For the purposes of fertilizer grades and recommendations, phosphorus (P) is measured as phosphate, or P2O5.
The amount of extractable P in a soil should not exceed the optimum soil test range to obtain the most economic return from P applications. To maintain water quality, extractable P should not exceed the Environmental critical concentration, which is much higher than the Optimum range, to maintain water quality. When extractable P exceeds the Environmental critical concentration, the risk of dissolved P loss in subsurface water flow or runoff is significantly increased. This P pollution can stimulate excessive growth of algae in lakes and ponds. When the algae die and their biomass is rapidly decomposed by microorganisms, oxygen levels are reduced below the level needed by fish and shellfish, resulting in large die-offs of aquatic life.
Excessive P amounts in soils are difficult to reduce because vegetable crops remove little P from the soil compared to N or K. For example, Table 4 (page 8) shows that sweet corn takes up about 155 lb N per acre and about 105 lb K per acre, but only about 20 lb P per acre. However, many growers apply about 100 lb P per acre annually. This is justified only if soil test P levels are below optimum. If the soil test level for P is above optimum, there is little if any crop response to additional P applications.
Plant uptake of P is extremely slow in cold soils. For this reason, when planting early into soils testing Optimum or lower, it is often advisable to apply up to 30 pounds of P2O5 as starter fertilizer in a band about 2" below and 2" to the side of the seed when planting, or as a liquid around transplants. Keep in mind that P availability is reduced in alkaline soils (pH >7.3) as it will bind with Ca, becoming unavailable to plants, and in acidic soils (pH <5.5) when it binds with Al, becoming unavailable to plants. Therefore, it is important to first balance the soil’s pH with lime applications into the range of pH 6-7 before making P2O5 applications.