Silsoe Research Institute, Wrest Park, Silsoe, Bedford, MK45 4HS, UK

The quantity of clay dispersed increased with increasing water content, in the absence of any mechanical pretreatment, the rate of increase rising sharply with declining SOC. Following simulated tillage, and at water contents above the plastic limit, clay dispersion increased in proportion to the energy of disruption, and also increased with decreasing SOC levels. Below the plastic limit all the soils were relatively insensitive to mechanical disruption. A simple empirical model was derived to link clay dispersion to SOC, water content and energy of disruption.

The proportion of WSA declined sharply with decreasing SOC, and to a lesser extent following tillage. The quantity of WSA following simulated intensive tillage (300 J kg^?1) of grassland (SOC, 2.8–3.2 g (100 g)^?1) was greater than that present, prior to tillage from fallow, arable and arable/ley rotation treatments (SOC 1.1–2.5 g (100 g)^?1). Aggregate tensile strength was found to be relatively insensitive to differences in SOC. However, variations of strength within treatments, an indicator of soil friability, increased in proportion with SOC. A turbidity index was derived in which the turbidity of natural and remoulded aggregates was compared. Variation of this index with increasing mechanical energy is used as an indicator of the sensitivity of soils to damage during tillage. A visual representation is constructed to link the sensitivity of soils to damage during tillage with both SOC and water potential. These experiments illustrate that management practices, which lead to a long term reduction in SOC, are responsible for an increase in aggregate strength and reduction in stability plus an increase in sensitivity of soils to structural decline following subsequent tillage.