Trace elements slowly accumulating, depleting in soils

Certain trace elements are essential for humans, plants and animals, but become toxic at higher concentrations. For many trace elements, the margin of safety between beneficial and harmful is narrow. Deficiencies of trace elements are common plant nutritional problems in crop production. While most trace elements in soils are beneficial to plant growth, a buildup of trace elements may have a negative effect on whoever eats the plant. Trace elements may also degrade water quality downstream. Some are added to soils from the atmosphere, irrigation water and agricultural inputs including chemicals, biosolids, manures and compost. On cropland, important trace elements may also be depleted from the soil profile through leaching, crop harvest, surface runoff and volatilization. We calculated the levels of trace-element accumulation and depletion on the West Side of the San Joaquin Valley. Although the accumulation of chromium, cadmium, lead, mercury, nickel, copper and zinc on cropland will increase significantly over time, the rate of accumulation is slow and the added trace elements are not likely to interfere with farming in the foreseeable future. At the same time, arsenic, boron, molybdenum and selenium are being depleted from West Side soils. Elements removed in drainage water are now accumulating in evaporation ponds. To ensure desirable levels, water and soil sources of these elements must be monitored, and research into methods for limiting their accumulation and depletion should continue