Abstract
A sandy and a sandy clay-loam soil from two dedicated (sacrificial) sewage sludge disposal sites were incubated with a total lime equivalent of 45 Mg·ha-1. Both these soils were acidified (pH 4.0 to 4.2) and had a significant accumulation of organic material (organic C of 2.9 to 3.7 %) compared to non-polluted soils (organic C of 0.6 to 1.0 %). The limed soils did not attain the desired pH of 6.5 after 6 months\' incubation due to a high buffer capacity. After incubation, soil samples were taken from the incubated pots and the levels of Al, Fe, Mn, Cu, Zn, Pb and Cd were determined by atomic absorption spectrophotometry (AAS) after extraction with NH4-EDTA and BaCl2. Most of the metals extracted with BaCl2 (except Mn in the sandy soil and Cd in both soils) decreased after liming. The EDTA-extractable Mn, Fe, and Cd in both soils and Cu and Pb in the sandy clayloam soil increased after liming, whereas Al and Zn, decreased in extractability (Statistically significant differences could not be determined for the trial due to the trial not having been designed for the results that were obtained). Similar results were reported in the literature for EDTA metal extraction but the phenomenon was not elaborated upon, except for Cr. The increased extractability of some of the metals after liming could negatively influence the use of EDTA as an extracting agent in proposed heavy-metal guidelines for similar sacrificial soils. Should liming be considered as a strategy to decrease metal mobility in sacrificial soils, the observed increase in extractability becomes a cause for concern and should receive attention in further research. Water SA Vol. 31 (2) 2005: pp.271-274
Highlights
In recent years, the trend has been to express heavy-metal levels in soils as plant- or potentially plant-available metal levels (Beckett, 1989; McLaughlin, et al, 2000) rather than “total” concentrations
Bruemmer and Van der Merwe (1989) stated that the NH4-EDTA-extractable heavy metal concentration gives a good estimate of those potentially plant-available, and suggested it to be used in the establishment of preliminary threshold values for heavy metals in South African soils
The metals Mn and Cd in Soil 1 and Cd in Soil 2 did not decrease in extractability after liming as was expected
Summary
The trend has been to express heavy-metal levels in soils as plant- or potentially plant-available metal levels (Beckett, 1989; McLaughlin, et al, 2000) rather than “total” concentrations. This implies that “weaker” extractants or chelating agents be used in heavy-metal studies. Bruemmer and Van der Merwe (1989) stated that the NH4-EDTA-extractable heavy metal concentration gives a good estimate of those potentially plant-available, and suggested it to be used in the establishment of preliminary threshold values for heavy metals in South African soils. There are no guidelines stipulating the maximum EDTA-extractable metal levels in South African soils
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