T.I. AshayeObafemi Awolowo University, Ile-Ife, Nigeria
Dark clays and associated soils cover extensive areas (estimates vary between 1 and 10 million ha) of northeast Nigeria. They are also found in the southwest part of the country (about 7000 ha)
This paper reports the results of studies on 12 soil profiles, 7 from the north and 5 from the south of Nigeria. The northern soils are Vertisols: they show the typical cracking pattern, gilgai and other morphological features of these soils, and their clay content is high (above 40%) throughout the profiles. The southern soils have variable clay contents-generally low in the topsoils and 31-57% in the subsoils: one soil has a low clay content throughout the profile. Despite their low clay contents, these soils show vertic properties: the clay mineralogy is mainly 2:1 lattice-type highly expanding clay, they are slightly acidic to alkaline, with Ca and Mg as the dominant cations in the exchange complex; and they have low permeability.
In the north these soils are used for growing wheat and sorghum, and for grazing. In the south they are used for grazing, and for growing a wide range of food crops, such as maize, yams and vegetables. The main management problems are flood control, maintaining a stable soil structure, correcting salinity problems (in the northern soils) and establishing cropping sequences and irrigation and drainage systems.
