Traditionally these materials fall into two categories - inorganic or organic. Inorganic fertilizers are chemically much simpler than organic fertilizers and are obtained from non-living sources. Most inorganic fertilizers are synthetic. For example, granular nitrogen fertilizer is made by combining nitrogen in the air with hydrogen from fossil fuels or water with various other substances. Fertilizers such as rock phosphate and potash occur naturally in deep deposits and are mined, sized and cleaned.
Rock phosphate is simply further acid-treated to produce highly soluble triple superphosphate.
Most plant nutrients in commercial fertilizers are in the form of moderately to very soluble salts. These salts dissolve rapidly in water to form ions. However some nutrients such as nitrogen remain very soluble when added to soil and become susceptible to leaching losses if applied in excess. The other 75 lbs consists of carrier and anticaking agents. Materials such as diatomaceous earths or clays are added to minimize dust and water sorption and evenly size particles for uniform spreading.