Plants absorb most of their phosphorus from the soil solution as orthophosphate (H2PO4-), regardless of the original source of phosphorus. Although orthophosphate’s negative charge prevents it from being attracted by the soil’s cation exchange capacity (CEC), it does react strongly in the soil, primarily with the large amount of iron and aluminum naturally in the soil, to form products that are very insoluble and thus unavailable to plants. A major factor controlling these reactions is the soil pH. At low or high pH, the solubility of phosphorus (and thus its availability) is very low. The maximum availability occurs in the 6.0 to 7.0 pH range. This is another important reason to lime regularly.
The solubility of phosphorus in fertilizer varies. The legal definition of available phosphorus in fertilizer is the sum of the phosphorus that is soluble in water plus that which is soluble in a citrate solution. Regardless of the actual chemical form of the phosphorus, the analyses of phosphorus fertilizers are given as phosphate (P2O5). The water solubility of this phosphorus can vary from 0 to 100 percent. Generally, the higher the water solubility, the more effective the phosphorus source. This is especially important for short-season, fast-growing crops, for crops with restricted root systems, for starter fertilizers, and for areas where less-than-optimum rates of phosphorus are applied to soils testing low in phosphorus.
The most common phosphate fertilizers are triple superphosphate (0–46–0), monoammonium phosphate (11–52–0), diammonium phosphate (18–46–0), and ammonium polyphosphate (10–34–0) liquid (Table: "Description of fertilizer materials"). All of these materials are highly water soluble. The ammonium phosphates also are excellent nitrogen sources. Monoammonium phosphate and ammonium polyphosphate, either alone or with some added potassium, make excellent starter fertilizers because of their high P-to-N ratios, high water solubility, and low free ammonia. Diammonium phosphate (DAP) is not recommended as a starter material because it produces free ammonia, which can harm the seed. Many starter fertilizers, however, contain DAP; thus it is critical, when that is the case, that the starter is accurately placed a safe distance (about 2 inches) from the seed, and that high rates are avoided. Because P is relatively immobile, placement where plant roots will have easy access to the fertilizer is especially important for P fertilizers.
Reference: The Agronomy Guide - Section 2: Soil Fertility Management