Bioavailability of certain heavy metals in selected organic products

Abstract

A short-term greenhouse pot experiment was conducted to determine the bioavailability of certain trace elements in organic fertilizers. Ni, Co, Cd, Cu, and Zn were selected for study. Lettuce (Lactuca sativa L. cv. Golden City), wheat (Triticum aestivum L. cv. Inia) and beans (Phaseolus vulgaris L. cv. Wintergreen) were used as test crops for metal uptake from 4 kg of soil during a 6 week growth period. A kraal manure and disinfected sewage sludge were applied at 5, 10, and 20 t/ha and a control treatment with no application of an OSA (Organic Soil Ameliorant) was also included. The soil pH was adjusted to just below 5 and above 65 by the application of 2 and 11 t/ha CaCO3, respectively. The total trace element content of the above-ground parts of the lettuce and wheat and the pods of the beans were determined while four extractants, N~-EDT A, EPA method 3050, DTP A, and AB-DTP A, were used to determine the concentration of the respective elements in the soil. The lettuce plants accumulated the most Co, Cd, and Zn, the wheat plants accumulated the most Cu and the bean pods the most Ni. An increase in soil pH mostly caused the plant concentration (mg/kg) of the trace elements to decrease, although there were some notable exceptions. The soil pH had different influences on the different extractant-metal combinations for a number of reasons. The EPA method 3050 did not predict soil loading and plant bioavailability of trace elements very well. The other extractants, however, predicted soil metal loading well even with very small increases in metal content of the soil. Good predictions of plant bioavailability was also obtained for certain extractant, metal and plant combinations but not in as many cases as expected. Legislation for the maximum permissible Ni content in soil and Cd content in foodstuffs was found to be higher than the average concentrations of these metals in the soil and foodstuffs, respectively. More research is therefore needed to clarify the situation.