Readily-extractable hydroxyaluminium interlayers in clay- and silt-sized vermiculite

V. C. FARMER, B. F. L. SMITH, M. J. WILSON, P. J. LOVELAND* AND R. W. PAYTON* The Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2Q J, and *Soil Survey and Land Research Centre, Silsoe Campus, Silsoe, Beds., MK45 4DT, UK (Received 24 February 1988; revised 25 May 1988) A B S T RA C T : The dominant source of oxalate-soluble A1 in the B horizon of a brown podzolic soil developed on andesite appears to be hydroxyaluminium interlayers in a dioctahedral vermiculite in the clay and silt fractions, of which the latter contributes most to the total extractable A1 and fluoride reactivity of the whole soil. Cold sodium carbonate solutions extract as much interlayer A1 as does oxalate, indicating that this fraction of the interlayer material is highly reactive, and sharply distinguished from more resistant interlayers which are also present. Polymeric hydroxyaluminium interlayers are a widespread feature of vermiculites and smectites in acid soils over a pH range of 4.0-5.8 (Barnhisel, 1977). A survey of the literature suggests that such interlayers are highly resistant to mild extraction procedures. Cold acid oxalate solutions are effective solvents for allophane and imogolite (Farmer et al., 1983), but are reported to have little effect on hydroxyaluminium interlayers (McKeague & Day, 1966; Iyengar et al., 1981; Aurousseau et al., 1983). Commonly, repeated treatments with hot citrate solutions or other more aggressive procedures are used to extract interlayer material (Barnhisel, 1977), but even these often effect only partial removal, as indicated by incomplete collapse of the interlayer spacing of K-saturated clays (e.g. Dixon & Jackson, 1962). The mobilization of A1 from acid podzolic soils to surface waters in the hill and upland environment is causing increasing concern within the general context of acid deposition from the atmosphere and damage to fish stocks. A complete understanding of the conditions under which AI is released to surface waters requires the characterization and quantification of all possible sources and sinks of mobile AI in the soils. Secondary weathering products in podzolized soil profiles are highly significant in this respect (Wilson, 1986), particularly proto-imogolite allophane, the precipitation of which is very pH-dependent (Farmer & Fraser, 1982). However, some podzolized soils are characterized by higher amounts of reactive AI, as assessed by solubility in oxalate and pyrophosphate solutions, than can be attributed to allophanic material. We report here, however, an example of a soil horizon in which interlayer hydroxyaluminium species appear to make a substantial contribution not only to the oxalate-soluble AI fraction, but also to A1 extractable by cold sodium carbonate solution. The latter extractant is considered to dissolve only very reactive AI species (Farmer et al., 1983). It is also of interest that the silt fraction, not the clay fraction, is the principal source of this reactive AI. 9 1988 The Macaulay Land Use Research Institute