Phosphorus enables plants to store energy and develop strong roots. It promotes flower and fruit development and encourages early maturity of plants. According to the Colorado State University Extension, optimal availability of phosphorus occurs in soil with a pH between 6.0 and 7.0.
Commercial fertilizer contains phosphorus in its formula, indicated by the second number in the formula. For example, 5-10-10 fertilizer contains 10 percent phosphorus. Organic sources include residue from the breakdown of animal parts, such as bone meal. Rock phosphate and colloidal rock phosphate mined from mineral deposits provide a good source of phosphorus, but break down slowly in soil. Super phosphate is created by treating rock phosphate with sulfuric and phosphoric acid to begin the process of breaking it down, making it quicker acting than rock phosphate.
Soil that does not contain sufficient amounts of phosphorus, or that has a high or low pH that renders the phosphorus useless, cannot support lush growth. It produces plants with poor root development that may fail to bloom adequately and fruit production is inhibited in size, number and quality. Plants may appear weak and growth may be stunted.
The ideal time to add phosphate fertilizer is when the soil is tilled and prepared for planting in early spring or late fall. To add phosphorus fertilizer during the growing season, sprinkle it around the base of plants, or in bands along the row. Work the fertilizer into the top 2 to 3 inches of soil with a hoe or other hand tools.
When worked into the soil, super phosphate or bone meal provide quick results, often within a week or two, and supply the growing plants with the nutrients they need to thrive. Rock phosphate, however, may require two to three years to break down adequately in the soil. The type of fertilizer that is best for your soil depends on your needs and whether crops will be planted immediately.
A soil test to determine the pH level and amount of phosphorus in the soil prior to preparing the garden bed provides you with the information needed to amend your soil for optimal plant growth. Not only do the results provide you with an analysis of your soil, a soil summary provides recommendations for soil amendments and their application rates. Your local cooperative extension provides soil testing for a minimal fee.