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Inland Salt-Affected Soils (Natraqualfs)

Salt-affected soils are soils that contain sufficient salt to impair the growth of crop plants. In fact, salt injury depends on species, variety, growth stage and environmental factors. It is difficult to define salt-affected soils precisely. For the sake of simplicity, however, salt-affected soils here refer to the soils that have enough salt in the root zone to give an electric conductivity in the saturation extract (Ece) of more than 4 mmho/cm at 25C. Apart from the salt-affected soils or saline soils along the coastal zone, referred to earlier as mangrove soils, salt-affected soils in this section refer to inland soils where the salt source is at the surface or as underground saline water. Salts that contaminate these soils are commonly introduced by capillary action, surface runoff or interflow accumulated in the soil profile. Most of the inland salt-affected soils of Thailand have a sandy loam or loamy sand, leached surface horizon overlying a very hard and impermeable Bt horizon (natric horizon). The subsoils are sandy clay loam or clay loam and are generally characterized by columnar or prismatic structure.

Due to the flat topography coupled with impervious layers, the soils show dominant signs of wetness. A gray color matrix with brownish or yellowish mottles is present throughout the soil profile. Salt and lime concretion, if present, is found in the subsoil. However, in the dry season, salt crusts are observable on the soil surface. In the profile, the dominant salt is sodium salt and its amount commonly increases with increasing depth. The sodium adsorption ratio (SAR) ranges from 20 to 50 ds/m. while the exchangeable sodium percentage (ESP) varies from 15 to 50 percent.

The management of these soils is very problematic. Furthermore, the measures strictly depend upon the degree of salinization and mode of salt-forming processes. As for general countermeasures, management of these soils involves leaching, proper drainage control, land leveling, surface mulching, organic amendments, deep plowing, and the use of salt-tolerant plant varieties. However, a number of management problems often occur. They are:
- The source of freshwater for leaching is normally scarce.
- Installation of a drainage system is always expensive, needing high inputs.
- The high evaporation rate during the dry season; more irrigation water is required to 
          suppress the evaporation rate.
- Drainage will cause substantial changes in the hydrological phenomena of the
          adjacent areas as well as the drained area. The change will affect the agricultural
          pattern of the overall area.

Therefore, the simplest practice being used in Thailand at the moment is the use of salt-tolerant crops and trees. In highly salt-affected areas, Eucalyptus, Sesbania, Acacia and Dixie grass have been introduced with great success.

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