Soil Nutrient Cycling
Phosphorus Cycle
In contrast to the carbon and nitrogen cycles, phosphorus does not have a significant atmospheric phase. Although phosphorus compounds do occur in the atmosphere, as trace quantities in particulates, the resulting inputs to ecosystems are small compared with the amounts available from soil minerals or from the addition of fertilizer to agricultural land. Unlike nitrogen, there is no biological process that can add phosphorus to soils. Phosphorus in soil exists in a variety of forms that differ in their availability to plants. In soils, phosphorus occurs in three pools: fixed or nonlabile pool, active or labile, and soil solution. The first pool is called fixed or nonlabile pool. This phosphorus pool is largest of all the pools. This phosphorus is unavailable for plant uptake and is comprised of primary minerals (insoluble inorganic phosphate compounds) and organic phosphorus compounds that do not mineralize easily.
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