RESPONSE OF FALL GREENHOUSE COS LETTUCE TO CLEAR MULCH AND NITROGEN-FERTILIZER

Application of clear plastic much with or without N fertilizer did not significantly increase (P > 0.05) yield of cos lettuce (Lactuca sativa L. cv. Paris Island), grown in Fall in a greenhouse in the Mediterranean mountains. Yield ranged from 31 to 38 kg/50 heads. Leaf NO3-N and total P levels were higher in mulched than unmulched plants, and in fertilized than in unfertilized plants, and always above the sufficiency level in all treatments. Soil levels of NO3-N were higher under mulched than unmulched plots, and under fertilized than unfertilized plots, which had more than 50 ppm NO3-N at harvest. This indicates ample supply of N and thus explains the lack of response to added N. It may be concluded that in mild climates and on soils with adequate N, lettuce will not respond to the use of clear mulch and N fertilizer.