Soil salinization from sea water intrusion is a big threat for sustainable rice production in the coastal Mekong Delta of Vietnam. An on-farm study quantified the effectiveness of flushing salinity from paddy soil and identified strategies for rice production under salinity intrusion, by applying oxygen isotopic technique combined with hydro-geological and bioeconomical methods. The study consisted of two on-farm trials. The first trial investigated salinity source and the effectiveness of soil desalinization using isotopic oxygen-18 technique. The second trial tested for approriate soil nutrient management practices for rice production under salinity. Three fertilizer application treatments were: (1) farmer’s fertilizer application practice, (2) applying 50% nitrogen (N) and 300% potassium of (1), and (3) applying 25% nitrogen (N) and 300% potassium of (1) plus 400 kg commercial organic fertilizer per hectare. Fifteen farm plots measuring 1000 m2 each were used for the experimentation.

Soil desalinization using rainwater and evaporation strongly influenced salinity in the root zone of rice. Rainwater was the important source while irrigation canal water was not reliable for soil desalinization. Only about 40% of saltwater in the root zone was flushed out in the rainy season. Potassium and organic fertilizer application to rice highly facilitated flushing sodium from the root zone and improving rice yields. Organic fertilizer application, however, was not  conomically feasible. Rice yields did not significantly change with nitrogen application reduction by 50%. The combined application of the isotopic technique, hydrogeological and bio-economical methods is useful to explain the
interactions among soil – water – crop for dealing with salinity intrusion in low land regions.