In a nutshell, phosphorus is an essential nutrient for all reproductive plant parts. It is found in high concentrations in seeds, roots, small seedlings, flower and fruiting structures. It stimulates root formation and growth, blooming and seed formation. Phosphorus is not mobile through the soil like nitrogen. This is important to know because for some crops, if it is broadcast on the surface of the soil, the phosphorus may never actually reach the root zone where it needs to be for uptake.
Research suggests that phosphorus be cultivated into the soil before planting for best effect.
In soils, phosphorus levels vary widely from one part of the country to another. As recommended before, a soil test from your local soil test lab will be able to help you with fertilizer application recommendations appropriate for your area. Potassium Like nitrogen, potassium is very mobile within plant structures and can be moved easily from one part of the plant to another. Potassium is associated with plant metabolism, photosynthesis, stem strength, fruit and vegetable quality, water regulation, and winter hardiness. In the soil, potassium is easily leached just like nitrogen.