part of nitrogen. Because only about one-third of the carbon metabolized by microbes is incorporated into their cells (the remainder is respired and lost as CO2), the microbes need to find about 1 g of nitrogen for every 24 g of carbon in their “food.” Therefore, a residue with a C:N of about 24:1 provides the perfect balance of energy and nutrients for soil microorganisms. If foodstuff such as mature alfalfa hay (C:N ratio of 25:1) is added to the soil (see Table 9.1), the soil microorganisms will consume it relatively quickly with essentially no excess carbon or nitrogen left over.
Managing Nitrogen Immobilization-Mineralization
Nitrogen tie-up can be avoided in several ways. One way is to plant after the nitrogen depression period is over—when the decay of previous crop residues is mostly complete. During the nitrogen depression period, nitrogen is tied up (immobilized) faster than it Is released (mineralized). Another is to fertilize with enough extra nitrogen to provide for the needs of the microorganisms. For instance, if manure is being used to amend soil, some additional nitrogen fertilizer can be added as well to meet the needs of both the microbes and the crop.
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