Supplemental source of nutrients. There are two kinds of fertilizers: 1. Synthetic mixtures of chemicals enriched in elements and compounds not found in abundance in natural systems. These manufactured products unavoidably contain trace elements that can accumulate to higher concentrations if the product is used frequently. Such is the case in the Imperial Valley of California where selenium, a trace element, is now found in the water table in some places at toxic levels for waterfowl. 2. Natural fertilizers include manures from various animal sources and composted non-edible portions of plants.