STUDY ON NITROGEN AND PHOSPHORUS REDUCTION AND CONTROL IN SHELTER GROUND - CULTIVATED LETTUCE IN THE DRAINAGE AREA OF DIANCHI LAKE

N, P and K were applied in different combinations to lettuce grown in sheltered ground in Dayu Township of the drainage area of Dianchi Lake. The yield of lettuce was increased by 29.6% when the rates of N and P were reduced to 48.7% of the conventional rates adopted by the local farmers. NPK,applied in proper combinations,helped to increase the compactness of lettuce and the size of its head and to decrease the nitrate content of its edible parts,thus improving the quality of the vegetable. Reducing N and P rates also decreased N and P accumulation and nitrate content in the soil and decreased the risk of N and P running off.