A berdeen) WITH PLATE SummaryExamination by infrared spectroscopy and electron microscopy of the fine clays (<0.5 pm) dispersed at pH 3.5 from HzO,-treated soil indicates that imogolite and proto-imogolite allophanes are concentrated in podzolic Bz and B, horizons, and make up at least 6 percent of one B, horizon soil, which contains virtually no layer silicate clays. It is argued here that imogolite-type components are the principal source of extractable aluminium and silicon in such horizons, that they may act as cementing agents in indurated horizons, and that proto-imogolite, a soluble aluminium-silicate complex, is the predominant mobile form in which aluminium is transported to B, and lower horizons of podzols. Comparison of the amounts of aluminium extracted by acetic acid with those extracted by EDTA indicates that extractable aluminium in Bhg, Bh, and organic-rich A, horizons is present principally in organic complexes. It is proposed that the aluminium fulvates concentrated in these horizons are formed in situ.

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Translocated (oxalate-soluble) A1 and Fe are present predominantly in inorganic forms in the B, horizons of the five pcdzol profiles examined: A1 as imogolite and proto-imogolite allophanes, and Fe as a separate oxide phase. Below the top few cm of the B, horizon, over 75 per cent of the extractable (acid-plus alkali-soluble) organic matter is present as Al-fulvates, largely sorbed on allophanic material. The Bh horizons of the Iron Humus Podzol and Iron Podzol intergrades are distinguished by very high levels of organically bound Fe (soluble in EDTA solution), five to ten times more than in immediately adjacent A, or B, horizons, and also by larger humic acid contents than in comparable B, levels in typical Iron Podzols. Inorganic forms of translocated A1 and Fe are probably absent from two of the three Bh horizons examined, and also from the Bhg horizon overlying the thin iron pan in the Peaty Podzol. The organic matter in this Bhg horizon is saturated with Al rather than Fe. Chemical and physical processes which could lead to evolution of a profile along the genetic sequence, Iron Podzol, Iron Humus Podzol, Peaty Podzol, are postulated.

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Analyses of 42 sewage sludges from rural and industrial towns in England and Wales show very large concentrations of certain trace elements. Total contents of 5 % Zn, nearly 1 % Cr and Cu and 0.5 % Ni were found in certain dried sludges.Total contents of Cu, Sn and Zn and, to a lesser extent, Ag, Bi and Pb were much greater than the levels of these elements in soils. Total Cr and Ni were very high in a small number of samples. Acetic acid-soluble Zn and Cu levels were generally very much greater than in soils. Overall, Mn, Ni and Zn had considerably higher percentage solubilities than Cr, Cu, Mo, Pb, Sn and V. Zinc, Cu and Ni appear to be the elements most likely to give rise to toxicity problems in plants due to the use of sewage sludge as a soil additive.