The soil covering most of Bihar is thick alluvium which shrouds the Siwalik and older tertiary rocks. The soil is mostly fresh loam replaced every year by intermittent deposition of silt, clay and sand by different rivers. It lacks phosphoric acid, nitrogen and humus but potash and lime are generally present in large amounts.
There are three important types of soil in Bihar:
Bihar falls within the tropical to sub tropical zone. Rainfall is the single most important cause that decides the type of vegetation. Bihar possesses a monsoon climate with an average annual rainfall of 1200 mm.
The sub Himalayan foothill of Someshwar and Dun ranges in Champaran comprise a broad belt of moist deciduous forests. The characteristic trees are Shorea Robusta (Sal), Shisham, Cedrela Toona, Khair and Semal.
The fertile alluvial plain of Bihar stretches from the foothills of the Himalayas in the north to a few miles south of the river Ganges. Extensive farmland and luxuriant orchards are scattered throughout the state. The most important crops are paddy, wheat, lentils, sugarcane, jute. Cane grows naturally in the marshes of West Champaran. The chief fruits are mangoes, banana, jack fruit and litchis. This is one the very few areas outside China which grows litchi.
The soil and vegetation of Bihar are important factors in fixing the occupation of a considerable part of its inhabitants.
