Nitrogen Leaching in Field-Grown Lettuce

Previous studies in southern Sweden have shown that during a season of intensive production of field-grown lettuce there may be a progressive accumulation in the soil profile of large amounts of residual nitrogen, which subsequently leach into the surrounding environment. In spring 2003 an observation field was established on the southern aspect of the Bjarehalvon peninsula. The measuring station was connected to the outflow pipe of a well-delineated and documented drainage system comprising approx. 0.8 ha. The water flow from the drainage system was measured using a triangular weir (90 degrees). Water was sampled automatically by extracting flow-proportional (biweekly) bulk samples (one sub-sample per 0.1 mm runoff) with a peristaltic pump connected to the datalogger. The water samples were analysed for nitrate-N and total-N. Annual leaching was calculated for the period 1 June of one growing year to 31 May in the following year. The first culture was planted in March and covered with horticultural fleece. Culture 2 was sown at the end of June or in the first half of July. On average for the three years, the total amount of N supplied was approx. 350 kg/ha bed area and year, or around 260 kg/ha field area and year. The reported annual yields of commercial produce were between 30 and 35 tonnes fresh weight per ha field area. On the same basis, the N-harvest was around 90 kg/ha field area and year, giving an N use efficiency of approx. 34%. Average for annual precipitation and irrigation was 676 mm respectively 62 mm, and annual mean runoff from the drainage system was 228 mm per season. The weighted mean concentrations of nitrate-N and total-N in drainage water were 38 and 42 mg N/L resp., giving an annual N-leaching of 87 and 92 kg N/ha resp.