Soil aggregates have been divided into seven classes by observing the coherence of the clay fraction after reacting aggregates with water. The reactions used were: immersion of dry aggregates in water, immersion of wet remoulded aggregates in water, and suspension of aggregates in water. One further class was distinguished by the presence of carbonate. Illite and montmorillonite clays were modified so as to exhibit the physical properties of some of the aggregate classes. The results with the clays were then used to explain the characteristic properties of aggregates derived from a wide variety of soils in the various classes. For example, class 2 aggregates show partial dispersion when placed, dry, in water. The minimum percentage of exchangeable sodium present in aggregates of this class was equal to that required for dispersion of the dry Na/Ca-clays immersed in water. Examples are given of where the detection of dispersion from aggregates can be useful in the field. As only simple tests are used, the proposed method of classification can be easily carried out under field conditions.
Samples taken from the two textural phases of the surface soil of an irrigated natrixeralf and its clay subsoil were dried at wilting point and in air. Water retention increased linearly with C content, with values extrapolated to zero C content proportional to clay content. Emerson et al. (1994) (Aust. J. Soil Res., 32, 939-51) had already shown that water held by the surface samples at 10 kPa suction increased approximately linearly with C content, independently of clay content. Similar linear relations were deduced for other soils using values of field capacity and wilting point reported in the literature. A constant amount of water is considered to be held by portions of the silt/clay matrix. As the C content of the soil is increased, polysaccharide gels gradually fill additional 1-3 �m wide pores within the portions. It was calculated that, after a long period in grass, gel present increases available water on a weight basis, by 34% and 125% in loamy sand and sandy clay A horizons respectively. Where farmyard manure (FYM) was incorporated, gel only formed from the added C. Nevertheless the large increase in field capacity of a sandy loam produced by rotavation was temporarily preserved by prior addition of FYM. It is suggested that the gel here was mainly on microbial filaments.
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 : I . 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. 2. 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. 3. Dispersion of Na-saturated crumbs is prevented-the linkages of the domains by organic matter restrain the diffuse double layer swelling of the clay. 4. 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 cNsc\hs byc&\m\ny t , $ thePTeyiQUs+Y S t & &qua'ft2-doma.m 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.
Compressed discs of an illite, a kaolinite, and a mixture of the two were formed from freeze‐dried dispersed suspensions of the clays, into which one level of Fe(OH)3 and two levels of Al(OH)3 had been precipitated. The precipitated Fe was found to consist of particles about 40Å in diameter and amorphous to X‐ray, rather than the acicular goethite that precipitates under similar conditions in the absence of clay. It was deduced from N2 and water vapor sorption measurements that the precipitated Al was present as very thin layers on the surface of the clay particles, which in the case of the illite could bridge some of the particles. Their form was generally indeterminate, in contrast to the well‐crystallized bayerite formed in the absence of clay.All three hydroxy treatments inhibited the double‐layer swelling of the Na‐illite and Na‐kaolinite/illite mixture in dilute NaCl solutions. Both on weight percent and on an equimolar basis, Al was more effective than Fe in reducing the slaking of dry clay discs and in increasing the resistance of clay to dispersion in pyrophosphate solutions. Possible bonding mechanisms between the precipitate and the clays are discussed.
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