Planosol

A Planosol in the World Reference Base for Soil Resources[1] is a soil with a light-coloured, coarse-textured, surface horizon that shows signs of periodic water stagnation and abruptly overlies a dense, slowly permeable subsoil with significantly more clay than the surface horizon.

Geological stratification and/or a pedogenetic process of destruction and removal of clay has resulted in the relatively coarse-textured, light-coloured surface soil abruptly overlying finer textured subsoil; impeded downward percolation of water causes temporarily reducing conditions with a stagnic colour pattern, at least close to the abrupt textural change.

Major areas with Planosols occur in subtropical and temperate regions with clear alternation of wet and dry seasons, as in South America (Uruguay, northeastern Argentina, Paraguay, eastern Bolivia, the Brazilian pampa, the Pantanal mato-grossense and northeastern Brazil), Africa (Sahelian zone, East and Southern Africa), the east of the United States of America, Southeast Asia (Bangladesh and Thailand), and Australia.

Planosols in their natural state support a sparse grass vegetation, often with scattered shrubs and trees that have shallow root systems that can cope with temporary waterlogging.

Root development on natural unmodified Planosols is hindered severely by oxygen deficiency in wet periods, dense subsoil and, in places, by toxic levels of Al in the rootzone.