In Queensland, Vertosols occupy an estimated area of 50 million hectares. A number of surveys have shown that wheel-induced soil compaction is a major problem in Vertosols. Current trends towards the use of heavier farm tractors and minimum tillage systems, have made research to determine effective farming systems to ameliorate wheel induced soil compaction, imperative.
A field trial was carried out on a Tognolini cracking clay, over two dryland winter wheat crops. The site was first uniformly compacted by a fully loaded harvester over the wet soil (water content> plastic limit), simulating the compaction that could occur during a wet harvest. At intervals of time corresponding to post-planting and post-harvest, of the two wheat crops, soil shear strength, penetration resistance, water potential measurements and clod samples were taken (for clod
shrinkage analysis), to a depth of 0.30 m, in order to assess the effectiveness of seven management treatments, in repairing wheel-induced soil compaction. To allow direct comparisons, a method was devised to allow soil strength measurements to be standardized to a matric potential of 1500 kPa .
Of the five parameters and two variables statistically analysed in the clod shrinkage analysis, PA (the specific volume of air-filled pores at the air-entry point) was the only one found to be sensitive to all seven management treatment effects, when comparing treatments within individual depths. PA, and standardized shear strength and penetration resistance measurements were found to be valuable tools to compare the degree of soil compaction and repair that occurs under a range
of tillage activities.
The comparative dearth of trials examining soil strength in Vertosols can be attributed to the difficulty of normalising soil strength for water content on these soils. The technique established in this trial can be regarded as a valuable tool to compare the degree of soil compaction and repair that occurs under a range of activities.
All seven treatments were only fully expressed in the in the second season, which makes it difficult to make broad recommendations on the best tillage systems to use to repair soil compaction, based on the results of this trial without futher long-term trials. However a clear trend has been the poor performance of the Zero
Tillage treatment (direct drilling at planting) in repairing soil compaction. This has significant implications for dryland farming systems due to the increasing trend towards use of no-till or minimum tillage systems.