Easy Pedology - Learning more about soil texture

Learning more about soil texture

Soil texture or granulometry is concerned to the soil particle size constitution, that is, the clay, silt and sand fraction proportion. The soil clay fraction shows a large specific surface, showing colloidal nature with high cation retention and phosphorus adsorption. The clay fraction represents the soil solid phase most part and it is constituted by several negatively charged minerals which are responsible for the clay cation exchange capacity (CEC).

Not only the soil classification but also the soil management depends on the clay content along the soil profile. In the Order hierarchical level of soil classification, the textural gradient is one of the main evaluated characteristics; and the soil management (fertilizer, lime, gypsum and herbicide rates) changes according to the soil compaction degree, water availability and CEC.

Brazilian soils do not usually present high silt levels, with exception of most Inceptsols. Sandy soils present limited water availability, but it does not mean that clayey or very clayey soils have greater water availability. As for instance, the Acric Oxisols present high clay contents, but are very dried soils, because of the strong particle micro-aggregation as a consequence of the high clay flocculation degree.

When fine sand is present in much higher proportion than coarse sand, it might contribute to increase the profile water availability. According to the Atterberg granulometry scale, the clay fraction corresponds to soil particle sizes (diameter) lower than 0.002 mm; silt = 0.002 to 0.02 mm; fine sand = 0.02 to 0.2 mm and coarse sand = 0.2 to 2.0 mm particle sizes. Figure 1 shows the clay fraction range and the respective soil classification.

Figure 1. Soil clay content range and soil classification.

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