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 husb andry practices which are consistent with limiting nitrate leaching fr om agricultural land. The high losses of nitrate associated with ploug hing of grass-clover leys are balanced by lower losses during the subs equent years of the arable and grassland phases of the rotation. Nitra te 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 nitr ate concentration in soil water (sampled using porous ceramic cup samp lers installed at 0.9 m soil depth) integrated with cumulative net dra inage. The rotations operated on the farms were 5 to 8 years duration, including grass/clover for grazing and conservation, winter and sprin g wheat, winter and spring oats and potatoes. Average annual losses fr om these rotations calculated using the data derived from the monitore d 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 pr otection zones.