Most farmers buy complete fertilizers, which, despite the name, usually contain only three main nutrients: nitrogen, phosphorus, and potassium. There are usually three numbers found on a fertilizer bag. The first number represents the mass percent N; the second, the percent P2O5; and the third, the percent K2O. So, “5-10-5” means that a fertilizer contains 5% N, 10% P2O5, AND 5% K2O. The rest is inert material.
A 100-lb bag of this fertilizer contains 5 lb of nitrogen, N. The actual mass of the nitrogen-containing compound, of course, depends on its formula, for example, whether it is NH4NO3 (ammonium nitrate) or CO(NH2)2 (urea). But what about the K2O and P2O5? There are no compounds with these formulas in the fertilizer. These percentages based on oxides are a holdover from the way compositions were reported in the early days of analytical chemistry, but we can convert them to actual % P and % K without difficulty.
Phosphorus can be supplied by several compounds; Ca(H2PO4)2 and (NH4)2HPO4 are common ones. The potassium is nearly always supplied as KCl, although azny potassium salt will furnish the needed K+ ion.
Fertilizers greatly increase the production of food and fiber, but they also can cause problems. The fertilizers must be water soluble to be used by plants. When it rains, the nutrients from the fertilizers are washed into streams and lakes, where they stimulate blooms of algae. These chemicals, particularly the nitrates, also penetrate the groundwater. In some areas the nitrates present in well water have reached levels that are toxic to infants.
