Soil and water indicators for optimal practices when reclaiming acid sulphate soils in the Plain of Reeds, Viet Nam

In the Plain of Reeds (Mekong delta, Viet Nam), water and soil characterist ics show a high temporal variability which can be explained by changes in r elative soil/water levels. Acid, aluminium and iron concentrations in canal water increase (pH drops from 6 to less than 3.5) when the annual river fl ood water recedes and the drainage/flushing: of the acid sulphate soils beg ins. The high intra-annual variability of soil chemical characteristics is explained by sensitivity to changes in redox potential. Due to the high concentrations of toxic ions, cropping conditions are favou rable for an extremely short period (2-3 months), which starts at the end o f the annual flood. Optimal time window for cultivation, and optimal croppi ng techniques are related to the microelevation of the fields and to their permeability, which is very high in the first years following reclamation, In newly reclaimed paddy fields, water control is very limited and, even wi th frequent irrigation, maintaining favourable conditions is not possible b efore a less permeable plough-pan has been created by cultivation, To recla im these soils, the only solution is then to start cultivation as early as possible. Farmers in the Plain of Reeds developed a technique consisting of sowing pregerminated rice seeds in deep water (20-40 cm) when flood water recedes. Determination of the sowing date is a key issue. Flood characteris tics, in particular the speed of recession and water turbidity, the elevati on and the age of the field as they determine permeability and water contro l should be taken into account. (C) 2000 Elsevier Science B.V. All rights r eserved.