Soil structures produced by tillage as affected by soil water content and the physical quality of soil

Tillage experiments were carried out in order to study the effect of water content on the aggregate size distribution produced by tillage, and to investigate the relationship between the soil structures produced by tillage and Dexter's index of soil physical quality, S. Tillage with a mouldboard plough was done on four different soils over a range of naturally occurring water contents. The aggregate size distribution and the specific surface area produced by tillage were obtained by sieving. We define the optimum water content for tillage, theta(OPT), as the water content at which the specific surface area of the aggregates produced is maximum. This is consistent with the water content at which the amount of small aggregates produced is greatest and the proportion of clods produced is smallest. For the four investigated soils, theta(OPT) was found to be close to the water content at the inflection point of the water retention curve, and in the vicinity of 0.8 theta(PL) (where theta(PL) is the lower plastic limit). At water contents either lower or higher than theta(OPT), the specific surface area produced was smaller. The specific surface area produced at theta(OPT) was found to be strongly correlated with the index of soil physical quality, S. The specific surface area produced is larger the greater S, i.e. the better the soil physical quality. Consistently, the proportion of small aggregates produced at theta(OPTS) is larger and the proportion of clods produced at theta(OPT) smaller, the greater S. No clods (> 50 mm) are produced on soils with good physical quality. (c) 2006 Elsevier B.V. All rights reserved.