Growth and nitrate concentration of lettuce as affected by total nitrogen and chloride concentration, NH4/N03ratio and temperature of the recirculating nutrient solution
For five years experiments were performed in which lettuce (Lactuca sativa capitata L.) was grown in recirculating nutrient solutions. Both in winter and in summer, ammonium/ nitrate ratios and total nitrogen concentration were varied so as to lower the nitrate content of the lettuce heads. Fresh weight of the heads was mainly determined by global radiation during growth. In winter, when light was poor, the nitrate concentration of lettuce was considerably higher than in summer. Nitrate accumulation by lettuce was reduced when 20% of the total nitrogen in the solution, present as N03-N, was replaced by NH4-N. A further increase in the amount of NH4 in the nutrient solution a few weeks before harvest decreased the nitrate concentration of lettuce even more, while fresh weight of the heads was unaffected. Temporarily omitting all nitrogen resulted in the lowest nitrate concentrations, but also in a sharp decrease in fresh weight. When the N concentration of the solution was decreased in winter from 10 to 2.5 mmol I“1, growth and the nitrate concentration of lettuce were scarcely affected. In spring and summer, however, growth was severely reduced, while the nitrate concentration of the heads decreased. Addition of chloride decreased the nitrate concentration of lettuce only when NH4 was present in the nutrient solutions. With a view to minimizing energy consumption, the effect of a simultaneous increase in temperature of the nutrient solutions and a decrease in temperature of the glasshouse air on the growth and nitrate concentration of lettuce was investigated. Raising the temperature of the nutrient solution and lowering the air temperature resulted in better growth, but increasd the nitrate concentration of the lettuce, partially nullifying the effect of ammonium in the nutrient solution.