Therefore, it is not only available P in the surface horizon but throughout the soil profile that contributes to plant uptake. As weathering of minerals occurs, soluble P is released. Soluble P may percolate through the soil, be used by plants or microorganisms, enter the reactive pool, be transformed into secondary minerals, or be lost in runoff. Total P is all the P that exists in soils. Solution P is a very small amount, can be drawn out with water or a dilute salt, and is mostly the plant available orthophosphate form.
Reactive P is the fraction most soil tests extract.
Occluded and P in primary minerals is relatively insoluble and resistant to weathering. Organic P is found in organic matter. This process is called mineralization and is most rapid in warm, moist, and well-drained soils. Assuming a plow layer weight of 2,000,000 pounds per acre, total P is 100,000 pounds. The average soil loss due to water erosion for cropland in Iowa is 5 tons per acre.