These clay particles release phosphorus very slowly. This capacity of soils to fix phosphorus is one reason why levels of phosphorus in soil solution are usually low. Some phosphorus is held in soil organic matter. This becomes available during decomposition when organic matter is broken down by soil microbes. In agricultural soils this process is accelerated if the soil is disturbed during cultivation.
Cultivation also increases the likelihood of phosphorus losses through soil erosion.
Studies, conducted by scientists from the Smithsonian Environmental Research Center on soils near Annapolis, Maryland, found that losses from corn fields were approximately seven times that from less disturbed forested areas on a per unit area basis. Regardless of the form in which it is added, phosphorus levels may build up in soils when applied in quantities that exceed removal by crop uptake and losses via erosion and runoff. Eventually the level of phosphorus in the soil will become sufficient for crop needs. Addition of further phosphorus at this point increases the chance of phosphorus loss. When soil phosphorus levels become excessive the capacity of the soil to fix it may be exceeded and levels of soluble phosphorus in the soil may rise and result in increased phosphorus losses through surface runoff and, to a much smaller degree, leaching.