Cultivation can benefit agriculture by controlling weeds and pests, and by creating suitably sized soil aggregates for a good seed bed. However, cultivation also disrupts microbiological activity and causes oxidation of organic matter. Its effect on soil organisms and organic matter has been likened to a fire through ploughed soil: cultivation causes a decline in organic matter, which can lead to a general loss of fertility, unless counteracted by actions such as using fertilisers and rotating crops or pastures to restore organic matter levels. Loss of organic matter often leads to soil structural problems, such as surface sealing and hard-setting. Excessive cultivation was widespread during the first half of the 20th century, and still remains a problem in some locations.
Conservation agriculture is a set of soil management practices that minimise the disruption of the soil’s structure, composition and biodiversity. During recent decades, techniques of conservation agriculture have been developed that emphasise retention of crop residues, appropriate rotations with legumes and reduced tillage, or even no tillage. In these systems, seed is drilled directly into the soil, minimising disturbance of soil structure and biota, oxidation of organic matter and the threat of erosion. Maintaining soil cover on sloping land is especially important to protect against erosive rainfall. These changes have a major influence on soil condition and trend. Although declines in uptake of conservation agriculture (e.g. direct drilling) have been reported (Darbas et al. 2013), in some jurisdictions the proportion of cropping land sown using no-till methods increased from 16 per cent in 1999 to 67 per cent in 2013.
Continuous dryland cropping increases run-off and causes erosion, and long-fallowing dryland cropping contributes to rising watertables. Irrigated agriculture also contributes to rising salinity levels, with run-off of sediments, nutrients and pesticides. Maintaining vegetation in riparian zones helps to reduce nutrient run-off, trap sediments and reduce erosion, particularly erosion due to summer rainfall (Darbas et al. 2013). There is also some data to suggest that wetlands have a role in sequestering nutrients from water, although this may involve accumulation in wetland soils, with the potential for remobilisation during flood events (McJannet et al. 2012).
Conservation agriculture practices have the potential to improve crop yields while maintaining soil ecological health. The minimum-tillage and direct-drilling practices of conservation agriculture, which reduce water erosion through minimal disruption of soil, are widely practised across central and southern New South Wales, south-eastern South Australia, Victoria and Tasmania (Peterson et al. 2014). Nationally in 2014–15, the most common land cultivation practice for crops and pasture was ‘zero or minimum till’ (i.e. no cultivation apart from sowing). Of the pasture land cultivated, 2.3 million hectares received no cultivation apart from sowing, and, of the crop land cultivated, 12.4 million hectares received no cultivation apart from sowing. The use of 3 or more cultivations was the least reported cultivation practice in 2014–15 and had the largest decrease of all cultivation practices, falling by 31 per cent to 660,000 hectares since 2013–14 (ABS 2016c).
The most common crop residue management practice reported in 2014–15 was for standing residue to be retained, which was undertaken on 7.4 million hectares of crops. This practice was followed by residue retained on the ground and residue grazed off, with each reported to be used on 4.8 million hectares of crops. There was a 16.7 per cent decrease in stubble being incorporated into the soil and a 3.5 per cent decrease in stubble being removed by hot burn in 2014–15 compared with 2013–14.
Rates of adoption of conservation agriculture have decreased in the Queensland Murray–Darling Basin catchments, in part as a result of reduced soil extension services, unclear profitability and, possibly, costs of practices. Similarly, although understanding of the farming practices that result in soil erosion and salinity is high in the New South Wales Murray–Darling Basin, adoption of conservation agriculture is low and sometimes decreasing (Darbas et al. 2013).