The diversified geological conditions, topographical characteristics, climatic situations and vegetation types have favour the formation of different types of soil in the state. The soils of Assam can broadly be divided into four main groups, viz. alluvial soils, piedmont soils, hill soils and lateritic soils.

The alluvial soils are extensively distributed over the Brahmaputra and Barak plain and are very fertile. The alluvial soils can further be divided into two main sub types-young alluvial and old alluvial soils. The young alluvial soils is characterized by modern alluvium deposits. The colour of these soils is generally gray to molted gray. On the other hand, the old alluvial soils occurs in some patches of Kokrajgar, Barpeta, Nalbari, Kamrup, darrang, Sonitpur, Lakhimpuir and Dhemaji district. Generally, the old alluvial soils are very deep with fine loams to coarce loams in texture. The piedmont soils are confined to the northern narrow zone along the piedmont zone of the Himalayan foothills. The soils are very deep and fine to coarse loamy in texture. The hill soils are generally found in the southern hill regions of the state. These soils are deep, dark grayish brown in colour and fine to coarse loamy in texture. The lateritic soils are extensively occurring in N.C. Hills district and in some parts of the southern Karbi Plateau. These soils are dark and finely textured with heavy loams.

Soils

Soil is the most valuable nature resource and serves as one of the prime requisite of life. Soils and in its turn the land through their relative fertility support all agricultural activity ant the plant growth and thereby the most important element of the natural ecosystem. As regards the soils of Assam, geology (parent material), topography and climate seem to play vital role in their formations. Therefore, under varying geological conditions, topographical characteristics and agro-climatic situations different types of soils are found to occur in the hills, piedmonts, plateaus and plains. The soils of Assam may thus generally be divided into four groups, viz.

a) Alluvial soils

b) Piedmont soils

c) Hill soils

d) Lateritic soils.

a) Alluvial Soils:

The alluvial soils are extensively distributed over the Brahmaoutra and Barak plain. These soils are very fertileas they formed from the alluvium deposits, deposited by the rivers Brahmaputra, Barak and their tributaries. The alluvial soils of Assam can be further be divided into two sub-types base on some micro differences in character such as – younger alluvium and old alluvium.

The younger alluvial soil occurs in an extensive belt of the north-bank and south-bank plains including the active flood plains of the Brahmaputra and the Barak rivers. This soil characterized by recent deposition of alluvium, moderately deep to very deep with grey to molted grey colour . It is mostly composed of sandy to silty loams and slightly acidic in nature. On the riverbanks it is less acidic and sometimes nutral or slightly alkaline. The soil lack in prifile development and is deficient in phosphoric acid, nitrogen and humus.

The old alluvial soil occurs in some patches of Kokrajhar, Barpeta, Nalbari, Kamrup, Darrang, Sonitpur, Lakhimpur and dhemaji districts between the northern piedmont soil belt and the southern new alluvial soils of the Brahmaputra valley. In the south bank districts of the valley it occurs in a narrow belt bounded between the southern hill soils and northern new alluvial soils. In the Kopili plain covering Nagaon district the old alluvium finds wider extension. The Barak plain, on the other hand, has some elongated patches of old alluvial soil confined between the new alluvial soils of the active floodplain and the hill soils boardering Mizoram. Generally the old alluvial soil is very deep, brownish to yellowish brown with texture of fine loams to coarse loams and is slightly to moderately acidic.

b)Piedmont Soils:

The piedmont soils are confined to the northern narrow zone along the piedmont zone of the Himalayan foothills. These soils comprise the Bhabar soil and the Tarai soil, covering respectively the Bhabar and the Tarai belt of the Brahmaputra valley. The Bhabar soil occurs in the narrow belt along the Assam-Arunachal boarder extending east up to the river Subansiri’ is characterized by unassorted detritus of boulders, pebbles, cobbles, sand and silts. This soil is deep and fine to clay loamy in texture. The Tarai soil occurring just south of the Bhabar soil extends up to Dihang river in some discontinuous narrow patches.This soil varies from sandy to silty loams that remain saturated and support tall grasses in a series of swamps.

c) Hill Soils:

The hill soils are generally found in the southern hilly terrains of the state. The fertility of these soils defers greatly in different regions. These soils are rich in nitrogen and organic matters. On the basis of the physical texture and chemical composition, the hill soils may be divided into –red sandy soils and red loamy soils. The red sandy soils are distributed covering as narrow belt along the Assam- Meghalaya border, the Karbi Plateau, southern part of Barail range of the N.C.Hill district and some parts of the foothills along the eastern border of the Cachar district. This soil is very deep and well drained, brownish to yellowish in colour, strongly to moderately acidic with high organic content. The red loamy soils, on the other hand, occurs in the narrow southern foothill belt running along the Assam’s boarder with Arunachal and Nagaland and also in the southern fringes of the Karbi Plateau and the Barail hills of N.C.Hills district. These soils are very deep, dark grayish brown to yellowish red and fine to coarse loamy. Red loamy soils are slightly to moderately acidic and these lack in nitrogen, phosphoric acid, humus and lime.

d) Lateritic Soils:

The lateritic soils in the state extensively occurs almost entirely over the N.C.Hills district covering some parts of southern Karbi Plateau while few patches are confined to eastern margin of the Hamren sub-division of Karbe Anglong district, southern boarder of Golaghat district and the northern part of the Barak plain along the foothills of the Barail range. These soils are dark and finely texture with heavy loams and deficient in nitrogen, potash, phosphoric acid and lime.

The soils of Assam are very rich in content of nitrogen and organic matter. The alluvial soils of the Brahmaputra and the Barak valley are highly fertile and are very much suitable for raising of varieties of crops round the year such as cereals, pulses, oilseeds, plantation crops etc. The well drained, deep, acidic alluvial soils of upper Assam with good proportion of phosphoric content are mostly for the plantation. New alluvial soils occurring in the charlands of the Brahmaputra are most suitable for growing oilseeds, pulses and rabi crops. The alluvium of the plains offers excellent opportunity for cultivating rice and vegetable. The soils occurring in the upper reaches of the hill slopes are very suitable for horticulture and plantation crops.