Rock phosphate

Rock phosphates are natural deposits of phosphate in combination with calcium. The material, as dug from the earth, is very hard and yields its phosphorus slowly. When finely ground and with impurities removed, the powdery material is only slightly soluble in water, but it may be beneficial to plants in subsequent seasons following application. The reaction of phosphate rock with acids from decaying organic matter in the garden or compost tends to make the phosphorus available to garden plants. Collodial phosphate is also available and widely used.

Bonemeal

Bonemeal is a byproduct of meat-packing plants, available at some nurseries and garden supply stores. It comes in raw and steamed forms, both of which are high in phosphorus. The raw form releases phosphorus slowly. The steamed form has a higher phosphorus concentration and releases phosphorus more quickly than the raw form. Both forms also contain some nitrogen. The steamed form is recommended, if available.

Apply rock phosphate or bonemeal at the rate of 2 pounds per 100 square feet of garden soil annually, or 10 pounds per 100 square feet every five years. In heavy clay soils, phosphate (phosphorus) levels decline more slowly than in sandy or loam soils.