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Marginal land and problem soil of Thailand


Mangrove soils
               Mangrove soils are basically alluvial in nature and mainly consist of fine-grained sediment in which silt and clay predominate. The soils are often clayey or semifluid with abundant organic soil material in parts. Since the soils are almost permanently under submerged saline conditions, they are strongly reduced, have a low oxygen content, but high salt content. Hydrogen sulfide can often be smelled in areas where sediments are enriched with pyrites (FeS2). If the soils are high in pyrite, under natural conditions, the soil pH is higher than 7.0, but oxidation of the pyritic compounds occurs (if pyrites are present in appreciable amounts), as in the following equation :
               Therefore, when mangrove soils are subject to strong acidification upon drying, they are generally called potential acid sulfate soils or saline acid sulfate soils. Regarding agricultural potential, the mangrove soils can be rated as ‘low’ in potential for crop production,
due to :
high salinity, derived directly from seawater intrusion or frequent flooding with seawater;
adverse soil conditions both prior and after drainage, i.e. poor physical properties unripe stage, and uneven land surface, and extreme acidification upon drying;
high monetary input for major engineering measures needed, namely dike building, empoldering, proper drainage and flood control system; and
fragile ecosystem of the mangrove complex that is susceptible to rapid deterioration under improper use and management.
Reclamation of these soils faces three major problems :
1
 flood protection
2
 leaching of salt
3
 proper water regulation to maintain water at a proper level to prevent oxidation (which produces extreme acidic conditions).The cost of reclamation is very expensive. Therefore, at present, it is imperative that these areas should remain as a nation’s resource. Although Thailand is likely to face a serious shortage of agricultural land, it is inadvisable to open new mangrove areas for crop production as long as the present utilization of this land remains inefficient. However, it is acceptable to utilize mangrove for the production of charcoal, timber and for fisheries. Nevertheless, proper management to maintain the highest possible production of the mangroves is required. Furthermore, management must ensure that the environment is not damaged.
          For simpler and more economical solutions, especially for small farmers, the use of resistant varieties is rather helpful. In Thailand, some improved varieties can adapt themselves, to some extent, to tolerate acidic conditions. The Kow Chan, Look Daeng, and RD 7 varieties are grown by small farmers in various parts of the Central Plain and the peninsula. It is interesting to note that some areas of these soils have been ridged for growing tree crops such as mango, citrus, banana and annual crops such as watermelon. However, they need large amounts of lime, N and P. Raised beds (ridge and ditch system) at present, are appropriate for reclamation of typical acid sulfate soils on coastal lowlands or deltaic plains. However, rather high monetary inputs are needed for construction of raised beds and an adequate water supply from the irrigation scheme. Therefore, high economic value crops need to be selected. For instance, tangerine (mandarin), and pomelo are claimed to be very profitable.