Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
FERTILIZER MATERIALS
Phosphorus fertilizers
The chemistry of phosphorus in fertilizer and in the soil is very complex. 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 readily in the soil, primarily with iron and aluminum, 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.
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). To convert the numerical value from phosphorus (P) to phosphate (P2O5), multiply by 2.3. 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 shortseason, 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 1.2-11). 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 that the starter is accurately placed a safe distance (about 2 inches) from the seed, and that high rates are avoided.