Assessment and maintenance of soil fertility in temperate organic agriculture.
2008
Watson, C. A. | Stockdale, E. A. | Rees, R. M.
The maintenance of soil fertility is a key tenet of organic farming and can be broadly defined as the ability of the soil to support the productivity of the system. The central concept of soil fertility in these systems is the use of legume-based multi-annual rotations together with the judicious use of on-farm manures. Rotations provide the opportunity for nutrient elements to be replenished, usually within a grass-clover ley phase with inputs of carbon and nitrogen (by the biological processes of photosynthesis and nitrogen fixation). We review evidence that shows that this 'building phase' of the rotations is also associated with increased biological activity and changes to the soil's physical characteristics. The sequence of crops within a rotation is designed to utilize changing levels of fertility and optimize the utilization of nutrient resources over the period of the rotation. Crops with high nutrient demand would therefore normally be placed at a point following the incorporation of a nutrient-rich ley phase. In some circumstances cover crops or undersown vegetation are appropriate to conserve nutrients and reduce losses at particular times within the cropping sequence. Where necessary, a small range of carefully controlled external inputs are allowed. The best methods of assessing soil fertility in organic farming depend upon longer-term and integrative assessments of the system such as crop yields or long-term changes in soil quality and organic matter rather than snapshot measurements of individual nutrient pools.
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