HEAVY-METAL CONTAMINANTS IN INORGANIC AND ORGANIC FERTILIZERS

Commercial phosphate (P) fertilizers contain small amounts of heavy-me tal contaminants which were minor constituents in phosphate rock (PR). Animal manures and sewage sludges (biosolids) are the main organic fe rtilizers and the latter also may contain heavy-metal contaminants. He avy metals in biosolids may be found in the inorganic form or may be o rganically complexed, which could affect their chemical reactions in s oil. These heavy metals may accumulate in soil with repeated fertilize r applications. Cadmium (Cd) is the heavy metal of most concern becaus e it may affect human health. Other heavy metals of possible significa nce are arsenic (As), chromium (Cr), lead (Ph), mercury (Hg), nickel ( Ni), and vanadium (V). Some countries have set tolerance limits on hea vy-metal additions to soil because their long-term effects are unknown . These limits usually are set for the tillage layer (surface 20-30 cm ) of soil where most root activity occurs. Controls on heavy-metal con centrations in sewage biosolids and their maximum total and annual loa ding rates to soil have been imposed in some countries. Regulations al so have been proposed for phased-in limits on maximum heavy metal conc entrations permitted in P fertilizers, or they are already in effect. Most of the fertilizer regulations relate Cd limits to P concentration s, so P application rates dictate Cd inputs to soil. Regulations affec ting sewage biosolids include a number of heavy metals, while those co ncerning P fertilizers only include limits on Cd at this time.