APlanosol in theWorld Reference Base for Soil Resources[1] is asoil with a light-coloured,coarse-textured, surfacehorizon that shows signs of periodicwater stagnation and abruptly overlies a dense, slowlypermeablesubsoil with significantly more clay than the surface horizon. In the US Soil Classification of 1938 used the name Planosols, whereas its successor, theUSDA soil taxonomy, includes most Planosols in theGreat Groups Albaqualfs, Albaquults and Argialbolls.
These soils are typically in seasonally waterlogged flat lands. They occur mainly insubtropical andtemperate,semi-arid andsubhumid regions. Planosols are formed mostly in clayeyalluvial andcolluvial deposits. 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.
Planosols cover an estimated 130 million hectares of the world. Major areas with Planosols occur in subtropical and temperate regions with clear alternation of wet and dry seasons, as inSouth America (Uruguay, northeasternArgentina,Paraguay, easternBolivia, the Brazilianpampa, thePantanal mato-grossense andnortheastern Brazil),Africa (Sahelian zone,East andSouthern Africa), the east of theUnited States of America,Southeast Asia (Bangladesh andThailand), andAustralia.
Planosols in their natural state support a sparse grass vegetation, often with scatteredshrubs andtrees that have shallowroot systems that can cope with temporary waterlogging.Agricultural land use on is normally less intensive than that on most other soils under the same climate conditions. Vast areas of Planosols are used for extensive grazing. Wood production on Planosols is much lower than on other soils under the same conditions.
In the temperate zone these soils are mainly in grass or planted tocrops such aswheat andsugar beet. Yields are modest even on drained and deeply loosened soils. Root development on natural unmodified Planosols is hindered severely by oxygen deficiency in wet periods, dense subsoil and, in places, bytoxic levels ofAl in the rootzone. Planosols inSoutheast Asia are widely planted withpaddyrice. Other crops met with little success.Fertilizers are needed for good yields. In climates with long dry periods and short infrequent wet spells the best land use aregrasslands. Strongly developed Planosols with a verysilty orsandy surface soil are best left untouched.
