Soil Types of Assam
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 suitable 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.
The diversified geological conditions, topographical characteristics, climatic situations and vegetation types have favour the formation of different types of soil in the hills, piedmonts, plateaus and plains. The soils of Assam may thus generally be divided into four groups, viz.
- Alluvial soils
- Piedmont soils
- Hill soils
- Lateritic soils.
The alluvial soils are extensively distributed over the Brahmaputra and Barak plain. These soils are very fertile as 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 neutral 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.
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.
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.
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.
Riverbank erosion during high flood period in the valley is a regular annual feature. Over bank flood due to breaches in the embankment render the fertile cultivable land unsuitable for crop production due to deposition of coarse sand on the surface to a variable depth. As per Assam Government Revenue Dept. records, an area of 6116 hectares of land was affected by soil erosion in Upper Brahmaputra Valley and North Bank Plain zone during 1994.
The highly productive and fertile soils of Assam are now facing the serious problem of soil erosion like other parts of the country. Under heavy precipitation and humid climate loss of topsoil through surface run-off is the most common type of soil erosion in the entire state.
The problem of topsoil erosion is severe in the plain during the flood season. It is estimated that nearly 3.2 million hectares of land of the plain districts of the state are vulnerable to topsoil erosion with varying intensity. Terrain deformation through mass movement is another type of soil degradation, which is primarily confined to the hill districts of Karbi Anglong and N.C. Hills covering an area of about 1.53 million hectares. Another important type of soil erosion in the state, which assumed serious proportion in the recent time, is the bank erosion by the rivers. It is observed that at some places, a few kilometers of bank along the villages, fertile agricultural lands and roads are being eroded by the rivers. Majuli, the largest river island of the world is now seriously affected by the erosion and virtually facing the threat to existence. The extent of loss to the bank erosion varies from year to year depending on the severity of floods in the state.
Land is laid waste by destructive means of plantation and polluted by the disposal of domestic and industrial waste. Jhum cultivation, new habitations and settlements, big reservoirs and dams made for various uses such as irrigation, water supply and power, etc. play a role is destroying and adversely changing the land surface. Unscientific mining and extraction of raw materials from the ground have lasting damage on land. Sludge from the sewage plant is deposited on the land surface and which affect the fertility of the soil.