Regolith and soils in Bhutan, Eastern Himalayas

Abstract

Summary Bhutan lies at altitudes of 100–7500 m on the steep, long and complex southern slopes of the Eastern Himalayas. Soil surveys show that, despite steep gradients, there are many moderately or deeply weathered soils. Many slopes are mantled with polycyclic, layered drift materials, so soil horizons owe as much to regolith heterogeneity as to pedogenesis. In the limited arable areas soil profiles are further complicated by rice cultivation and the construction, maintenance and irrigation of flat terraces on steep slopes. Some natural pedogenic horizonation is apparent, and there is an altitudinal zonation of soil types. Although the climate is warm and seasonally wet, most soils on the subtropical southern foothills are not particularly weathered and leached. The foothills are seismically active, and many soils are formed in unstable landslide debris. Elsewhere the regoliths are more stable. The main soils up to about 3000 m in the inner valleys are moderately weathered and leached, and have bright subsoil colours and thin dark topsoils. Above these there is a zone of bright orange‐coloured non‐volcanic andosolic soils. Further upslope there are acid soils with thick surface litter, stagnogleyic topsoils, and drab brown subsoils with organic cutans. These grade to weak podzols, which extend from about 3500 m up to the treeline, around 4000 m. Above this, alpine turf soils, with deep, dark, and friable topsoils and yellowish friable subsoils, are intermixed with unweathered glacial deposits. The interactions between pedogenesis and the deposition of the varied and layered drift materials complicate mapping and classification of the soils.

Geochemical investigation of soils developed in different lithologies in Bhutan, Eastern Himalayas

“…In contrast to podzolisation, clay illuviation occurs in Bhutan only to a minor degree (Baillie et al, 2004). In the present synopsis, the only horizons to show initial lessivation in the form of weakly pronounced clay cutans, are PK 139/2-3 and PK 158/4.…”

“…Soils of this zone often qualify as Cambisols according to WRB classification (ISSS, 1998), however orange-coloured non-volcanic Andosols frequently occur between 2200-3500 m (Bäumler et al, 2005). Leaching, acidification and podzolisation increase with altitude, and podzolised soils are most frequent from 3500 m to the tree line at about 4000 m (Baillie et al, 2004). Above that, we find alpine turf soils and unweathered glacial deposits, which mostly qualify as Regosols.…”

“…The steep and rugged Bhutanese terrain in combination with summer monsoonal rains and frequent earthquakes provides a framework for regolith truncation and interrupted soil formation. Thus, the soils on the southern foothills are not as weathered and leached as could be expected under humid subtropical conditions (Baillie et al, 2004). Soil development in the temperate inner valleys, up to approximately 3500 m, is characterised by moderate to advanced weathering and leaching.…”

“…In case of PT 053, these processes are weak: Not even the slight podzolisation process, which is typical for Bhutanese soils at 3500-4000 m (Baillie et al, 2004), and associated low pH values (Table 2) are expressed in the chondrite-normalised REE patterns (Fig. 5a).…”

Organic agriculture in Bhutan: potential and challenges

“…They are mostly underlain by thick sheets of highly metamorphosed gneisses (~70 %), quartzites, schists and marbles (Baillie et al 2004). They are mostly underlain by thick sheets of highly metamorphosed gneisses (~70 %), quartzites, schists and marbles (Baillie et al 2004).…”