Changes in aggregate stability and associated organic matter properties after direct drilling and ploughing on some Australian soils

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Abstract
After 3-8 years' continuous cropping with conventional cultivation and direct drilling, five Australian soils were examined for changes in structural stability attributable to tillage methods. Some increase in organic carbon was found in four direct drilled soils, and the proportion of carbon in the greater than 2 µm fractions was also higher in these soils after short ultrasonic treatment. Direct drilled soils had significantly greater stability with at least one of the stability tests used, but no soil showed consistently greater stability to five different tests. One direct drilled soil was more stable than its ploughed equivalent after alkaline and neutral salt extraction of metal ions and associated humic substances. One direct drilled soil was more dispersed by polysaccharide extraction. Three direct drilled soils retained greater stability after selective extraction of trivalent metal ions complexed to humic fractions. Structural improvement takes place in Australian soils after several years of direct drilling, but may be at a slower rate and to a lesser extent than has been reported for wetter environments.