In South Australia, clay delving has become a popular practice for increasing the productivity of contrast texture soils.

Clay delving brings to the surface sub-soil clay to be mixed with the sandy top-soil and, unlike clay spreading, it combines the addition of clay material with a ripping-like effect; disrupting the boundary between top-soil sand and subsoil clay. In this case study we evaluate the effects of this soil modification on soil water behaviour and potential effects on soil carbon storage when compared to the original (pre-delving) conditions of the soil.

Two sites, near Coonalpyn and Bordertown were studied using a combination of experiments to evaluate soil water behaviour in terms of infiltration, spatial distribution of water infiltrating the soil, root growth and the effect of clay addition on water storage. Random sampling and collection of images of the modified (delved) soil profiles have also been used to estimate the effect on soil carbon storage of clay delving when compared to the original characteristics of the soil.

Results from the experiments have shown that the drastic soil modification after clay delving has a clear impact on soil water movements and storage. Deeper and more even water infiltration was found in delved profiles when compared to the control sites (undelved). The effects on water infiltration of clay delving are particularly clear when the soils are in summer dry conditions.

The results of water retention of different cores from the site in Coonalpyn have shown that the increase in clay content due to clay delving increases the potential water storage and plant available water. Nevertheless, the variability of the results suggested that other factors, rather than just clay content, have to be taken into account for comprehensive and realistic evaluation of water storage.

Carbon storage appears to increase in the top 0.1 m of soil in the delved profiles, probably as a result of greater yields and root growth, and more organic material can be found at depth along the delving lines.

Although there are no doubts on the benefits that clay delving can deliver in the amelioration of texture contrast soil, different results were achieved in the sites and the great potential for an increase in productivity of soils with clay delving depends on several factors such as pre-delving soil characteristics and delving methods

By Giacomo Betti, School of Agriculture Food & Wine, The University of Adelaide, Adelaide South Australia.