Phosphorus is adsorbed by plants in the ionic forms H2PO4– and HPO4=. General knowledge of ion exchange in soils would predict that these anions are not retained by the negative charged soil colloids, but move in the soil similar to nitrogen. However, phosphorus does not leach. In fact, it moves very little, even with large amounts of precipitation or irrigation. The reason for this apparent anomaly is that the soil solution contains only a very small amount of available phosphorus in these ionic forms at any one time. In fact, most soils contain less than 0.00005 grams phosphorus per liter or 0.0000068 ounces phosphorus per gallon of soil. It has been estimated that the phosphorus in the soil solution must be replenished on an average of about twice every day for normal crop growth. This is the basic phosphorus problem — to adequately re-supply the soil solution as the crop roots remove available phosphorus from the soil solution. It is the soil’s ability to re-supply the soil solution that dictates whether the crop will need additions of fertilizer phosphorus and whether those additions will be effective in the forms applied.