Journal zyxwvutsr of Soil Science, zyxwvutsrq 1986,31,329-335 zyxwvuts Studies on aggregate stability. I. Re-formation of soil aggregates K. CHANEY*& R. S. SWIFT? Edinburgh School o$Agriculture, West Mains Road, Edinburgh EH9 3JG, U.K. SUMMARY When natural soil aggregates were destroyed by crushing, techniques traditionally used for re-forming aggregates, such as wetting/drying and freezing/thawing cycles, did not produce any stable re-formed aggregates. Incubation without amendment, was similarly unsuccessful, whereas incubation with glucose amendment did produce stable aggregates, and their stability was related both to the natural soil organic matter levels and to the ori- ginal stability of the natural aggregates. However, the stability induced by incubation with glucose was of a transient nature and declined over a period of 12 weeks. This behaviour was attributed to the production of microbial, extracellular polysaccharides and their subsequent decomposition. Addition of microbial polysaccharides of known structure confirmed that such polymers were capable of producing stable re-formed aggregates without the assistance of further microbial activity. Longer term incubation showed that the stability of the re-formed aggregates also declined as soil micro- organisms broke down the polysaccharide material. Neither the glucose incubation nor addition of extracellular polysaccharide was very successful in producing stable aggregates when used with soil which had been washed with salt solutions, and dialysed, to form mono-ionic soils. INTRODUCTION Traditionally, physical processes, such as wetting and drying or freezing and thawing cycles, have been associated with the improvement of soil structure in agricultural soils. These processes, which have been studied on field samples collected at various times throughout the year (Chepil, 1954) and under laboratory conditions (Willis, 1955; Richardson, 1976; Utomo zyxw & Dexter, 1982) are seen as factors involved in the formation and stabilization of soil aggregates. The importance of polysaccharides in the formation of stable soil aggregates has been reviewed by Cheshire (1979). Readily available substrates, such as glucose or polysaccharide materials have been shown to produce water-stable aggregation which is transient because the glues are decom- posed readily. Treatments with periodate indicate a dominant role for polysaccharides (Tisdall & Oades, 1982). From such studies it has been postulated, and is now generally accepted, that whilst soil microorganisms metabolize most of the organic amendment they also produce extracellular polysaccharides (including gums and mucilages) which are capable of forming and stabilizing soil aggregates (Cheshire, 1979). The aim of this study was to develop a procedure for the re-formation of aggregates from a soil in which the natural aggregates had been destroyed by mechanical crushing. It was intended that the properties of the re-formed aggregate should approximate to those found in the field. In addition, attempts were made to re-form aggregates from mono-ionic soils, prepared by washing *Present address and correspondence: Dr K. Chaney, Norsk Hydro Fertilizers Ltd, Levington Research ?Present address: Department of Soil Science, Lincoln College, Canterbury, New Zealand. Station, Ipswich, zyxwvuts Suffolk IPlO OLU, U.K. 329