Along with nitrogen and potassium, phosphorus is considered one of the three essential nutrients for garden plants. Complete fertilizers contain all three nutrients, but some soil does not need additional nitrogen and potassium. If only phosphate is needed, then organophosphate fertilizers, such as rock phosphate and bonemeal, can be used to supply phosphorus. Manures high in phosphorus also can be used as soil supplements and supply nitrogen and potassium as well.
The term "organophosphate" refers to organic compounds that contain phosphorus. The phosphorus-containing fertilizers rock phosphate, bonemeal and manures can be used individually, or manure can be combined with either finely ground rock phosphate or bonemeal. Mix them at a rate of 2 to 3 pounds rock phosphate or bonemeal per 25 pounds of manure. Apply up to 1 pound of that mixture per 1 square foot of soil surface in your garden.
Rock phosphate is the source of most commercially available phosphate fertilizers. It is usually refined into superphosphate or ammonium phosphate before used in fertilizer mixes. Unrefined, finely ground rock phosphate can be applied to garden soil to supply phosphorus and calcium at a rate of 2 to 5 pounds per 100 square feet of soil surface. It is considered a slow-release fertilizer, and releases nutrients only when the soil where it is applied has a pH level below 6.0, which can be determined by a soil test.
Bonemeal consists of ground bones and can come from a variety of sources, most commonly from processed cattle bones left over from the beef industry. Like rock phosphate, it is a slow-release source of phosphorous and calcium. Use of bonemeal in soil is more common than rock phosphate, and it is often used when planting bulbs. Apply bonemeal at a rate of 1 tablespoon per 2 square feet of soil, or by adding 1/2 teaspoon of bonemeal to each planting hole for bulbs. Bonemeal releases phosphorus best when the soil pH is below 7.0.
Manures from different sources contain different amounts of phosphorous. The highest levels of phosphorus are found in poultry manure, such as chicken, duck, goose and turkey manure. Sheep manure also has higher levels of phosphorus than cow, horse and hog manure. Manures should be aged at least 30 days or composted before applied to soil. Before planting, apply at least 12 pounds of aged or composed manure per 100 square feet of soil surface, and work the manure into the soil. If you want to apply aged or composted manure to established plants, then spread up to 5 pounds of it per 100 square feet of soil surface.
- Merriam-Webster: Organophosphorus
- Clemson Cooperative Extension: Fertilizers
- Colorado State University Extension: Phosphorus Fertilizers for Organic Farming Systems
- University of Wisconsin and University of Wisconsin Cooperative Extension: Soil and Applied Phosphorus
- University of Florida IFAS Extension: Phosphorus Sources and Risk Potential -- Organic and Inorganic Fertilizers
- University of Florida IFAS Extension: Organic Vegetable Gardening