Phosphorus
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 and to avoid negatively impacting water quality. When extractable P exceeds the environmental critical concentration, which is much higher than the soil test optimum range, 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 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, and in acidic soils (pH <5.5) when it binds with Al, in both cases 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.
Potassium
Potassium (K) is measured as potash (K2O), similar to the way P is measured as P2O5. Crop need for K varies considerably as can be seen in Table 4. It is important that the soil K plus the applied K is enough to meet crop needs. However, excessive levels should be avoided because K can interfere with the uptake of Ca and Mg (see Cation Exchange Capacity and Base Saturation in the next section). K is subject to leaching on sandy soils low in organic matter, so if high amounts are needed, split applications should be used. Very high application rates of K are also known to suppress Mg uptake, and when soil test Mg levels are low, may cause Mg deficiency.
Calcium
Calcium (Ca) is usually supplied in sufficient quantities by liming if appropriate liming materials are chosen (see Soil Acidity, pH, and Liming in the next section). If soil pH is high and Ca is needed, it can be supplied without affecting pH by applying calcium sulfate (gypsum) which contains 19-23% Ca. Small amounts can also be applied as calcium nitrate fertilizer (19% Ca) or superphosphate (18-21% Ca). See Table 5.
Magnesium
Magnesium (Mg) is most economically applied as dolomitic or high-mag limestone (see Soil Acidity, pH, and Liming). If liming is not needed, Sul-Po-Mag (11% Mg, 22% K) can be used. Blended fertilizer containing Mg can also be ordered.