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 permeable which greatly limits water control in the years following reclamation. After irrigation, the water-table level returns to groundwater table level within 24-48 h. Using a semi-quantitative model explaining variability of rice growth and yield in these conditions, water management practices are compared for fields at different elevations. In fields higher than 85 cm above mean average sea level (m.a.s.l.), with Typic Sulfaquepts, dry conditions and consequent acidification at the end of the plant cycle are a major constraint. Water management should, therefore, aim at maintaining wet conditions in the topsoil as long as possible. However, keeping the soil permanently submerged is neither feasible nor recommended. Dry conditions at the end of the plant cycle also can be prevented by sowing earlier while the land is still flooded. Sowing too early (in more than 30 cm of water) is not recommended 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 soil type are similar to those in 'high' fields but to a lesser extent, as water recession is slower, allowing better water control. In 'low' fields (lower than 75 cm above m.a.s.l.) on Hydraquentic Sulfaquepts, waterlogging and deep reduction are the main problems. Slight oxidation of the topsoil is favourable and greatly enhances plant growth and increases yield. Sowing rice seeds on wet soil after pumping water out of the field greatly reduces plant mortality and gives highest yields (3.5 t/ha, as compared to 2-3 t/ha when sowing in water). Year by year, with repeated cycles of land preparation and cultivation, a plough-pan is created and toxic ions are flushed. Water control and soil conditions in fields at high and medium elevation gradually improve. In these fields, it becomes possible to delay sowing, while maintaining wet conditions and a favourable redox potential until the end of the plant cycle, with three to four irrigations. Within 3 years, yield can reach 5 t/ha. On the contrary, cropping conditions and yield in low fields improve very slowly as soils have a relatively high organic matter content and can hardly be drained. (Résumé d'auteur)