THE IMPACT OF ROTATIONAL PRACTICE ON NITRATE LEACHING LOSSES IN ORGANIC FARMING SYSTEMS IN THE UNITED-KINGDOM

The characteristics of organic farming involve the application of husbandry practices which are consistent with limiting nitrate leaching from agricultural land. The high losses of nitrate associated with ploughing of grass-clover leys are balanced by lower losses during the subsequent years of the arable and grassland phases of the rotation. Nitrate leaching data are presented from a monitoring programme carried out on 10 field sites on three commercial organic farms in the UK during the period 1988-92. Fields in arable and grassland phases of the farm rotations were monitored. Nitrate leaching was estimated from the nitrate concentration in soil water (sampled using porous ceramic cup samplers installed at 0.9 m soil depth) integrated with cumulative net drainage. The rotations operated on the farms were 5 to 8 years duration, including grass/clover for grazing and conservation, winter and spring wheat, winter and spring oats and potatoes. Average annual losses from these rotations calculated using the data derived from the monitored fields ranged between 10-21 kg Nha(-1) yr(-1). It is concluded that organic farming systems in the UK may have a valuable role to play in limiting nitrate leaching and should be considered for use in water protection zones.