Changes in soil structure under different cropping systems

Despite a large number of experimental investigations, it remains difficult to predict, or even interpret, changes in soil structure under different cropping systems. The principal objective of this investigation was to devise and test a conceptual model which could provide a framework for the design and interpretation of experiments concerning the effects of cropping systems on soil structure. The model concentrates on processes that change E100 (the volume of pores > 100 µm in effective diameter per unit volume of total soil). The processes identified are tillage, slaking and slumping, earthworm activity as well as root growth and decomposition. A glasshouse experiment demonstrated that root growth and decomposition could significantly change E100 and soil transmission properties, but attempts to mathematically model these changes were unsuccessful. In addition, the presence of living roots of wheat and perennial ryegrass both helped to maintain pore stability, as indicated by the way in which saturated hydraulic conductivity changed with time. Field experiments were conducted at Lincoln College, Canterbury, New Zealand to provide sufficient data to apply the conceptual model in the form of a budget equation for the 5-15 cm soil depth. In one experiment, perennial ryegrass and winter wheat were managed identically within two cultivation systems (conventional cultivation and direct-drilling) on a silt loam initially in long term pasture. Many of the soil measurements indicated a deterioration in soil structure over the two years of the experiment, regardless of treatment. There was little indication of differences between the effects of winter wheat or perennial ryegrass on soil structure. Nevertheless, as in the glasshouse experiment, the presence of living roots helped to arrest the decline in soil structure. In another field experiment using rhizotrons, it was found that an instantaneous measurement of the maximum amount of root material present in the soil could underestimate the real amount returned over one season by as much as 60% of the harvested root dry weight. Visual observations indicated a half-life of 254 days for perennial ryegrass root decomposition under Canterbury conditions. Analysis of the field experiment results using the conceptual model suggested that tillage and slumping were primarily responsible for changes in E100, whereas the actions of roots and earthworms themselves were insufficient for causing appreciable short-term increases in E100. Tillage appears the only practical way to achieve a rapid increase in E100. This increase may be best achieved if tillage is carried out when a crop is established so that the effect of living roots on macropore stability will be utilised to its fullest extent. Development and application of the conceptual model has emphasised a major need for methods to estimate macropore stability.