Each of these reactions will result in very insoluble compounds of phosphate that are generally not available to plants. The conversion of available P to fixed P is partially the reason for the low efficiency of P fertilizers. Most of the P fertilizer applied to the soil will not be utilized by the crop in the first season. Continued application of more P than the crops utilize increases the fertility of the soil, but much of the added P becomes fixed and unavailable. Occasionally the question of how much phosphate a soil can hold is asked, especially when high loading rates of P are expected or have occurred.
Soils differ in their phosphate holding capacity.
In addition the subsoil of many soils often has an even greater capacity to hold phosphate than does the corresponding surface soil. Increased amounts of phosphate in solution will potentially cause more phosphate to be lost to water running over the soil surface or leaching through the soil. Loading soils with very high levels of phosphate will generally not hurt crops but may result in increased phosphate movement to nearby bodies of water. Predicting the Availability of Phosphorus in Soils Soils may contain several hundred to several thousand pounds of phosphate per acre. However, much of the phosphate in soils is not available to growing plants.