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The relationships between fractions of soil organic carbon (C) oxidised by varying strengths of potassium permanganate (KMnO4) and important soil physical and chemical properties were investigated for Queensland Ferrosols. These soils spanned a wide range of clay contents (31-83%), pH values (4·4-7·9; 1 : 5 water), and total C contents (12· 1-111 g/kg). Carbon fractions were derived by oxidation with 33 mM (C1), 167 mM (C2), and 333 mM (C3) KMnO4, while organic C and total C were determined by Heanes wet oxidation and combustion, respectively. Aggregate stability was determined by wet sieving soil from the surface crust after 30 min of high intensity (100 mm/h), simulated rainfall on disturbed samples in the laboratory. The proportion of aggregates <0·125 mm (P125) was used as the stability indicator because of the high correlation between this size class and the final rainfall infiltration rate (r2 = 0qa86, n = 42). The soil organic C fraction most closely correlated with P125 was C1 (r2 = 0·79, n = 42). This fraction was also highly correlated with final, steady-state infiltration rates in field situations where there were no subsurface constraints to infiltration (r2 = 0·74, n = 30). Multiple linear regression techniques were used to identify the soil properties determining effective cation exchange capacity (ECEC, n = 89). Most variation in ECEC (R2 = 0 ·72) was accounted for by a combination of C1 (P < 0·0001) and pH (P < 0·0001). These results confirm the very important role played by the most labile (easily oxidised) fraction of soil organic matter (C1) in key components of the chemical and physical fertility of Ferrosols. Management practices which maintain adequate C1 concentrations are essential for sustainable cropping on these soils.

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Change in soil infiltration characteristics with cultivation can result in reduced soil water storage, increased runoff and erosion, and reduced crop production. We measured changes in infiltration characteristics associated with years of cultivation for 5 soil groups in south-eastern Queensland. Soils were grouped according to soil type and texture into Sodosols, light and heavy Vertosols, Red Ferrosols, and Red Chromosols/Kandosols. Soil infiltration characteristics were determined from measurements of permeability and water-holding properties of the cultivated layer (0–0·1 m deep) and the layer immediately below the plough layer (0·1–0·2 m deep). A rainfall simulator was used to measure the hydraulic conductivity of surface seals and infiltration of bare, tilled soil in the field. Hydraulic conductivity of the soil matrix and macropores and water-holding properties of the 0·1–0·2 m layer were measured with disc permeameters and pressure plate apparatus. Hydraulic conductivity of surface seals decreased exponentially in all soil groups with period of cultivation; half of the decline occurred within 2–6 years of first cultivation. Hydraulic conductivity, macroporosity, and moisture characteristic of the 0·1–0·2 m layer were similarly affected by longer periods of cultivation in all but light-textured soils. Cultivation of light-textured, hardsetting soils (18% clay) did not adversely affect hydraulic conductivity or macroporosity of the 0·1–0·2 m layer and the loosening effect of tillage was somewhat beneficial for the water-holding properties of this layer. The low hydraulic conductivities of the surface or 0·1–0·2 m layer after long periods of cultivation reduced infiltration of rainfall in the field.