Agriculture Reference
In-Depth Information
Current standards for organic practice may best suit chemically fertile soils in temperate
climates rather than more impoverished soils in other regions. Globally, poor soils are gener-
ally dominant in tropical latitudes (Leonardos et al. 2000) and the most chemically fertile
soils primarily occur in temperate areas (Huston 1993). Although differences in distribution
of basic plant nutrients inf luence the extent to which farming systems are dependent on
inputs, local exceptions occur within these global generalisations. Certified organic farming
practices evolved in regions with chemically fertile soils (especially in Europe and North
America) (Treadwell et al . 2003). In contrast, precursors of modern organic farming practices
have been successful on chemically infertile soils for hundreds of years (e.g. in tropical soils
in Africa, South America and Asia) and may include high inputs of organic matter (including
animal and human waste). At the other end of the scale, great expanses of arable land are cer-
tified 'organic' primarily because of the impracticality of using synthetic inputs (e.g. vast
areas of arid Australia). On a global scale, inherent soil chemical infertility is associated with
the greatest plant species diversity (Huston 1993).
The results of field trials and paired comparisons (e.g. 'organic' versus 'conventional'
farms) evaluating effects of organic farming systems on components of soil fertility need to be
interpreted with caution (Gosling and Shepherd 2005). Long-term studies provide an oppor-
tunity for soil biological processes to stabilise. However, few of the published field trials evalu-
ating the effect of farming systems on soil fertility have been run for longer than eight years
and most have been conducted in regions with naturally chemically fertile soils and temperate
climates (Table 2.1). Also, the evaluation of soil fertility in organic farming systems needs to be
made at the level of specific management practices (Kirchmann and Bergström 2001, Shepherd
et al. 2002, Stockdale et al. 2002, Stockdale and Cookson 2003) but this is not always practical.
Both direct and indirect effects of specific management practices on the components of soil
fertility should be considered to allow more complete interpretation of the impacts of soil dis-
turbance resulting from changes in agricultural practices on soil fertility.
In this review, we consider components of soil fertility in the context of organic farming
systems and how certified organic practices are used to manage soil fertility. Data from long-
term field studies evaluating organic farming are discussed in terms of the components of soil
fertility. The concept of soil fertility is also viewed in relation to its relevance to the application
of organic farming practices across landscapes and soil types in different climatic zones.
Components of soil fertility
The use of the term 'soil fertility' is of little value unless the physical, chemical and biological
properties of soil that contribute to its fertility are all considered (Abbott and Murphy 2003).
There is widespread agreement as to general standards for 'good' soil physical and soil chemical
fertility (which take into account different soil types), but standards for 'good' soil biological
fertility are more difficult to define (Abbott and Murphy 2003). Furthermore, in contrast to
chemical fertility, there are few commercial services available for farmers to measure soil
physical and biological properties (Price 2001), and so the use of this information is not wide-
spread. The greater emphasis on biological processes in organic farming means that the
chemical fertility of the soil depends to a large degree on how management practices facilitate
beneficial biological processes.
Soil physical fertility
Soil physical fertility contributes to the sustainability of organic farming systems by creating
the framework in which biological and chemical processes supply nutrients to plants and
protect soil from erosion. Soil physical fertility in organically managed systems is generally
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