order of 2 and 3 [extremely acidic]. The ferric sulphate ions are further hydrolised into straw-coloured jarosite (basic ferric sulphate material) and accumulated in the soil, giving it characteristic bright yellow mottles. (Sanchez, 1976)
In the Okoroba-Nembe district these soils are of three main types:
- Cat-clays: the more recently deposited alluvial muds. When exposed to the air, they oxidise and release bubbles of hydrogen sulphide, producing the typical rotten-egg smell of mangroves at low tide.
- Chicoco Soil: older, peaty clay soils, formed over time from the thick mat of mangrove secondary roots.
- Saline Sandy Soils: containing far less organic matter and far fewer nutrients, being about 75% sand. Ogonny describes them as having been developed by the erosion of sandy deposits, presumably the beach-ridge barriers and the Sombreiro-Warri delta plain remnants near Degema, Buguma and Port Harcourt, as well as the remnants of the coastal terrace near Ogoni and Andonni.
#Udic Oxisoils - the better drained freshwater soils
On the higher ground of the Delta, such as the old coastal terrace, the higher levees and the ridges of the barrier islands, soils are better drained and downward movement of water is not impeded by an impervious layer. Soils here are described as Udic Oxisols (U.S. taxonomy).
The profiles of oxisols are easily visible in the borrow-pits and gas pipeline trenches at Botam-Tai in Ogoni. They generally have deep B horizons consisting of kaolinite, iron oxides and quartz. They are generally red or yellow, with a good and uniform structure but of low fertility.
'Udic' describes the soils typical of tropical rainforests. They form in hot, rainy climates where there is an almost consistent downward movement of water through the soil horizons, and where there are large amounts of added biomass. In natural rainforests this gives a high surface concentration of humus. However in the Niger Delta, where sand is a major parent material and where heavy rain causes leaching for much of the year, the Udic Oxisols are more sandy and depend on biomass being returned to the soil to maintain fertility.
#Higher levee soils
These are closer to Oxisols than Inceptisols, inasmuch as they are not waterlogged. However they are not deep, and high water tables limit the leaching of nutrients.
ERA define them as 'young shallow oxisols'; the soils become exhausted by agriculture, as do the Udic oxisols.
4.5.6 THE INFLUENCE OF HUMAN ACTIVITY ON SOILS
The soils described above are the natural soils of the Niger Delta; that is, before their alteration by human activity.
However, an important feature of the Oxisols is their dependence on massive additions of added biomass. Being sandy, they depend on humus to hold the sand together and give structure, as well as to maintain fertility. When forest is cleared for agriculture, the soils cultivated and the remaining biomass burnt before fallow years, humus content declines rapidly and is not adequately replaced. Nutrients and clays are leached, and poor sandy soils are the result.
