Figure 22 – soil texture and structure

The soil structure is the way the soil particles are arranged. Because of cementing action of ions in the soil, individual particles in a soil tend to aggregate together in lumps. According to the shape of the lumps, soils can be described as having a platy, prismatic, crumby and Granular structure (figure 22(ii)). The presence of humus helps the formation of a crump structure. The soil structure has an important bearing on its east of cultivation. Soils with a crumb structure are best for seed germination. Forking, raking, ploughing and harrowing are few techniques to improve the soil structure.

Soil colloids - tiny particles with unusual chemical properties – may be organic (very finely divided humus) or mineral (minute thin flakes called clay mineral). Together, the two types make up a clay-humus complex. Clay minerals have a vast surface area in relation to their weight and are net negatively charged. This is invariably neturalised by the attraction to their surface of positively charged ions (cations) of calcium, magnesium, potassium and sodium (bases). They are only held loosely in an exchangeable position by the clay minerals and may be given up in the process of exchange to plants in forms of nutrients which require them for growth. These cations are generally replaced by hydrogen ions. Over a period of time, this process makes soil more acid, unless the bases are replenished in some way. It is possible naturally with decomposition of animals and plants or artificially in form of fertilizer.

Soil acidity is a property related to the proportion of exchangeable hydrogen in the soil in relation to other elements. A pH value of about 6.5 is normally regarded as the most favourable for the growth of cereal crops.

Colour varies considerably in soil and can tell us much about how a soil is formed and what it is made of. In recently formed soils, the colour will largely reflect that of the parent material, but in many other cases, the colour is different from the underlying rocks. Soils can range from white to black, usually depending on the amount of humus. In cool humid areas, most soils contain relatively high humus content and are generally black or dark brown, wheras in desert or semi-desert areas, little humus is present and soils are light brown or grey. Reddish colours in soills are associated with the presence of ferric compounds and usually soil is well drained. In humid climates, grayish colours relect poor drainage conditions.