Soils > South African Soils

South African Soils

Soil properties can be in part described by the soil texture, composition type and regional occurrence. In this section key features will be highlighted as well as the effect on the capillary pressure which it exerts in the vadose zone. Soil texture is an approximation of the relative quantities of sand, silt and clay particles in any given soil. Soil structure is a measure of the arrangement of these soil particles and the spaces between them and can also be related to packing effects.

Soil texture refers to the relative proportions of sand, silt and clay in a given sample. The sand, silt and clay percentages are also useful because we can use these measurements to classify the soil into general categories, which can then be used to give approximate infiltration rates. In the Table below a general comparison of particle sizes are given.

Table: Ranges for particle sizes of clay, silt and sand

Material	Particle Size (mm)
Very Coarse Sand	2.00-1.00
Coarse Sand	1.00-0.50
Medium Sand	0.50-0.25
Fine Sand	0.25-0.10
Very Fine Sand	0.10-0.05
Silt	0.05-0.002
Clay	<0.002

A loamy soil contains these three types of soil particles in roughly equal proportions. A sandy loam is a mixture containing a larger amount of sand and a smaller amount of clay, while a clay loam contains a larger amount of clay and a smaller amount of sand (see Figure below for the classification of the different soil types).

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Classification of different soil types according to sand, silt and clay composition.

A second fundamental property of soils is the structure, it is described the aggregates in soil in terms of shape. The different shapes of aggregates in the soil define the pattern of pores and packing arrangements. This pattern defines the soil structure which greatly influences water movement, heat transfer, aeration and porosity in soils. Agricultural activities on farmlands (disturbed), tillage and liming, impact soils largely through the effect on the soil structure; especially in the surface horizons.

The effect of soil structure on the saturated hydraulic conductivity of soils is clearly illustrated in the Table below.

Table: Range of hydraulic conductivities in different soil types.

Soil Type	Saturated Hydraulic Conductivity, Ks (cm/s)
Gravel	3x10-2 – 3
Coarse Sand	9x10-5 – 6x10-1
Medium Sand	9x10-5 – 5x10-2
Fine Sand	2x10-5 – 2x10-2
Loamy Sand	4.1x10-3
Sandy Loam	1.2x10-3
Loam	2.9x10-4
Silt, Loess	1x10-7 – 2x10-3
Silt Loam	1.2x10-4
Till	1x10-10 – 2x10-4
Clay	1x10-9 – 4.7x10-7
Sandy Clay Loam	3.6x10-4
Silty Clay Loam	1.9x10-5
Clay Loam	7.2x10-5
Sandy Clay	3.3x10-5
Silty Clay	5.6x10-6
Unweathered marine clay	8x10-11 – 2x10-7

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