Adoption of conservation agriculture (CA) principles in Australia increased rapidly during the 1990s and it now boasts the highest adoption rates worldwide. These principles of (1) diverse rotations (2) reduced (or no-) till systems and (3) the maintenance of surface cover make good sense in extensive, mechanised, rain-fed cropping systems on erosion-prone, structurally-unstable soils. Indeed reduced fuel and labour costs, soil conservation and moisture retention are the most commonly stated reasons for adoption of CA principles by farmers in Australia. Yet even in Australia, while broadly applicable, the adaptation and application of CA principles within specific farming systems remains pragmatic due to the diverse biophysical and socio-economic factors encountered. Most "no-till" adopters continue some strategic tillage (~30% cropped area) for a range of sound agronomic reasons, intensive cereal systems dominate, and partial removal of crop residues as hay or by grazing livestock is commonplace within the largely mixed-farming systems. Although this challenges the notion of "ideal" CA principles (zero-till with no soil disturbance, full stubble retention and >3 species in rotations) this high degree of flexibility in CA principles as practiced in southern Australian mixed farming systems makes sense to optimize both economic and environmental outcomes. In addition, some proposed ecosystem service benefits of CA such as soil carbon sequestration and energy efficiency have been recently questioned. Though the socio-economic factors of small-holder farming systems in Africa and south Asia are more diverse and clearly different to Australian farms, some of the biophysical challenges and economic realities are shared (infertile soils, variable and extreme climates, relatively low input levels, integrated crop-livestock systems, small profit margins, highly variable income). It is therefore useful to consider from a biophysical standpoint why a pragmatic approach to CA principles has been necessary, even in a relatively high-adopting country like Australia, and why we should expect similarly 'imperfect' adoption of CA (if at all) in the diverse smallholder systems of Sub-Saharan Africa and South Asia. We review aspects of CA adoption in Australia in an effort to draw out important lessons as CA principles are adapted elsewhere, including the smallholder farming systems of Sub-Saharan Africa and South Asia.
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