Lateritic soils are rich in iron and aluminum and formed in wet and hot tropical areas. Nearly all lateritic soils are rusty red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock. They cover about one-third of the earth’s continental land area with the majority of that in the land areas between the tropics of Cancer and Capricorn. It is a highly weathered material rich in secondary oxides of Iron, Aluminum, or both. It is void or nearly void of bases, and primary silicates, but may contain large amounts of quartz and kaolinite. They are formed in hot, wet tropical regions with an average annual rainfall of at least 1200 mm and a daily temperature in excess of 250C and typically occur in humid tropical climates within 300 N and 300 S of the equator. They usually develop beneath the surface in soil zones, unconsolidated sediments, or decomposed rocks where the interrelations of groundwater, soil/water table, and topography are favourable. A significant factor in its formation is an alternating or variable moisture cycle and it is formed in association with grasslands and forests on lowland soil surfaces in tropical and temperate regions. They have been found to be suitable construction materials and are therefore extensively used in construction.
There are soils that may have a similar nature as the laterites. To know whether a soil is truly laterite, it is imperative to investigate the degree of laterization of the soil. Rossiter (2004) compiled a soil classification according to the degree of lateralization by evaluating the silica-sesquioxide (S-S) ratio. Accordingly, soils having an S-S ratio greater than 2 (S-S ˃ 2) are considered non-lateritic soils. If the S-S ratio lies between 1.33 and 2 (1.33 ≤ S-S ≤ 2), the soil is lateritic soil whereas for true laterites, the ratio falls below 1.33 (S-S ˂1.33). This ratio can be evaluated by the expression given below.
S-S ratio = SiO2/ (Fe2O3 + Al2O3)
The parameters in the expression can be obtained from laboratory tests. The standard test used is the X-ray fluorescence (XRF) test.
Example
Table 1 and Figure 1 below show the constituents of soil assumed to be lateritic. By calculation, show whether the soil is lateritic soil or another soil.
Table 1: Constituents of a soil as determined by XRF test
From the table, SiO3 = 57.9596%, Fe2O3 = 12.7063%, and Al2O3 = 26.6295% show whether or not the soil is laterite using silica-sesquioxide expression.
S-S ratio = 57.9596/ (12.7063 + 26.6295) =1.47.
This value of 1.47 lies between 1.33 and 2. Therefore the soil is a lateritic soil.
