Plinthite As Indicator of Soil Degradation: An Experience from the Savannah Ecology of South Western Nigeria.

Soil degradation is a very serious land resources management issue in sustainable land use. It may be slow and gradual, but may be very difficult to control once it occurs. Degradation may occur naturally or as a result of misuse of land by man. One of natural process of land degradation is plinthization. Soils with hardened plinthite are some of the major problematic soils in the world. There is no good estimates of the areas occupied by such soils under cultivation in Nigeria, especially south western Nigeria. Nevertheless, population has put pressure on the land to the extent that these marginal lands are been put to use for large scale mechanized farming. Regretfully, these soils have not received much research attention despite the FAO estimates of over 20 million ha in Nigeria. A semi detailed soil survey of the savannah region of south western Nigeria was carried out.  Using 1 x 3 km detailed sampling blocks cutting across all identified land facets, pedons with hardened plinthite were estimated, characterized and evaluated. The results show that 35% (913,000ha) of the area were occupied by hardened plinthite soils. Of these, 16% (410,000ha) were occupied by petroferric contact soils and 29% (503,000ha) were occupied by petroplinthic soils. The thickness of the pan layer varies from 30cm to 170+cm where the pan can be dug out. The soil texture over the pan ranged from sand to sandy clay loam, while the texture under the pan layer ranged from sandy clay to clay. The soils are not suitable (N) to marginally suitable (S3) for the crops evaluated. The soils are only sustainable with high input (20-29) with the present land use, while soil quality evaluation also revealed that the soils are low (29 – 56%) in quality for agricultural uses. Soil management options are given for sustainable land use.