An assessment of the vulnerability of soil structure to destabilisation during tillage .1. A laboratory test

A simple laboratory apparatus, consisting of a falling weight, was developed to impose different specific mechanical energy inputs to soil aggregates, Disruption or damage to the soil aggregates was quantified in terms of the amount of mechanically-dispersible clay (particle size 0.7 mu m or less), measured by turbidimetry. Natural aggregates, collected from the field were used to determine the sensitivity of three soils to damage as a function of specific energy input and soil water content. Results show that the amount of mechanically-dispersible clay is a function of both soil water content and specific energy input. Below a certain soil water content threshold, approximating to the plastic limit, even relatively large specific energy inputs (in tillage terms) have little effect on clay dispersion. However, as soil water content increases above the plastic limit, the soil becomes increasingly sensitive to mechanical disruption. For soil water contents between the plastic and liquid limits, the amount of clay dispersed is proportional to mechanical energy input, As soil becomes wetter, the rate of increase in dispersion becomes greater. A simple empirical model was developed to predict the amount of dispersible clay from soil water content and mechanical energy input. The laboratory method provides a convenient technique for rapid assessment of soil to mechanical damage, particularly that caused during tillage.