Abstract :
Ammonium acetate extractable Cr, identified as Cr(VI) using s-diphenylcarbazide, was detected as the toxic agent in certain stored ultramafic subsoils, that severely affected the growth of maize. The subsoils contained Fe–Mn concretions, which was evidence of oxidation-reduction cycles and had high total Cr contents and significant levels of easily reducible Mn. Before dry storage 8 years previously along with some 100 other bulked soil samples, these subsoils had not been toxic to maize. Some change such as oxidation had occurred during storage. Certain other subsoils containing high total Cr but no concretions (i.e. well aerated subsoils) and topsoils containing high total Cr, were treated with various Mn compounds in an attempt to induce Cr toxicity. Only subsoils treated with oxidizing KMnO4 induced toxicity in maize owing to a measurable release of extractable Cr. The topsoils were not susceptible to oxidation and were not toxic to maize. In order to explain the apparent slow oxidation of Cr in storage in some soils, induced oxidation of Cr in other soils and insusceptibility to oxidation of Cr in yet other soils, a two-stage mechanism is proposed. Firstly, slow hydrolysis under moist field conditions of Cr(III) in chromite or from isomorphically substituted Cr, to Cr(OH)3. Secondly, slow oxidation of Cr(OH)3 to Cr(VI) by the reduction of easily reducible Mn oxides. The first stage pertains to subsoils and not topsoils, presumably because subsoils are generally moister. Only concretionary subsoils that contain specific easily reducible forms of Mn (poorly aerated) are apparently capable of oxidising Cr(III) to Cr(VI) that is soluble and toxic. Where there is an absence of specific easily reducible Mn in subsoils (well aerated), oxidation may be induced by the addition of an oxidising agent. The forms of Mn oxide likely to be present in the different subsoils and topsoil are discussed.
