How plants use Phosphorus:
Phosphorous is found in most soils with most phosphates being located at the soil surface. Phosphorus must go through the same type of cycle as Nitrogen, soil bacteria must break phosphorus down so plants can absorb it. Without this bacterial activity most phosphorus will go unused by plants.
Soil alkalinity also helps determine whether or not plants get enough phosphates. Acid levels that are too high or too low will cause a chemical reaction to take place that locks up the phosphates and makes them unusable by plants.
Other factors that prevent plants from taking up phosphorus include: Extreme temperatures, Not enough organic material, Lack of Oxygen, or Too little soil moisture.
Light Application: 7 lbs per 1,000 square feet, 1/4 cup per plant or 1 lb per 20 ft row
Normal Application: 12 lbs per 1,000 square feet, 1/3 cup per plant or 2 lbs per 20 ft row
Heavy Application: 25 lbs per 1,000 square feet, 1/2 cup per plant or 3 lbs per 20 ft row
Derived from: Rock Phosphate