Some physical properties of structural aggregates separated from organic waste-amended soils

Surface (0–20 cm) samples from five soils amended with pig slurry, sewage sludge or cattle slurry were separated into four macro- and three micro-aggregate fractions by dry sieving. Relative to the controls, these amendments decreased the apparent density and dispersibility of the aggregates, increased slightly their water-retention capacity at —0·03 MPa tension, but had no significant effect on intra-porosity and particle-size distribution of the aggregates. On average, pig slurry reduced aggregate dispersibility by 34% in the sandy loam and 8% in the sandy clay loam Modena soils. Sewage sludge and cattle slurry reduced dispersibility by 41% and 26%, respectively, in the sandy loam Lamporecchio and sandy clay loam Cremona soils. The organic carbon contents of the aggregates accounted for 73–98% of variability in their tendency to disperse, whereas their silt plus clay contents accounted for 38–96% of variability in the moisture they retained at —0·03 MPa tension.