The dark-colored organic residue which resists decomposition and which is responsible for much of the property of organic matter is humus'. In fact, some of the very stable materials in the organic matter complex can remain in the soil for hundreds or thousands of years. The ratio of carbon to nitrogen of organic matter is an important factor in soil quality. These crops decompose rapidly and provide nutrients in excess of microbial needs. Residue from grain, grass and cotton are low in nitrogen but high in carbon and thus take longer to decompose.
Most soil microbes obtain the energy necessary to grow from carbon of the soil organic matter.
Since nitrogen is an essential component of cellular components, micro organisms must obtain nitrogen from organic matter, other microbes, or from the air. When organic materials with high carbon concentrations relative to nitrogen concentrations are added to the soil, soil microbes utilize most of the available nitrogen in the soil, thus creating deficiency conditions for higher plants. This is called nitrate depression, sometimes referred to as the priming effect', which will persist until activity of the soil micro organisms decreases due to a lack of carbon. Hence, it is often necessary to add supplemental nitrogen fertilizer when carbon-rich organic materials are added to the soil.