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Calphos Soft Rock Phosphate:
Soft rock phosphate, also known as colloidal phosphate is a clay material that is surface mined from the old settling basins of former hard phosphate rock mining operations in Florida. It contains about 20 percent P205 as well as over 25 percent lime and other trace minerals. It is a very fine material, but can be applied with all common fertilizer spreaders. Natural phosphate stays where it’s put when applied and does not move or dissolve into the soil solution. It needs to be plowed under or tilled into the soil. Unlike chemically made phosphates, soft rock phosphate is insoluble in water, will not leach away, and therefore is long-lasting. Has 18% phosphorous and 15% calcium as well as trace elements. Florida is the primary source. As annual plant takes up to 60 percent of its total phosphates needs the first few weeks of its life. If it doesn’t get phosphate then it is always behind and never catches up. Soft rock phosphate used directly under the seed or transplant at planting time is the very best method of application, especially in low acid or high alkaline soils. It is not as critical but still beneficial in slightly acid to neutral soils. It is almost impossible to overuse soft rock phosphate, you can grow beautiful plants directly in it without any harmful effects.
Soft phosphate rock should be applied at a rate about two tons per acre with other organic amendments. This phosphorous source will soon be gone. It is a byproduct of the making of 0-18-0 and only one company still bags it. In alkaline soils, apply the phosphate directly under the seed or transplants so the small roots don’t have to search for it. This is especially important in the spring.
*Information derived from www.dirtdoctor.com