Changes in the properties of a Vertisol and responses of wheat after compaction with harvester traf®c

Soil compaction has been recognised as the greatest problem in terms of damage to Australia's soil resource. Compaction by tractor and harvester tyres, related to traf®cking of wet soil, is one source of the problem. In this paper an array of soil properties was measured before and immediately after the application of a known compaction force to a wet Vertisol. A local grain harvester was used on soil that was just traf®cable; a common scenario at harvest. The primary aim was to determine the changes in various soil properties in order to provide a `̀ benchmark'' against which the effectiveness of future remedial treatments could be evaluated. A secondary aim was a comparison of the measurements' ef®ciency to assess a soil's structural degradation status. Also assessed was the subsequent effect of the applied compaction on wheat growth and yield in the following cropping season. Nine of the soil properties measured gave statistically signi®cant differences as a result of the soil compaction. Differences were mostly restricted to the top 0.2 m of the soil. The greatest measured depth of effect was decreased soil porosity to 0.4 m measured from intact soil clods. There was 72% emergence of the wheat crop planted into the compact soil and 93% in the uncompact soil. Wheat yield, however, was not affected by the compaction. This may demonstrate that wheat, growing on a full pro®le of stored soil water as did the current crop, may be little affected by compaction. Also, wheat may have potential to facilitate rapid repair of the damage in a Vertisol such as the current soil by drying the topsoil between rainfall events so increasing shrinking and swelling cycles. If this is true, then sowing a suitable crop species in a Vertisol may be a better option than tillage for repairing compaction damage by agricultural traf®c. # 2000 Elsevier Science B.V. All rights reserved.