Soils widely vary in their characteristics and properties. In order to establish the interrelationship between their characteristics they require to be classified. Understanding the properties of the soils is important in respect of the optimum use they can be put to and for their best management requrements. Classification helps to reduce the study of the number of individuals to a few well defined units. It helps to develop a legend for mapping soils during the surveys. It helps to group together such soils as have comparable characteristics so that the knowledge regarding them is presented in a systematic manner.
To classify soils and group them together in a meaningful manner different systems of soil classification have been used from time to time. these systems have varied over a period of time having been drawn up to meet the rquirements and the immediate purposes of their use. As a knowledge of the soils which helps one to understand their genesis has developed the systems of classification have also developed.
The modern system of classification "Soil Taxonomy" developed by the USDA has been recommended for adotion all over the world and in this country as a result of the decision taken at the All-India Workshop held in 1969. This is a multi category system which has six categories: order, sub-order, great group, sub-group, family and series. The system id heirarchical in that an order is divided into sub-orders and sub-orders into great groups, great groups into sub groups, etc upto the series level. Thus the number in the higher categories is fixed whereas in the lower categories it varies. The soil series is the basic unit of classification and in all our surveys the soil series are first identified and described.
Whereas phases of soil series are used as mapping units in detailed soil surveys the associations of soil series are used as mapping units in reconnaissance soil surveys. The progressive abstraction and compilation of reconnaissance maps help one to construct small scale maps at the state and country levels with the associatons od higher categories as the mapping units. Thus, for example, great groups, or the associaton of great groups are the units delineated at the state or country level.
Brief descriptions of the major soil groups along with their modern nomenclature are as follows:
Alluvial soils. These include the deltaic alluvium, calcareous alluvial soils, coastal alluvium, and coastal sands. This is by far the largest and most important soil group of India contributing the largest share to the agricultural wealth.
In this immense tract though a great deal of variation exists, the main features of the soil are derived from the deposition laid by the numerous tributaries of the Indus, the Ganges, and the Brahmaputra systems. These streams , draining the Himalayas, bring with them the products of weathering of rocks constituting the mountains in various degrees of fineness and deposit them on the plains.
Geologically, the alluvium is divided into khadar and bhangar. The soils differ from the drift sands to loams and from fine silts ti stiff clays. Impervious clays obstruct the drainage and promote the accumulation of injurious salts of sodium and magnesium and make the soils sterile.
The formation of hard -pans at certain levels in the soil profile due to binding of soil grains by silica or calcerius matter forming an impervious layer is often observed in these alluvial soils. Layers of kankars in the Indo-gangetic alluvium of Uttar Pradesh and West Bengal and alsi occassionally layers composed of impure iron oxides are instances of the formation of hard pans.
The most important charateristic of the soil of Assam is its acidity. Generally, those on the old alluvium and hills are more acidic than the new alluvial soils along the river banks which are neutral or alkaline.
In West Bengal, the portions Murshidabad, Bankura the whole of Burdwan and thw western half of Midnapore comprising the rarh region, are composed mainly of the old alluvium. There is hardly any regularity in the deposition of river-borne materials. Deposits laid down earlier, subjected to climate and other influences have led to soils varying from one another in colour, texture, chemical, physical and other properties.
The alluvial soils of Bihar may be divided into 2 main divisions (a)the alluvium north of the ganges; and (b)the alluvium north of the ganges.
(a)The northern alluvium comprises the area between the Himalayas in the north and the ganges in the south. The soil is alluvial with a calcerous belt in the form of a triangle in the west and inundated areas in the middle; these areas remain flooded for different periods in the year. The soils are sandy loams to clayeye loams and neutral to alkaline. Their CaO ranges from 0.5 to 20 %. They are rich in total and available potash but deficint in phosphorus.
(b)The southern alluvium comprises the area between the Ganges in the north and the hilly region in the south. The soils vary in color and texture from light greyish loams to heavy black clays. The area has been divided into (i)highland soils; (ii)soils liable to inundation; (iii)saline soils, and (iv)diara land soils.
The soils of Uttar Pradesh are diveded into four classes: (a) the alluvium of the west and north-west, lighter in texture, (b) the alluvium in the centre , intermediate betrween light and heavy texture, and (c)the alluvium in the north-east developed on a calcerous parent material.
The soils contain varying amounts of CaCO3 and soluble salts and are neutral to alkaline. The lime content increases at lower depths. They are generally poor in P2O5, nitrogen and organic matter.
On the coast of Orissa, there are stretches of sand and sand-hills alternatin with deltaic swamps. Behind this is an area of cultivated alluvium. Soils are both sandy and fine; with enough potash but not enough P2O5.
The alluvial soils of Tamil Nadu are found in the delataic regions and along the coasts. The prifile reveals alternate layers of sand and silt. The composition of the strata varies with the nature of the silt brough in by the rivers. The stratified deposition is confined to ares very near the river courses. But away fromthe rivers the soils are heavy throughout, the texture ranging from clay loam to heavy clay through silty clay. Sandy layers occur at very low depths.
In the Gujurat state, the alluvial soils are confined to the northern Gujurat tract, Ahmedabad and Kaira districts and are locally known as goradu. The gorat soil of Baroda corressponds to the older alluvium consisting of brown clay with kankar. Those from the recent depositions are known as bhata.
The light-sandy, red and yellow soils found in the Mahanadi basin (Madhya Pradesh) including the Balaghat and three districts of Durg, Raipur and Bilaspur are of alluvial origin.
The soils of the Punjab and Haryana plains belong to the same class as the Idogangetic plain. They are loams or sandy loams consisting of a soil crust of varying depth, with soluble salts in amounts. The lower layer contains kankar nodules. Due the presence of sodium, the soils are alkaline, with adequate phosphorus and potash, deficient in organic matter and nitrogen.
In kerela, there are 2 types of alluvial soils viz. the coastal alluvium and the alluvium. In central Kerela, the width of the coastal alluvial tracts increases, in the north and south they are comparirively narrow. The alluvial soils of Kuttanad form a low lying area, once a part of the sea, later filled up by the silt carried down by the pampa and other rivers. The coastal alluviums are sandy with low water retention and low nutrient status. The alluviums on the river banks are fertile.