Texture is the most important single feature of soil because it has profound effects on all physical and chemical properties. It is the permanent condition referring to the coarseness or fineness of the soil particles. Soils can be classified according to texture. This results from the inherent and marked property differences between sand and clay.


Sand grains remain in the soil because they are composed of inert minerals resistant to decomposition or change. Sand grains do not absorb ions and contribute nothing to chemical soil fertility.


Clay particles are derived from the chemical alteration of rock minerals such as orthoclase. Clay particles in soil:

In a given volume of soil the minute clay particles have a much greater surface area than the same volume of sand and therefore, retains more moisture. There are many kinds of clay, their composition depending on the amount of desilicification which has occurred however, clay particles are usually a mixture of magnesium, potassium, hydrogen, calcium, sodium, and other elements. Clays like all colloids have the power of retaining water by imbibition and the amount of water imbibed varies with the type of clay. Clays in dry areas generally absorb more water than clay in wet areas.


When a soil is saturated with water and allowed to drain for 1-2 days it is then holding the maximum amount of water under practical conditions. Plants can extract this water easily and grow rapidly but as water is used and the soil dries out, the plant is less able to extract the water remaining in the smaller pores.


The amount of water in wet ground is said to be the "available moisture". Heavy clays may have a very high amount of "available water" but it is less easily extracted compared with clay loam soils. On the other hand sandy soils allow water to enter rapidly and move readily through the soil.


The aeration of soil is essential in providing oxygen for plant roots. Although clay soils have a higher proportion of pore spaces than sands most are filled with water so that generally, aeration is poorer in clays.


Silt is intermediate between sand and clay and therefore, is intermediate for most of the above properties. Some silt soils are very fertile because plant nutrients are being released from the weathering of silt particles. Other silts fill the spaces between the sand grains making the soil very dense. There is a difference between "texture" and "structure" so that soils with the same texture may have very different structures.


Texture is assessed by rubbing the soil between the fingers. This is the standard test in the field:


Coarse sand may contain not only quartz but also fragments of mica and felspar. On the other hand, fine sand will contain almost only quartz. Sand can be classified into:


A loam contains roughly equal proportions of sand, silt, and clay. Loams can be classified into:


In clay soil the particles are very small and can be described as dust. As soils age their bedrock characteristics disappear. Soils from shale or sandstone parent rocks may improve with age. They can be classified into:


DIAGRAM 1 shows the "texture triangle" which represents the various combinations of clay, sand and silt.


A soil sample will contain obvious organic matter and mineral particles. The mineral particles can be classified as follows:

The international agreement on soil particles recognises the following sizes:

> 2mm diameter = stones
2.0 - 0.2 mm = coarse sand
0.2 - 0.02 mm = fine sand
0.02 - 0.002 mm = silt
<0.002 mm = clay.


Structure is a temporary condition referring to the arrangement of the soil particles. Farmers often describe soil by depth, structure, texture, colour, and mode of origin. For example, a farmer may say that his soil is "deep, friable, heavy red basalt". The valuer should understand such everyday terms and the "jargon" of the farmer as the valuer is concerned with the opinion of the market place when determining market value. Structure is the arrangement of soil particles into aggregates or PEDS. Structure can be described as:

The most desirable structure is a crumb structure which is friable enough to cultivate early but cohesive enough to remain stable when wet. Crumb structure is promoted by the presence of calcium and magnesium on the clay particles which causes them to flocculate or stick together and by organic matter which produces glues to bind sand and clay together. Soil structure can be destroyed by mechanical means such as excessive cultivation, compaction, heavy rainfall or irrigation. Soils with good structure have a high proportion of large pores resulting in high infiltration rates and permeability, good drainage, and aeration. Well structured soils are easy to cultivate and relatively resistant to erosion. Structure does not affect water availability as this depends solely on texture.


In wet climates excess water percolates through the soil so that over a long period of time, leaching occurs of all soluble material from the soil.

In arid regions soluble materials are drawn upwards with the capillary rise of moisture and tends to concentrate at the surface. In semi arid regions salts are washed down in the wet season and drawn up in the dry season. They tend to concentrate as nodules or concretions at certain depths in the soil.

Nodules of lime, gypsum or ironstone are most common. Sometimes they join together to form hard pans which limit root growth, water penetration and drainage.