Properties and Management of Smectitic Soils

Smectitic soils of the tropics are medium- to fine-textured alluvial soils containing moderate to large amounts (20% or more) of smectite, a shrinking and swelling clay mineral, in the clay fraction. Small to moderate amounts of other layer silicate minerals, such as illite, chlorite, vermiculite, and kaolinite, are also present in the clay fraction. Smectitic soils have moderate to high values of CEC (10-50 cmol/kg of soil), high base saturation, and high water-retention capacity. These soils are usually developed on alluvial materials rich in basic cations, especially Mg. Smectitic soils commonly occur on alluvial plains in river valleys and deltas as well as in inland depressions. In the wetter tropics, large areas of smectitic soils are found in tropical Asia, especially Vietnam, Thailand, and Myanmar (Burma). These young alluvial soils are rich in nutrient-bearing weatherable minerals, such as micas, feldspars, and hornblende. Smectitic soils on the alluvial plains and inland valleys have a shallow groundwater table, and some soils are flooded during the rainy season. Thus, they are best suited for rice cultivation. For example, in the flood plains along the Mekong and Chao Phraya rivers of the Indo- China peninsula, mineral-rich deposits from annual flooding are able to maintain relatively high rice yields with little or no additional nutrient inputs. Smectitic soils occurring in seasonally flooded coastal mangrove swamps are known as acid sulfate soils. These soils are used for cultivation of swamp rice and floating rice during the rainy season, depending upon the depth of flooding by fresh water. In drier regions, clayey smectitic soils (mainly Vertisols) often exhibit large cracks during the dry season and become very sticky and difficult to work with during the rainy season. In the drier tropics, large areas of clayey smectitic soils are found in central India, central Sudan, southern Ghana, and in the Lake Chad region of central Africa. Clayey smectitic soils are usually found in the inland depressions scattered throughout the drier regions of West, East and Central Africa. Because of their high chemical fertility, these soils are important soils for cropping and grazing in the drier tropics.