Northeast India is a place of heavy rainfall, high relative humidity, and high temperatures, which cause rapid weathering of rocks. As the weathering of surface rocks of the hills and mountains proceeds, the sheet flood caused by heavy rainfall removes the weathered materials and carries them down to the headward streams of the rivers. The rapidly flowing rivers then carry the sediments, big and small, down to the foothills and plains.
Soils found in northeast India are mainly four major groups. These are Alluvial Soil, Red Soil, Laterite Soil, and Mountain Soil.
In the Northeast Region, soil and environmental degradation is a big problem. This is further aggravated due to ecologically unsuitable shifting cultivation by the local tribes/indigenous people who are socio-culturally and economically very much attached to this form of land use.
Types of Soils Found in Northeast India
There are four major soil groups found in Northeast India
Alluvial Soil
Old Alluvium
New Alluvial Soil
Red Soil
Laterite Soil
Mountain soil
1. Alluvial Soil
The alluvial soil may be divided into Old Alluvium and New Alluvium on the basis of the period of their genesis
Old Alluvium
The Old Alluvium was deposited earlier in the late Pleistocene and Sub-Recent periods. The soil is more consolidated, acidic, and less fertile. Old alluvium is found in patches generally along the foothills.
In the Brahmaputra valley, there is a long patch of old alluvium along the Himalayan foothills from Kokrajhar District up to the Subansiri River of Lakhimpur district.
There are patches of old alluvium in the Semphema-Diphoma area of Kohima, along the foothills of the Cachar and Hailakandi districts, and along the foothills of Tripura.
New Alluvial Soil
The new alluvial soil consists of recently deposited silt and sand and is rich in organic content. New alluvial soil is found all over the Brahmaputra and Barak valleys, Manipur plain, and Tripura plain.
2. Red Soils
Red Soils are normally found on all the hills slopes and hilltops except in very high altitudes. These soils become red because of the oxidation of the iron content present in them.
These soils are commonly found in Meghalaya, the hills of Tripura, Mizoram, the Hills of Manipur, Arunachal Pradesh, and Karbi Anglong and Dima Hasao of Assam.
They are bright red in Meghalaya because of the presence of a greater proportion of oxidized iron in their parent rocks, i.e., the acidic rocks of Precambrian origin.
3. Laterite Soils
Laterite is found only in small patches at the medium altitude of the hills. This soil contains considerable quantities of iron and aluminum and hence is hard when dry and red in color.
Laterite is distributed in small patches over higher hilltops and slopes of Karbi Anglong and N.C hills of Assam. Relatively extensive patches of laterite are found in the Cherrapunji region of Meghalaya over sandstone deposits.
In Manipur, laterite is confined to higher hilltops of the south-eastern region. In Mizoram and Tripura, it is found on the ridges of higher altitudes. In Arunachal Pradesh, laterite is found in patches at mid-altitude on sandstone rocks.
4. Mountain Soil
In Northeast India, mountain soil is found at very high altitude above 4500m. Such soils are, therefore, confined to the higher altitude in Arunachal Pradesh and over the summit region of the Patkai Range.
Soil Distribution of Northeast India States
In Assam, we mostly found alluvial soils over the plains, red soils in most of Karbi Anglong and Dima Hasao, and only patches of laterite over the highest part of their hill ridges.
In Arunachal Pradesh, the limited river valleys contain alluvial and diluvial soil, while all the lower hills contain red soil. The area above 4500m has mountain soil mixed with glacio-fluvial deposits.
Meghalaya has red soil all over the hills except the higher central part where laterite is present.
Mizoram has mostly residual soil weathered from shale. Red soils predominate the state with only thin patches of laterite over the higher ridges.
Manipur plains contain alluvial soils, and its hilly areas have residual red soils that are sandy.
Nagaland also has red soils, but these are by and large fine-grained.
Tripura has alluvial soils over the piedmont plains and red soil over the hills. There are a few small patches of laterite soil found in the higher hills of Tripura.
Soil Erosion in Northeast India
The northeast region is unique in its climate, crops, and people diversity. Jhum cultivation is prevalent in the region for food production and sustaining livelihood.
One major constraint in sustainable crop production is problem soils, those soils which have serious physical and chemical limitations to crop cultivation.
The problem of soil erosion and land degradation in northeast India is widely visible both in the valleys and in the hills.
After the great earthquake that visited the region in 1958, the frequency and intensity of siltation and floods have taken a new dimension due to the disturbance and imbalance caused to the river courses and surrounding landmass.
Soil erosion, silt deposition, and the rising of the river beds level have also been accelerated due to the changes taking place in the hills. An increase in population growth and its ever-rising demand for basic necessities for survival as well as for other comforts of life necessitated the expansion of Jhum lands in the hills.
Soil erosion with Jhum cultivation having a slope (60–70 %) in the first, second, and third years as abandoned was recorded at 147, 170, and 30 t/ha/year, respectively.
The region has mostly problems of land degradation, acidity, and severe erosion because of existing Jhum farming by tribal farmers.
Severe soil acidity problems (pH <5.5) are prevalent in the region, which accounts for ~54 % of the acid soils of India.
The significance of soil erosion and resultant siltation in the river beds is manifold. There is a vertical and a horizontal increase in river beds, and increasing frequency and intensity of floods are perennial problems in the plains of the northeast.
The problem of waterlogging, especially in tea plantation areas, has also been alarmingly increasing and turned out to be an acute problem in the plains of Northeast India.