The reaction of P fertilizers and P crop availability depends on application methods and soil conditions, including soil minerals, soil acidity, moisture, and temperature. Soil test values for crop production are obtained from the surface horizon because that is generally the most active area of P uptake by plants. 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.