CHEMICAL STUDIES OF PODZOLIC ILLUVIAL HORIZONS

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Abstract
Summary: A study of the effects of pH and of carbon/aluminium ratio on the flocculation of podzol humus by aluminium is reported. Portions of resin‐treated humus extracts were adjusted to selected pH values (which varied from 1·8 to 7·0) and mixed with equal volumes of AlCl3 solutions at the same initial pH. The C/Al ratios employed were varied from 1·5 to over 20.At C/Al ratios of 16, using B‐horizon humus from an Australian podzol, complete precipitation of C and Al occurred only at pH 4‐4·5, i.e. at about the precipitation pH of hydrous aluminium oxide. As the pH was decreased below 4, increasing amounts of C (up to 25 per cent. of that added) and Al (up to 95 per cent.) remained in the supernatant liquid. As the pH was increased above 4·5, all, or nearly all, the added C and Al remained unflocculated. At C/Al ratios of 20 or more, no precipitation of either constituent was found at any pH employed. At a C/Al ratio of 16, only partial precipitation of humus and Al occurred which was maximal at pH 2·5. Similar results were obtained for humus from a New Zealand podzol although it was less sensitive to flocculation.After mixing the two phases, changes in pH occurred which were independent of C/Al ratio, degree of flocculation, or humus type, but which depended on the mean pH of the C and Al systems before mixing. Negative pH changes were recorded below pH 4·5 while above this value positive differences were found. A detailed analysis of these changes suggested the participation of polymeric hydroxy‐aluminium complexes in humus flocculation at pH 3·5‐4·5, whereas below pH 3·5 flocculation is probably brought about by free Al ions. Evidence is presented for the exchange of humus ‘anions’ for hydroxyl on the surface of freshly formed Al hydrous oxide at pH values over 4·5.A brief study of the sorption of podzol humus by Al‐bentonite and by Na‐H‐bentonite is described. Sorption by Al‐clay was maximal and complete from pH 2·0 to pH 3·0, but above this increasing amounts of humus remained unsorbed. Interpretation of the pH changes in these systems was complicated by lack of equilibrium conditions. Pure H‐clays (prepared by resin treatment of clay suspensions) showed decreasing sorption from 50 per cent. at pH 2 to practically zero at pH 4·5 or greater. It is suggested that sorption of organic matter by clays in acid soils is facilitated by the presence of exchangeable or surface‐sorbed Al.Humus suspensions showed considerable deflocculating properties towards H‐ and Al‐clays. This factor may be important in clay migration in soils.

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