Studies on aggregate stability. I. Re‐formation of soil aggregates

Abstract

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 original 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.