Concentrations of extractable Cu, Zn, Fe and Mn in a group of soils as influenced by air- and ovendrying and rewetting

Abstract

The effects of air- and oven-drying a group of field-moist soils and subsequently remoistening them on concentrations of Cu, Zn, Fe and Mn extractable with EDTA, DTPA and HCl were investigated. Drying increased the amounts of extractable micronutrients and organic matter from all the soils and oven-drying had a more pronounced effect than air-drying. The increases in micronutrient extractability varied with different elements, different extractants and different soils. When EDTA was used as the extractant, the drying effect persisted after 6 sequential two-hour extractions, after the extraction time was increased from 2 to 48 hours and after the EDTA concentration was increased from 0.005M to 0.16M. When the more powerful extractants pyrophosphate and oxalate were used drying had no effect on extractable micronutrients. When air- and oven-dried soil samples were rewetted to their original field-moist conditions, concentrations of EDTA-, DTPA- and HCl-extractable Cu and Zn and EDTA- and DTPA-extractable Fe rapidly decreased to near their original field-moist concentrations. In contrast, the increased organic matter extractability was only slowly reversed following rewetting. Increases in the concentrations of HCl-extractable Fe and of EDTA-, DTPA- and HCl-extractable Mn induced by drying were not generally reversible upon rewetting. When soils were rewetted and incubated in a chloroform atmosphere, concentrations of EDTA-extractable micronutrients were not significantly different from soils rewetted and incubated in open-air. The flush of microbial activity that occurs following rewetting therefore did not appear to influence micronutrient extractability. It is suggested that the reversible drying effect on micronutrient extractability is related to the physical disruption and reformation of organomineral associations whilst the irreversible effect is due to chemical changes in mineral structure (e.g. of Mn and Fe oxides).