Abstract

Light-textured soils (<35% clay) make up more than 80%, by area, of cropping soils in Australia. Many have inherent soil physical problems, e.g. hardsetting, sodicity and low organic carbon levels. Maintenance and improvement of soil organic carbon levels are crucial to preserving the soil structure and physical fertility of these soils.

A review of field trials on conservation tillage (3–19 years duration) on these soils in southern Australia revealed that significantly higher soil organic carbon levels compared with conventional tillage were found only in the wetter areas (>500 mm) and the differences were restricted to the top 2.5–10.0 cm. The average magnitude of the difference was lower than that reported in the USA. The lack of a positive response to conservation tillage is probably a reflection of a number of factors, namely low crop yield (due to low rainfall), partial removal of stubble by grazing and the high decomposition rate (due to the high temperature). There is evidence suggesting that under continuous cropping in the drier areas, the soil organic carbon level continues to decline, even under conservation tillage.

Better soil structure and soil physical properties, namely macro-porosity, aggregate stability and higher infiltration have been reported under conservation tillage when compared with conventional tillage. However, little information on long-term changes of these properties under conservation tillage is available. As many of these soil qualities are associated directly or indirectly with soil organic carbon levels, the lack of significant increase in the latter suggests that many of these improvements may not be sustainable in the longer term, particularly in the drier areas. Continuous monitoring of long-term changes in the soil organic carbon and soil quality under conservation tillage in different agro-ecological zones is needed.