The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient. mainly polysaccharides, (b), temporary, roots and fungal hyphae, and (c) persistent. resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. The effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate. Roots and hyphae stabilize macroaggregates, defined as > 250 pm diameter; consequently, macroaggregation is controlled by soil management (i.e. crop rotations), as management influences the growth of plant roots, and the oxidation of organic carbon. The water-stability of microaggregates depends on the persistent organic binding agents and appears to be a characteristic of the soil, independent of management.

An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 �m diameter breaking down to microaggregates 20-250 �m diameter before particles <20 �m were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 �m diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 �m but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.

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Summary The stability of pores and particles is essential for optimum growth of plants. Two categories of aggregates macro-(> 250 }.Lm) and micro-« 250 }.Lm) depend on organic matter for stability against disruptive forces caused by rapid wetting. Dispersion of clay particles from microaggregates is promoted by adsorption of complexing organic acids which increase the negative charge on clays. The acids are produced by plants, bacteria and fungi. However, the dispersibility of clay in microaggregates is offset by the binding action of polysaccharides, mainly mucilages produced by bacteria, but also by plant roots and fungal hyphae. The stability of microaggregates is also enhanced by multivalent cations which act as bridges between organic colloids and clays. Macroaggregates are enmeshed by plant roots, both living and decomposing, and are thus sensitive to management, and increase in number when grasses are grown and the soil is not disturbed. Lack of root growth, i.e. fallow, has the opposite effect. Various implications for management of soil structure are discussed.

A simple densimetric method for the separation of free and occluded particulate organic materials was developed and applied to five virgin soils. The free organic matter was isolated by suspending the soil in sodium polytungstate solution (d = 1.6 Mg m-3) and decanting the light material. The remaining soil was disaggregated by sonification for liberation of occluded organic materials. The free light fraction consisted of large, undecomposed or partly decomposed root and plant fragments. This fraction comprised 0.59-4.34% of soil dry weight and accounted for 6.9-31.3% and 5.9-22.1% of total soil carbon and nitrogen respectively. Identifiable components of the occluded fraction were small particles of incompletely decomposed organic residues, pollen grains, particles of plant tissue such as lignin coils and phytoliths. This fraction comprised 0.69-1.81% of soil dry weight and represented 9.2-17.5% and 6.2-14.1% of the total soil carbon and nitrogen. The proportion of soil organic carbon recovered as the occluded fraction was high in soils with high clay contents. The chemical composition of occluded and free organic materials was investigated by solid-state 13C CP/MAS NMR spectroscopy. Despite the differences in soils, environmental conditions and vegetation, the organic structure of the free light fraction was similar in four of the five soils. This fraction consisted of 55-63% O-alkyl C, 18-25% alkyl C, 14-18% aromatic C, and 5-7% carbonyl C. In the other soil, this fraction showed a higher proportion of alkyl C (31%) and lower O-alkyl C (46%). Most of the differences between soils were associated with organic materials contained in the occluded light fraction. The differences in chemical structure between the occluded light fraction and free light fractions were similar in all examined soils. The NMR data showed that the proportion of O-alkyl C was lower and alkyl C higher in the occluded light fractions than in the free light fractions. The proportion of aromatic and carbonyl carbon was higher in the occluded fractions of three soils while the percentage of these two types of carbon remained unchanged in the two other soils. It is considered that the occluded organic matter is an old pool of carbon that has been accreted within aggregates during decades of root growth and it is that pool which is lost due to cultivation.