2. SOILS AND TOPOGRAPHY
The topography of Bhutan is characterized by rugged mountains separated by river valleys. Elevations range from just below 200m in the south to almost 8,000m in the north. Geologically, most of Bhutan consists of crystalline sheets with large masses of tertiary granite intrusions towards the north. For details of the geology of Bhutan see Ganser (1983) who defined 5 geological zones: the Sub-Himalaya; the Lower Himalaya; the High Himalaya; the Tibetan Himalaya, and the Indus-Tsangpo zone.
Information on Bhutan’s soils is very scarce. The FAO/UNESCO soil map (FAO/UNESCO, 1977) classified about 27 percent of Bhutan as having either cambisols or fluvisols (cambisols are most common in the medium- altitude zone, while fluvisols mostly occur in the southern belt). Less fertile acrisols, ferrasols and podzols were estimated to cover 45 percent of the country. The same study also reports that 21 percent of the soil-covered area suffers from shallow depth with mostly lithosol occurring on steep slopes (Roder et al., 2001). Bhutan’s forest soils have been classified by Sargent et al. (1985) and Okazaki (1987) who delineated 5 major soil groups (yellow soils, yellow brown forest soils, brown forest soils, podzols and alpine meadow soils) based on 69 samples collected from sites between 150 and 5,300 m. Altitude and prepcipitation were the main factors used in the classification. Currently a Soil Survey Project has embarked on a systematic soil survey of Bhutan.