NRCS National Conservation Practice Standard, Nutrient Management 590. P recommendations for corn grain production. The recommended amounts of P 2 O 5 for the optimum soil test category are based on nutrient removal for the reported yield. The amount shown in the table for the optimum soil test category is for 140 lb of corn grain per acre. Although P 2 O 5 is not recommended at the high soil test category, a small amount equivalent to that contained in 100 pounds of a common complete NPK grade, applied as a starter fertilizer banded to the side and below the seed row, may be advantageous under conditions of limited soil drainage, cool soil conditions, or crop residues on the soil surface.
None is recommended for the very high soil test category.
There are significant advantages and problems in using the soil test approach to modifying P applications for water quality purposes. From the standpoint of optimal agronomic and economic P resource use and protection of soil and water resources, the soil test P risk assessment method makes a lot of sense. There is also the issue of where and how to collect soil samples for best prediction of P loss. This assessment method is very similar to the soil test method. P threshold could be determined from a routine soil P test, an environmental soil P test, P saturation of the soil, or some other soil test.