Water management for rice cultivation on acid sulphate soils in the Plain of Reeds, Vietnam

In the Plain of Reeds, Vietnam, severely acid sulphate soils are highly per meable which greatly limits water control in the years following reclamatio n. After irrigation, the water-table level returns to groundwater table lev el within 24-48 h. Using a semi-quantitative model explaining variability o f rice growth and yield in these conditions, water management practices are compared for fields at different elevations. In fields higher than 85 cm a bove mean average sea level (m.a.s.l.), with Typic Sulfaquepts, dry conditi ons and consequent acidification at the end of the plant cycle are a major constraint. Water management should, therefore, aim at maintaining wet cond itions in the topsoil as long as possible. However, keeping the soil perman ently submerged is neither feasible nor recommended. Dry conditions at the end of the plant cycle also can be prevented by sowing earlier while the la nd is still hooded. Sowing too early tin more than 30 cm of water) is not r ecommended since it reduces plant density, and thus the yield. Problems in fields at 'medium' elevation (75-85 cm above m.a.s.l.) with intermediate so il type are similar to those in 'high' fields but to a lesser extent, as wa ter recession is slower, allowing better water control. In 'low' fields (lo wer than 75 cm above m.a.s.l.) on Hydraquentic Sulfaquepts, waterlogging an d deep reduction are the main problems. Slight oxidation of the topsoil is favourable and greatly enhances plant growth and increases yield. Sowing ri ce seeds on wet soil after pumping water out of the field greatly reduces p lant mortality and gives highest yields (3.5 tma, as compared to 2-3 t/ha w hen sowing in water). Year by year, with repeated cycles of land preparation and cultivation, a p lough-pan is created and toxic ions are hushed. Water control and soil cond itions in fields at high and medium elevation gradually improve. In these f ields, it becomes possible to delay sowing, while maintaining wet condition s and a favourable redox potential until the end of the plant cycle, with t hree to four irrigations. Within 3 years, yield can reach 5 t/ha. On the co ntrary, cropping conditions and yield in low fields improve very slowly as soils have a relatively high organic matter content and can hardly be drain ed. (C) 2000 Elsevier Science B.V. All rights reserved.