Arunachal Pradesh, the largest mountainous state of India, is situated in the northeastern part of the Himalayan region and characterized by high annual rainfall, forest vegetation and diversity in soils. Information on the soils of the state is essential for scientific land use planning and sustainable production. Physiographically, Arunachal Pradesh can be divided into four distinct zones:
- snow-capped mountains (5500 m amsl);
- lower Himalayan ranges (3500 m amsl);
- the sub-Himalayan Siwalik hills (700 m amsl); and
- the eastern Assam plains.
In vast varied terrain with numerous rivers it is difficult to provide any generalization about the type of soils. However, considering the sand stone rocks the granite and geneiss formation interspersed with calcareous limestone and slate and other minerals tempered largely by swift flowing hilly rivers the broad group could possibly be
(i) Soils of the hills,
(2) Soils in the valleys and mid hills and
(3) soils in the foot –hills.
(4) Podzols
(5) Forest Soil
Soils of the hills
This soil type is found in high altitude meadows found near the snowline in all parts of the higher and trans Himalayas. Negligible rainfall occurs in these areas and the main form of precipitation is snow. The soil found here is very thin and fragile. Since the texture of the soil is very coarse with high gravel content, they are prone to displacement due to slides and avalanches. These soils are dark in colour and have a high content of humus.
Soils in the valleys and mid hills
It is a collective name given to a number of soil types occurring at very high altitude areas under sub-tropical and temperate conditions under a wide variety of forests. These soils have been termed as dabar soils in some parts of Garhwal-Kumaon. Mountain and hill soils are very thin, fertile and may be less than a centimeter deep on steep slopes. These may occur at elevations above 2750 m in the western Himalayas, over 2500 m in Nepal and over 2400 m in the eastern Himalayas. These soils are mixed with pebbles and gravel in many regions. The texture varies from sandy to sandy loam. The soil reaction varies from acidic to neutral and the organic matter content of these soils ranges from 1 to 5 percent.
Soils in the foot-hills
This type of soil is found in the sub-mountain and foothill tracts of the Himalayas ranging from the western regions in Jammu and Kashmir to the eastern most stretches in Arunachal Pradesh. This soil has a high content of organic matter and nitrogenous compounds, but lacks phosphate compounds. The soil is also found in the foothills of Darjeeling, where it has been transported by hilly rivers and streams at altitudes as high as 1500 m. Here, this soil is acidic and poor in plant nutrients and is made up of sandy material and raw humus.
Podzols
They are a class of infertile acidic soils that develop under coniferous forests in humid temperate conditions, where the rate of precipitation exceeds the rate of evaporation. A continuous percolation of water causes leaching of minerals like iron and alumina from upper layers, resulting in the formation of a grey bleached zone on the upper surface. This makes the layer look like ash, giving the underlying soil its name: podzol (pod=under, zola=ash in Russian). The lack of nutrients and acidic nature of podzols make them poor agricultural soils. The percolated minerals often form an impermeable layer at the lower end of the soil profile, which restricts water drainage. These soils occur in high altitude areas near snowline areas.
Forest soil
This soil type is formed either under conditions of high acidic content (where there is acidic humus) or under mildly acidic conditions. The top soil of the soil profile is very rich in humic matter formed by twigs, bark, leaves and needles. An important variety of forest soil is brown forest soil, which occurs under dense deciduous forests. Organic debris from the dense canopy contributes to the topsoil in the form of a thick layer of humus. This soil can be classified into mildly acidic with high base content and acidic with low base content.