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.