In the Plain of Reeds (Mekong delta, Viet Nam), water and soil characteristics show a high temporal variability which can be explained by changes in relative 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 ̄ood water recedes and the drainage/ ̄ushing of the acid sulphate soils begins. 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 favourable for an extremely short period (2±3 months), which starts at the end of the annual ̄ood. Optimal time window for cultivation, and optimal cropping techniques are related to the microelevation of the ®elds and to their permeability, which is very high in the ®rst years following reclamation. In newly reclaimed paddy ®elds, water control is very limited and, even with frequent irrigation, maintaining favourable conditions is not possible before a less permeable plough-pan has been created by cultivation. To reclaim 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 ̄ood water recedes. Determination of the sowing date is a key issue. Flood characteristics, in particular the speed of recession and water turbidity, the elevation and the age of the ®eld as they determine permeability and water control should be taken into account. # 2000 Elsevier Science B.V. All rights reserved.
10 Figures and Tables
Fig. 1. Map of the study area in Tan Lap village.
Fig. 2. Water level at low and high tides in canal `̀ 2000'', 1992±1996.
Fig. 3. Water level, pH and EC in canal `̀ 2000'', 1994±1995.
Fig. 4. Water level and changes in chemical characteristics in canal `̀ 2000'', 1995±1996: (a) soluble aluminium; (b) total iron.
Fig. 5. Water table level and changes in soil chemical characteristics, 1995±1996. EC in mS cmÿ1, extractable aluminium in meq per 100 g and ferrous iron in ppm 1000 ppm. Bulking of 10 samples before analysis.
Fig. 6. (a) Changes in ferrous iron; (b) extractable aluminium and (c) soluble sulphate, in topsoil as a function of time and microelevation. Moving averages of period 3. First cultivation after reclamation.
Fig. 7. Time series of water table level in relation to microelevation (adapted from Bil, 1994).
Fig. 8. Changes in redox potential in relation to water table level and microelevation, 1994±1995.
Fig. 9. Evolution with cultivation of vertical hydraulic conductivity at depth 10±30 cm for three main soil types. Average of three replications.
Fig. 10. Major constraints for cultivation and identi®cation of optimal cropping period for three major topographic positions, 1994±1995.
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