Deposits of rock phosphate form naturally in geological formations. It is harvested, ground to powder and added to soil as a source of phosphorus. A Michigan State University bulletin, titled "N-P-K Fertilizers," reports a steady de cline in the availability of rock phosphate on the market. The author cites the material's high transportation costs and poor water solubility as catalysts for its diminished use in agricultural practices. Rock phosphate still has limited use in domestic cultivation and is refined as an ingredient in other advanced fertilizers.