Vertisols of the semi-arid tropics [a review]
1989
Blokhuis, W.A.
Vertisols, as classified at the highest category in Soil Taxonomy (USDA 1975) and in the French Classification System (CPCS 1967), are a remarkably homogeneous group of soils with a specific morphology, and with mineralogical, physical, and chemical properties that vary within a narrow range. The central concept of the order is a Typic Ustert belonging to the fine clayey, montmorillonitic, isohyperthermic family. The typical morphology of this soil is a structure profile that shows surface mulch, cracks, wedge-shaped peds with shiny faces in a multiple compound structure, and slickensides. Next to this central concept, arid subhumid, and sodic variants can be conceptually defined. The behavior of soil water has the greatest effect on agricultural use of Vertisols. The physical soil factors are difficult to measure making estimates of the available water-holding capacity less reliable. These factors are also affected by the nature and concentration of the soil solution and by the composition of the exchange complex. The relative saturation with Na, compared with that of Ca and Mg, more strongly influences shear strength, swelling pressure, plasticity index, and hydraulic conductivity. Sodic Vertisols cover only a relatively small area in the semi-arid tropics. Identification of sodic Vertisols requires reliable data on ESP (exchangeable sodium percentage) and CEC (cation-exchange capacity). The standard method for determining CEC (using ammonium acetate, NH4OAc, at pH 7) may give faulty results when free carbonates are present; the use of a more recent method developed by Begheijn using LiEDTA and LiBaEDTA is suggested
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