DRY-MATTER AND NITROGEN ACCUMULATION BY 3 LEGUMINOUS GREEN MANURE SPECIES AND THE YIELD OF A FOLLOWING WHEAT CROP IN AN ORGANIC PRODUCTION SYSTEM

Organic farming systems often include livestock to use the leguminous forages which supply nitrogen (N) to the grain crops in the rotation. An alternative approach (especially relevant to farms with 'set-aside' ) may be to manage leguminous green manure crops by repeatedly cutting and mulching them directly in the field, An experiment (carried out o n an organic farm in the UK) compared the dry matter and N accumulatio n of legumes grown for periods of between 6 months and 2 years, compar ed with a non-leguminous rye-grass (Lolium spp.) control. The performa nce of a subsequent wheat (Triticum aestivum L.) crop was also measure d. Red clover (Trifolium pratense L.), white clover (Trifolium repens L.) and trefoil (Medicago lupulina L.) green manures and the rye-grass control were cut to maintain a height of no more than 30-40 cm, and t he cut material was left on the soil surface ('mulching'). Of the legu mes, white clover accumulated the most dry matter (12.2 t DM ha(-1) ye ar(-1)) and red clover the most N above ground (371 kg N ha(-1) year(- 1)) over a 1 year period of green manuring. Both these species accumul ated significantly more dry matter and N than trefoil and ryegrass. Yi elds of following wheat crops varied considerably. Spring wheat (grown after 6 and 18 months of green manuring) did not yield at a commercia l level due to poor establishment. Winter wheat generally yielded well following 1 year of green manuring with red (6.0 t ha(-1)) and white clover (5.2 t ha(-1)) and significantly more than following trefoil an d the nil-legume ryegrass control (3.3 t ha(-1) and 2.1 t ha(-1), resp ectively). The results suggest that red clover is the optimum species for use as green manure. This is relevant in the context of set-aside management. There was no indication that winter wheat yield was improv ed following a second year of green manuring with either red or white clover. Approximately one third of the total N accumulated by red clov er above-ground was lost by leaching (measured using porous ceramic cu p samplers) following cultivation of the 1-year green manure in Septem ber prior to establishing a winter wheat crop, Delaying cultivation un til the spring substantially reduced leaching due to uncultivated soil over winter. The environmental risks (and agronomic benefits) associa ted with farming systems relying on natural N fixation and soil microb ial activity should be more fully evaluated.