Large areas of arable land in the eastern inland regions of Australia are dominated by cracking clay soils (Vertisols). The occurrence of compaction on these soils and its effect on crops is not well understood. This work investigated the effect of compaction on the growth and development ofsunflowers and the response of a range of cracking clay soils to compactive forces.
Compaction on Vertisols was expressed as bulk density corrected to a reference water content assuming the soil undergoes normal shrinkage. This presents a useful technique to quantify compaction of Vertisols in the field. Field experiments were carried out on a Vertisol (black earth) at the University of Queensland, Gatton College at Lawes, to investigate the effect of compaction on the growth ofsunflowers and its influence on soil water contents during two growing seasons in 1984/85 and 1985/86. Treatments included an uncompacted control, low compaction and severe compaction. The uncompacted treatment was achieved by deep ripping to 40 cm followed by fallowing for several months. Compaction was induced using a road roller with a contact pressure of lOOkPa; 1 pass and 4 passes with the roller were used for low and severe compaction respectively. In both years the compaction treatments wereapplied before sowing. A third experiment was conducted in 1985/86 on the site compacted in the previous year without re-inducing the compaction treatments to investigate the residual effects of compaction after selfamelioration of the soils in the absence of machinery traffic.
In both years severe compaction reduced the early growth of the crop and amount and depth of water depletion was decreased. However, as the season progressed differences between treatments decreased. Final seed yields were reduced by 15% in 1984/85 under high compaction. This was related to bulk density values exceeding 1.39Mgm-3(at 0.3 gg-1) below the cultivated layer at 20 to 25 cm depth. Compaction only affected plant growth when it was induced at the start of the cropping season. Wetting and drying cycles over the period of one year in the absence of machinery traffic did eliminate soil compaction in the field experiments of this study.
The response of clay soils to compactive forces was evaluated in the laboratory for soils found in major cotton growing areas in Queensland. A total of 70 sites were sampled from a wide range of soil types with clay contents varying from 30 to 73%, organic C from 0.8 to 5.7% and dispersible clay contentsfrom 2 to 22%. Atterberg limits, water content at permanent wilting point, exchangeable cations and clay content of the soils used were strongly related and could be predicted from each other.
A uni-axial compression test, using pressures and loading times similar to those occurring during the passage of agricultural vehicles, was developed and used to determine the response of these soils to compaction at different soil water contents. It was expressed using the relationship of bulk density to the soil water content at which compaction was carried out (compaction curves). Bulk density values reached a maximum value at an optimum water content depending on the magnitude of the compactive pressure applied. It was increased as uni-axial pressures increased and clay content decreased. Organic carbon decreased bulk density but its effect was small. The bulk densities at maximum compaction as well as bulk densities at water contents lower than the optimum water content were related to applied pressure, texture and water content at which the soil was compacted and could be predicted with a high degree of reliability. The equation for bulk density at water contents less than the optimum is:
p = 1.425 - 0.986 clay + 0.144 In p + 0.530 θg
n - 507, r2 = 0.71 and where
p = bulk density [Mgm-3]
clay = clay content [gg-1]
p = uni-axial pressure [kPa 10-2 (Bar)]
6g = gravimetric water content [gg-1]
This equation should be able to be developed into useful field model and a management tool The potential effects of compaction on soil productivity were associated with the change in the soil pore size distribution. Compaction reduced the volume of large pores and slightly increased the volume ofsmall pores. This resulted in slightly lower amounts of plant available water but a strong decrease in saturated hydraulic conductivity in soils at high bulk density values. Compaction can be expected to become harmful to crop growth when a threshold bulk density was exceeded. The threshold bulk density for growth and yield ofsunflower on vertisols (blackearths) at the University of Queensland, Gatton College at Lawes was 1.39 Mgm-3 (corrected to a water content of 0.3 gg-1). This value was close to that of maximum compaction achievable corresponding to an airfilled porosity of about 6% which is likely to be simularfor other vertisols and crop types.