NUTRIENT ACCUMULATION AND TIPBURN IN NFT-GROWN LETTUCE AT SEVERAL POTASSIUM AND PH LEVELS

'Buttercrunch', 'Grand Rapids', and 'Summer Bibb' lettuce (Lactuca sativa L.) seedlings were grown with the nutrient film technique (NFT). The influence of two K concentrations (150 and 225 mg.liter-1) and four solution pH levels (5.0, 5.5, 6.0, and 6.5) on lettuce tipburn was investigated in four experiments. Additionally, the influence of pH on foliar nutrient concentration was examined. Even though tipburn was observed in 'Buttercrunch' and 'Summer Bibb' lettuce, neither K nor pH level consistently affected tipburn incidence. No tipburn was observed in 'Grand Rapids'. Solution pH generally did not affect concentration of total N and NO3-N in lettuce tissue. Increasing the pH increased K concentration and resulted in increased proportions of K compared to Mg or Ca. Although the influence of solution pH on P, Ca, and Mg concentration was significant, nutrient accumulation differences were not reflected in lettuce fresh-weight differences. The influence of K solution concentration and pH on lettuce yield was not significant. Tipburn incidence in NFT-produced lettuce appears to be primarily affected by environmental conditions maintained during greenhouse growth.