Trace elements slowly accumulating, depleting in soils

Abstract

Certain trace elements are essen- tial for humans, plants and ani- mals, but become toxic at higher concentrations. For many trace elements, the margin of safety be- tween beneficial and harmful Is narrow. Deficiencles of trace ele- ments are common plant nutri- tional problems in crop produc- tion. While most trace elements in soils are beneficial to plant growth, a buildup of trace ele- ments 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 atmo- sphere, irrigation water and agri- cultural inputs including chemi- cals, biosolids, manures and compost. On cropland, important trace elements may also be de- pleted from the soil profile through leaching, crop harvest, surface runoff and volatilization. We calculated the levels of trace- element accumulation and deple- tion on the West Side of the San Joaquin Valley. Although the ac- cumulation of chromium, cad- mium, lead, mercury, nickel, cop- per 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, molyb- denum and selenium are being depleted from West Side soils. El- ements removed in drainage water are now accumulating in evapora- tion ponds. To ensure desirable lev- els, water and soil sources of these elements must be monitored, and research into methods for limiting their accumulation and depletion should continue.