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Phosphate fertilizer is a fertilizer that is high in phosphorous. Most phosphate fertilizer comes from phosphate rock, a mineral mined in massive quantities of millions of tons from locations around the world. This mineral provides one of the three main nutrients needed by all plants for vigorous growth; the other two are nitrogen and potassium.
Phosphate rock is most often mined in large open pit mines; notable deposits are in Morocco, China, Florida and South America. This raw ore is occasionally used as a fertilizer without any further processing, especially in acid soils, where it serves the dual purpose of raising pH. Rock phosphate fertilizers are becoming less common, as the raw rock provides relatively little phosphorous for its weight and transportation costs make it more expensive than refined phosphate fertilizer.
The process by which phosphate rock is converted to phosphate fertilizer involves treatment with sulfuric acid; the result is often called "super phosphate." This treatment with sulfuric acid draws the phosphates from the raw ore and creates a water soluble form. This is mixed with water to create a number of similar fertilizer compounds in a concentrated liquid form which is easy to apply to fields and crops.
The water soluble super phosphate fertilizer is sometimes packaged and shipped in granular form to reduce shipping costs. This type of phosphate fertilizer is then mixed with water by the end user for application. Phosphate fertilizers are sometimes combined with other primary fertilizers to create complete fertilizers. Potassium phosphate and ammonium phosphate are phosphates combined with potassium and ammonia, respectively.
Complete fertilizers contain amounts of all three of the primary plant nutrients; these are commonly combined into a granular mix as well and packaged for shipping. These fertilizers are labeled according to the relative percentages of the amounts of the three ingredients in the mix by weight, and these proportions will be denoted on the label by a series of three numbers separated by dashes. These numbers refer to the amounts of nitrogen, phosphorous and potassium respectively and are generally represented by their individual chemical symbols N,P and K.
A complete fertilizer may be labeled as having a formula of 10-15-20 N-P-K. This means that the fertilizer has 10% nitrogen by weight, 15% phosphorous by weight, and 20% potassium by weight. The rest of the product is made up of other ingredients, which may include other nutrients plants used in lesser amounts like sulfur, iron, magnesium and others.
The NPK explanation is incorrect. 10-15-20 is 10 percent N but the P and K values are based on P2O5 and K2O. P is 43 percent of P2O5 and the amount of elemental P is 15 x 43 percent = 6.5. The conversion for K is 0.86 x 20 = 16.6 percent. It would be far more sensible if the NPK straightforwardly expressed the elemental percentages directly - as is the case for N.