Phosphate is responsible for the deep green color that is much desired in turf. Determining the correct amount of phosphate to use to adequately supply the needs of a plant is much more difficult than for nitrogen. The problem stems from the fact that the phosphate ion (the plant available form) is highly reactive and combines with calcium from the soil to for insoluble and unavailable compounds. Organic growers use Rock Phosphate for this, a natural calcium phosphate mineral. The phosphate ion is slowly etched away from the rock complex in the presence of weak natural acids generated in the process of microbial decomposition of organic matter. This phosphate ion may be taken up by plant roots, used by soil microbes or, more commonly, recombined with calcium. An equilibrium is achieved with only about 1/10 the total phosphate in rock phosphate available for plant uptake at any one time. As phosphate ions are absorbed more are etched into the soil water. The rate at which phosphate ions are released to the soil will determine whether the crop meets its needs for phosphorus or not. The process is quicker in acidic soils and soils with high levels of organic matter and microbial activity; slower in alkaline soils where free calcium is abundant and available to combine with the phosphate ions.

The theory of applying rock phosphate every 4 or 5 years is not practical or advised as it came from the old days when farmers applied over a ton to the acre. This method was from the chemical industry and should not be used by organic farmers.

I feel the proper application rate would be 300 to 500 lbs. a year per acre, to slowly build up your phosphate levels over time. Remember an organic method is to have a biologically alive soil to help release the phosphate. A proper balance of nutrients applied is very important. I recommend a proper soil test each year to establish nutrient needs.