Is an enhanced soil biological community, relative to conventional neighbours, a consistent feature of alternative (organic and biodynamic) agricultural systems?

A review was conducted of studies which compare the soil biological community on Farms where plant nutrition is managed in a 'conventional' manner (addition of synthetic easily-soluble fertilizers) with farms which adapt 'alternative' fertilizer strategies (organic or biodynamic). Such alternative strategies include additions of natural minerals, composts or manures, and growth of green manure crops or inclusion of legumes in the rotation. Four groups of soil organisms are examined (soil micro-organisms, soil fauna, soil-borne plant pathogens and soil micro-organism/plant symbioses which enhance plant nutrient uptake) and a case study of biodynamic and conventional dairy farms in southern Australia is presented. It is concluded that the total soil microbial biomass and the biomass of many specific groups of soil organisms will reflect the level of soil organic matter which will, in turn, be largely determined by the volume of recent organic matter inputs. Hence, when alternative farms include regular inputs of organic matter in their rotation, they will tend to have a larger soil community than conventional neighbours whilst these inputs are decomposed. Non-transient increases in the soil community may take many years to appear as relatively stable soil organic carbon tends to increase slowly. Conventional practices, such as addition of soluble fertilizers and pesticides, may affect some groups of soil organisms, but their overall effect on the soil community will generally be small. However, if alternative farming systems do not include large inputs of organic matter, if their production is strongly limited by low levels of nutrients such as phosphorus, or if they are of a low intensity nature, the soil community may not differ from a conventional neighbour. This is often the case in Australia and may occur with increasing Frequency in other regions if alternative farms have negative nutrient balances. Hence, an enhanced soil community relative to conventional neighbours should not be considered as a definitive feature of alternative agricultural systems.