NITRATE ACCUMULATION IN GREENHOUSE VEGETABLE CROPS

Four cultivars each of lettuce (Lactuca sativa L.), radish (Raphanus sativa L.) and spinach (Spinacia oleracea L.) were grown at three NH4NO3 levels (0, 14.7 and 29.4 g N/m2) in a muck soil as a commercial greenhouse crop under winter conditions (low light intensity and short photoperiod). Analysis of the tissues for NO3-N showed that all three vegetable crops contained high concentrations of NO3-N when grown under these conditions: radish highest, followed by lettuce, then spinach. Total N and NO3-N were increased by N fertilizer only in radish roots, and yield was promoted by the additional N only in lettuce. Lettuce cultivars Domineer and Korrekt contained significantly less NO3 than Grand Rapids or Noran and were higher yielding than the standard cultivar Grand Rapids. The NO3 content of radishes was extremely high, especially in the root of the white cultivar Icicle (1.68% NO3-N). Yield of roots from Icicle was not significantly different from Champion, Early Scarlet Globe and Red Boy, cultivars which did not contain as much NO3-N. Smooth-leaf spinach cultivar Northland was higher yielding with significantly less NO3 than savoyed cultivars America, Winter Bloomsdale or Savoy. High NO3 tissue concentrations can be reduced in these crops by growing cultivars that accumulate less NO3.