Soil in focus4

Humic Podzol on old coastal ridge, mainly derived from adamellite (biotite granite and granodiorite), West Sarawak, Malaysia.

Soils in focus :4  IYS2015

In the northern hemisphere, the podzol is a common soil in the sandy landscapes of, amongst others, the ice pushed ridges and cover sands. Although typical for zones, the layered and contrasting podzol also occurs in tropical landscapes, where they form in specific environments, especially in quartz rich sediments.

The podzol is recognizable by a top layer of humus-rich soil with a bleached, ash-grey subsurface horizon, the leaching layer (the eluviation or E-horizon), which is referred to in Russian as 'Zola' (= ash). The leached humic acids and iron and aluminium compounds precipitate at some depth, around and between the sand grains, creating a dark accumulation layer; the illuviation (B-) horizon). The illuviation layer may cement at the lower boundary by iron oxidation, thus forming a hard layer (hard pan). The podzolization process can only occur under conditions of precipitation excess and acidic conditions in the soil. That is to say, the release of soluble, organic humic acids, under forest- or heath vegetation, and a soil environment in which the acids are not neutralized because of a lack of easily weatherable minerals or positively charged ions such as, for example, calcium. The organic acids cause the leaching of the iron and aluminium through which a eluviation layer is formed. Below the illuviation horizon, the original sandy parent material, the C-horizon is found. When disturbed through management, in the case of ploughing for example, the bleached eluviation layer is not always recognizable.

Tropical podzols are prone to severe erosion when left bare. This image shows a degraded podzol in East-Kalimantan. Image: S. Mantel.
Tropical podzols are prone to severe erosion when left bare. This image shows a degraded podzol in East-Kalimantan.
Photo: S. Mantel.
Black water stream.
Photo: Tim Ross.

Podzols are limited in extent in tropical regions and are most commonly found in lowlands areas where sandy parent material, in combination with high rainfall and specific vegetation, favours the podzolization process. Hardon used the scientific term "podzol" for tropical soils in 1936 for the first time in his description of quartz-rich sands with poor vegetation on the island Bangka in the South China Sea.  Rivers colour ‘coffee-brown’ in these landscapes from the humic acids washed through the soil and drained to the waterways. Podzolization can progress so strongly in these soils that the eluviation layer may eventually disappear altogether and sterile, white sands remain.