Roughly two-fifths of Thailand is covered by mountains and hills, the steepness of which generally precludes cultivation. Nevertheless, perhaps as much as a tenth of this area might also be converted to agricultural purposes once detailed information was obtained through surveys. Estimates in the 1970s of overall land-use suitability classified roughly 58 percent of mountainous and hilly regions as cultivable (compared with 24 percent 2 decades earlier), of which about 19 percent was usable for paddy, 28 percent for upland crops, and 11 percent for both paddy and upland agriculture. Actual holdings of agricultural land--not all of which was under cultivation at any one time--were estimated in the mid-1970s to occupy about 43 percent of the total land area.
Soils throughout most of the country are of low fertility, largely as a result of leaching by heavy rainfall. Differences between the various soil types are the result of differences in parent rock material, variations in the amount of rainfall, length of wet and dry seasons, type of vegetable cover, and other natural factors. In general, stony and shallow soils characterize the hill and mountain terrain of the North. Large portions of this mountainous area were traditionally used by hill peoples for thailand/th_glos.asp#shifting"> shifting cultivation (see Glossary). The Lua (also called Lawa) and Karen cultivated for short periods, then permitted the land to lie fallow for long periods, which allowed forest regrowth and restoration of soil fertility (see The Non-Tai Minorities , ch. 2). As a result of population pressures, however, other groups sometimes failed to follow this practice. The principle crop of many hill peoples was upland rice; maize was an important secondary crop. The Hmong, Lisu, and certain other hill peoples cultivated the opium poppy as a cash crop , but this activity had important implications for internal stability as well as major international repercussions (see Criminal Activity and the Narcotics Trade , ch. 5). Thai authorities, with substantial international assistance, increased efforts in the 1980s to redirect these people to other cash crops, including tobacco and coffee.
Many inhabitants of the lowlands in the North also practiced shifting cultivation in hill areas lying not far above the valleys. The valleys usually had better soils, some of fairly high or moderate fertility, which were used mainly to grow irrigated rice. In places where population pressures had developed, the higher areas were often turned to shifting cultivation to supplement lowland production. The principal crop was usually upland rice, although other crops were also grown.
Shallow sandy loams cover a large part of the Khorat Plateau. Their generally low fertility partly explains the lower economic level of the region. Soils along the main rivers are more fertile, and alluvial loams of high fertility are found along the Mekong River. Lowland soils covering about a fifth of the Northeast (some 3.5 million hectares) had been converted to rice paddy.
The central plain rice-growing area and the delta of the Mae Nam (river) Chao Phraya has clayey soils of high to moderate fertility. Low-lying and flat, much of the area is flooded during the rainy season. Higher areas on the edges of the plain are generally well-drained soils of high to moderate fertility that are suitable for intensive cultivation. These lands are used extensively for maize and sugarcane. Among other highly useful soils are the well-drained clayey and loamy soils in parts of the peninsula where rubber is grown.