Biological indicators of soil quality in organic farming systems

The health of the soil, recognized by its active role in the linked processes of decomposition and nutrient supply, is considered as the foundation of agriculture by the organic farming movement. Nutrient management in organically managed soils is fundamentally different from that of conventional agricultural systems. Crop rotations are designed with regard to maintenance of fertility with a focus on nutrient recycling. Where nutrients are added to the system, inputs are in organic and/or non-synthetic fertilizer sources that are mostly slow release in nature. Hence a greater reliance is placed on soil chemical and biological processes to release nutrients in plant-available forms. In this respect, nutrient availability in organically farmed soils is more dependent upon soil processes than is the case in conventional agriculture. The development and use of biological indicators of soil quality may therefore be more important in organic (and other low input) farming systems. The aim of this paper is to evaluate current evidence for the impact of organic farming systems on soil biological quality and consider the identification of appropriate biological indicators for use by organic farmers and their advisors. Organic farming systems are generally associated with increased biological activity and increased belowground biodiversity. The main impacts on biological fertility do not result from the systems per se but are related to the amount and quality of the soil organic matter pool and disruptions of soil habitat via tillage. Even within the constraints of organic farming practices it is possible for farmers to make changes to management practices which will tend to improve soil biological quality. It is, however, by no means clear that distinct indicators of soil biological quality are needed for organic farming systems. It is important not only to identify the most appropriate indicators but also