The term “marginal land” in a broad sense refers to land that returns barely enough to meet expenses in a specific use. By this definition, marginal land therefore groups together a wide range of land that consists of the following types of soil, with severe constraints for agronomic production.

  Acid sulfate soils occur in parts of the coastal lowlands. It has been recorded that in the Chao Phraya Delta of Thailand, most of the acid sulfate soils have a very dark clayey surface, with a thickness of 15-30 cm. Subsoil consist of light brownish gray clay with a weak prismatic structure or massive structure. Mottles of brownish and reddish color are common. In addition, jarosite mottles [KFe3(SO4)2(OH)6] with straw yellow color are common in the subsoil. The underlying material, the so-called ‘C horizon’ is generally dark gray, and if highly organic it is dark brown or dark olive brown and clayey.In general, the acid sulfate soil is poorly drained and subject to flooding during periods of heavy rainfall. Since it has a clayey texture, high water holding capacity, and low permeability, physical soil conditions are favorable for rice production. However, due to some adverse chemical conditions, these soils become less productive. The chemical conditions affecting crop growth include extreme acidity, aluminum toxicity, iron toxicity, hydrogen sulfide toxicity, low base status, and inadequate supply of N and P. Therefore, to use these lands for rice production the following amelioration measures are practiced in Thailand.

  - Leaching and drainage control in order to remove acidity and prevent oxidation
    of pyritic material at the same time.

  - Liming. Marl and lime dust at the rate of 13-25 t ha-1 have been used
    extensively in Thailand to increase the pH of the soil. Several research works

    have revealed that the combination of leaching and liming had the greatest effect
  - Use of N and P. Acid sulfate soils are well known for their severe deficiency of

    N and P. Application of rock phosphate and ammonium sulfate at the rate 1,250 g ha-1
    and 150 kg ha-1 respectively increased the grain yield of rice grown on extremely
    acid sulfate soils.

  - Use of resistant varieties.

  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.