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Soil texture is the "feel" of the soil when a moist quantity is manipulated between thumb and forefinger. Some soils are sticky, others will not stick together at all, and others feel "doughy" or "spongey". Some soils can be manipulated like plasticine. These differences in properties gave rise, in agriculture, to soils being called clays, loams or sands. Clays stick to your boots, loams are easily moulded but non-sticky, sands are not cohesive at all and cannot be moulded when moist.
There are 19 grades of texture that can be simplified into six major groups: Sands, Sandy Loams, Loams, Clay Loams, Light Clays and Medium to Heavy Clays. These texture differences are the result of fineness or coarseness of particles in the soil. The particles are divided into three size classes:
Silt 0.002 - 0.02 mm
Sand 0.02 - 2 mm
Sand, silt and clay percentages are related together in a triangular graph. The graph shows groupings of soils and the name that is given to that grouping. So, silty soils always have more than 25 % of silt sized particles (organic material makes soils feel silky and therefore more silty). The loams all have less than 40 % clay sized particles. When moist soil is manipulated in the hand, sands, loams and clays feel very different (refer Quick Reference Guide – Assessing Soil Texture).
The effect of particle size
The relative size of particles is important. For example, the finest sand particles are 10 times the diameter of the largest clay particles. The surface area of a spherical particle 0.02 mm diameter is 100 times greater than a spherical particle of 0.002 mm diameter. Clays have an even greater surface area than spherical particles are they are made up of sheet-like structures stacked together. This difference in surface area contributes to the differences in adhesion and cohesion of the texture groups.
Figure 1. Texture triangle
|Medium and coarse sand||Easily felt|
|Fine sand||Felt and heard by manipulating the soil close to the ear|
|Silt||Silky or smooth feeling similar to that of talcum powder|
|Clay||Stocky, cohesive and plastic|
|Clay type||Clay mineralogy affects trafficability. Montmorillonite is very fine and encourages ribboning. Kaolinite is very coarse and will inhibit ribboning|
|Organic matter||Cohesion of sandy textures and greasiness of clays|
|Oxides||Cementation (Al & Fe) masks fine textures|
|Carbonates||Cohesion in sands and loams, but inhibits ribboning in clays|