NITRATE LEACHING FROM A SANDY SOIL - THE EFFECT OF PREVIOUS CROP AND POSTHARVEST SOIL-MANAGEMENT IN AN ARABLE ROTATION

Agronomic practices can be modified to decrease autumn soil nitrate an d nitrate leaching. This experiment aimed to measure the effectiveness of such practices when integrated into a farming system under UK cond itions. The experiment started in autumn 1988 on a sandy soil in Notti nghamshire, UK, and comprised a four-course rotation of potatoes-cerea l-sugarbeet-cereal. Three husbandry systems were superimposed, ranging from current commercial practice to most nitrate retentive. Plots wer e split further to receive either half or full recommended rates of ni trogen (N) fertilizer. Soil mineral N (Nmin) and nitrate leaching (usi ng porous ceramic cups) were measured on selected treatments; this pap er presents the findings after five winters. Autumn Nmin and N leached were strongly influenced by the previous crop, consistently following the order potatoes > cereal > sugarbeet. Pre-harvest management (chie fly N fertilizer input) affected Nmin, and post-harvest management als o modified N loss. Cover crops (winter rye or forage rape) after cerea ls removed 10-30 kg/ha N, depending on previous N management, time and method of establishment. They decreased leaching and were particularl y effective if they were able to establish fully before significant dr ainage occurred. Nmin following sugarbeet, which had received 125 kg/h a N, was less after November lifting than after October lifting (16 an d 28 kg/ha N, respectively, as a mean of autumns 1989-92). Potatoes le ft most Nmin (a mean of 60 kg/ha for autumns 1989-92, receiving 220 kg /ha fertilizer N), and their late harvest gave little scope for decrea sing leaching losses by establishing green cover before the start of w inter. After late harvested root crops (both beet and potatoes), it wa s often preferable to leave the land fallow over winter, rather than p loughing and drilling a winter cereal. We show that nitrate leaching c an be decreased by simple and inexpensive modifications to an existing crop rotation. Averaged over five winters, adopting such practices de creased the mean N concentration in drainage from 22.3 to 14.5 mg/l.