Problem hydromorphic soils in north-east Thailand. 3. Saline-acid conditions, reclamation, improvement and management.

Abstract

Saline-acid conditions have developed in patches in the irrigated areas on the low terrace in north-east Thailand. There are also traditionally uncultivated, virtually barren, saline-acid strips adjoining higher terrace remnants, in spite of the excess of monsoon rainfall over evapotranspiration. Calculations show that the salts in the shallow groundwater of the low terrace may have originated from rainfall, but that salts in the main rivers are mainly derived from salt beds. The local surface salinity, mainly of NaCl, is caused by continual evapotranspiration during the dry season and locally impeded leaching. The latter is due to a combination of a shallow water-table, slow vertical permeability and in some cases the slight elevation above the normal level of monsoon flooding. The high salt concentrations in and on the soil surface bring originally exchangeable aluminium into solution, which lowers the pH. In extreme cases even some ferric iron is dissolved at the soil surface. Reclamation, improvement and management practices on these soils should include leaching, for example under two rice crops per year; judicious liming, to eliminate most of the exchangeable aluminium but not to exceed the small buffer capacity of these soils; and emphasis on paddy rice, both in the monsoon season, and, irrigated, in the dry season. If, however, dry-season dryland crops are to be grown, physical problems of different kinds may necessitate further land improvement and management practices. These include, principally, lowering and keeping down the water-table, for example by control of irrigation water losses from canals and ditches; ploughing or disking in chopped crop residues with added nitrogen; and locally, chiselling the upper part of a dense subsurface horizon. (Abstract retrieved from CAB Abstracts by CABI’s permission)