THE STRUCTURE OF SOIL CRUMBS

Abstract

Summary: A model of a soil crumb is proposed in which the crumb consists of domains of orientated clay and quartz particles. A clay domain is defined as a group of clay crystals which are orientated and sufficiently close together to behave in water as a single unit. In normal agricultural soils, where calcium is the dominant exchangeable cation, the clay crystals in the crumbs are all aggregated into domains by drying. Drying to the wilting point is apparently sufficient. In the model the domains and the quartz particles may be linked to each other by organic matter or soil conditioners. The domains may also be held together by electrostatic forces. The model satisfies four conditions derived from experiment on the effect of organic matter on soil crumbs: The crystalline swelling of the clay in the crumb is unaltered—the individual crystals within the domains are unaffected by organic matter and are free to take up water. Slaking is prevented—the strains set up in the crumb on wetting are shared between several domains through their common linkages by organic matter to quartz particles. Dispersion of Na‐saturated crumbs is prevented—the linkages of the domains by organic matter restrain the diffuse double layer swelling of the clay. pF—moisture content curve is unaltered—the stabilization by organic matter does not require a rearrangement of the structure of the crumb. On the basis of the model, conventional wet‐Sieving or dispersion tests assess the strength of the organic‐matter linkages. Remoulding crumbs breaks many of the Previously Stable quartz‐domain bonds, so that on drying and re‐testing the stability of the crumbs has decreased. The model has been shown to be consistent with present knowledge of the action of cement and h e in stabilizing soil, cement or lime being substituted for organic matter in the model. It also accounts qualitatively for the sensitivity of Soils, that is the decrease in shear strength after remoulding. On the basis of the model the amount of organic matter resistant to microbial attack should be proportional to the surface area of the clay domains. Remoulding soil (or tillage) by breaking the organic‐matter bonds joining the clay domains to the quartz will free organic matter for microbial attack. This model does not apply to soil crumbs in which the crystalline swelling of the clay is prevented, for example by heat treatment.