While the International System is commonly used to designate textures, the US system is necessary for classifying soils according to Soil Taxonomy which is being adapted in the soil-survey organizations all over the country.

Textural classes The varying proportions of particles of different size groups in a soil constitute what is termed as a textural class.

The principal textural classes are: Clay, sandy clay, silty clay, clay loam, sandy clay loam, silty clay loam, loam, sandy loam, silt loam, sand, loamy sand and silt.

The physical properties and the chemical composition of the small and large particles differ greatly. the coarse fraction, gravel and sand which are mainly composed of rock fragments or primary minerals act as individual particles. They have low specific surface and are relatively non-reactive. They do not hold large amounts of water or nutrients. Owing to large voids between them, they transmit air and water easily.

The silt particles are intermediate between sands and clays. Mineralogically, the particles of silt are similiar to those of sand, as they are largely composed of primary minerals. They are more reactive than sands because of the higher specific surface.

The clay fraction controls of the important properties of the soils. They are chiefly composed of secondary minerals--crystalline alumino silicates. They have high specific surface and are most reactive. They have high capacity to retain water and nutrients.

The textural classes differ not only in the particle size analysis, but also in their bearing on some of the important factors affecting plant growth, such as
(i)the moveability and availability of water,
(ii)aeration,
(iii)workability, and
(iv)the content of plant nutrients.

Sandy soils are very permeable and well drained but are less water retentive and hence need more frequent irrigation for successful crop growth than fine textured soils. The clayey soils can hold more moisture, but they have high wilting percentage. The rate of water intake of these soils is low. They are subject to water-logging resulting in poor aeration and workability. The moderately fine-textured soils e.g. loams, caly loams, or silt loams are by far the excellent soils for plant growth, since they have the advantages of both sands and clays.

Colour.Colour gives a ready clue to soil conditions and some important properties. It is due either to mineral or organic matter and mostly to both. Red, yellow or brown colors are usually related to the different degrees of oxidation, hydration and diffussion of iron oxides in the soil. dark colors of a soil are associated with one or a combination of several factors, including impeded drainage conditions, content and state of decomposition of organic matter, the presence of titaniferous magnetite etc.

Uniformity in nomenclature of colors is possible by comparing the soils with charts containing standard colours. One such chart is the Munsell Colour Chart. It consists of coloured chips arranged according to hue, value and chroma, the three simple variables that combine to give the colour. The hue refers to the dominant spectral colour, the value to relative lightness of colour and chroma to relative purity.

Density. Soils having larger particles are usually heavier in weight per unit volume than those having smaller particles. True density of a soil is based on the individual densities of soil constituents and according to their proportionate contribution. The bulk density or apparent density is the weight per unit volume of dry soil as a whole i.e. particle and pore space and hence it is lower than the true density. The relationship between the true density (T) and the apparent density (A) and the pore space (P) is as follows:

In most mineral soils the true density varies within narrow limits of about 2.5 to 2.7 and the apparent density between 1.4 and 1.8.

Pore space.The pore space of soil is the portion occupied by air and water and it is determined largely by structural conditions. Sands have low pore space of about 30%, whereas clays may have as much as 50-60%. Although clays possess greater total porosity than the sands the pore spaces in the latter being individually larger are more conducive to good drainage and aeration.

Plasticity and cohesion. Plasticity is the property that enables a moist soil to change shape on the application of force and retain this shape even when the force is withdrawn. On this basis, sandy soils may be considered to be non-plastic and clayey soils to be plastic. Cohesion is the tendency of the particles to stick to ine another. Plastic soils are cohesive. Plasticity and cohesion reflect the soil consistency and workability of the soils.

Soil temperature and heat. Soil temperature is one of the important factors that control the microbiological activity and all the processes involved in the growth of plants. Heat is necessary for seed germination, root growth and other biological activities.

The temperature needed for germinaton and root growth varies with crops and varieties. Crops, e.g. wheat , barley, and peas grown in India during winter germinate at relatively low temperatures as compared with maize, and those at which groundnut or cotton germinate. Microbiological activities are retarded by low soil temperature. As a result, the nitrification processes in the soil are slowed down and plant nutrition and growth adversely affected.

Soil temperature is to be considered so important that in the soil taxonomy there is a provision to use it as a differentiating criterion at the family level of categorisation.

Soil air.It is well known that restricted soil aeration adversely affect root development, processes of respiration and other essintial biological processess. It is therfore, important to know the content of the soil air and its composition. It has already been mentioned that, depending on its texture, the soil may have pore space of 30-60 percent.The pore space not fill by water is occupied by air