A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 1999

An experiment was conducted for fifteen weeks at the Asian Institute of Technology in Thailand, to determine the depth distribution of salts over soil profile under low salinity irrigation regime and the impact of the produced salinity on rice yield. Supanburi-90 rice seedlings were planted in twelve experimental plots of 13m2 each (6.5 x 2m), with 5 plants in each hill. Rice plots were irrigated with water of 2ppt, 5ppt, 1 Oppt and Oppt as treatments. Water levels were maintained by refilling the plots twice a week. Salinity, conductivity, pH and cations of irrigation water and soil were monitored biweekly. Results show that 100% mortality of rice plants at 1 Oppt salinity during the fourth week of experiment. The lowest value for accumulated exchangeable Na+ in soil was in 30-60cm depth and the highest value in the top 5cm layer. A 2-week delay in flowering was observed in rice plants cultivated with 5ppt salinity compared with plants of control and 2ppt. There was found a significant (p<0.05) effect of all the treatments over the yields except for the ones treated with 2ppt and 5ppt salinity levels. The present experiment demonstrated that the major loss of exchangeable Na+ in irrigation water was in the top 30cm soil profile and the top 1 Ocm of the soil accumulated 50% of total cations. This experiment demonstrated that the rice yield had no significant difference using brackish water of 2ppt and 5ppt.