LETTUCE RESPONSE TO LEACHATE RECYCLING IN AN ARID ZONE GREENHOUSE

Lettuce response to salinity has been studied in open irrigation systems but not under solution recycling conditions. The objective of this study was to determine effects of threshold EC values for solution replacement on stalk weight and quality, solution chemistry and quantities of water and nitrogen removed from the system. Results showed that lettuce yield response to threshold EC in recycled solutions was similar to response to steady salinity in open irrigation systems reported in the literature. The salinity induced yield decline was stronger in the spring than in the autumn growing season. Accumulated NaCl in solution reduced Mg, Ca and K concentration in leaves in the spring experiment, but only Mg was significantly correlated with untrimmed and trimmed stalk yield. Recirculation reduced water and nutrients discharge and hence their input into the system. Increasing the threshold EC from 2 to 4-5 dS/m reduced water discharges by 60-to-100%, while the yield decline was ~10% in autumn and ~20% in spring.