Human Ecosystems: Okoroba-Nembe
valleys, its plains and its plateaux like any other: it is only the vertical emphasis which is limited.
The Okoroba-Nembe district is a young, pioneer landscape of mangroves turning mud flats into land and of forests creating soil. It is a dynamic landscape: rivers meander, forming levees and ox-bow lakes, frequently capturing each other, becoming drier or suddenly (in geological terms) carrying much more water by river capture, and thus eroding the older levees.
The difference between the situations of Nembe and Okoroba represent the two topographical poles of the district. Okoroba is in an area of swampy plains, interspersed with dry and seasonally dry plateaux formed by levees and fossil barrier islands, so that it is possible to walk, for much of the year, to Ogbia and thence to Yenegoa. Nembe is at the Southernmost tip of this area. It borders an arc-shaped plain created by the most recent alluvial deposits of the rivers that cut it. And, it is a waterlogged plain, large parts of which are submerged at high tide, scattered about with small, low, organic islands, like the two that make up Nembe itself.
20.3 SOILS
There are three soil types in the district: soils of the Fresh-water ecosystems; Brackishwater (mangrove) soils; and soils that are subject to both Brackish and Fresh-water regimes.
20.3.1 THE FRESH-WATER SOILS
The fresh-water soils are of three main sub-types: levee soils; flood-plain soils; and fresh-water swamp soils.
The levee soils develop where the fine sand and silt deposited by floods create land that is high enough to be largely free of floods. They are fine sandy loams and silty loams with a low permeability so that if they are not flooded in the wet season there may be standing water at times. The soils are shallow because of the permanently high water table but there is some leaching.
Where the water table of the levee soil is one metre or more below the surface they tend towards soils similar to the Oxisols on the ridge tops of Akassa (but with a higher silt content). That is, soils which develop in hot, rainy climates where there is a regular downward movement of water, and where there are large amounts of added biomass - the typical soils of the African lowland tropical rainforest.
The flood plain soils are typically high water-table silty clay and clay gley (seasonal reduction) soils. They are criss-crossed with flood streams which work their way around the buttresses, stilt and knee roots that are typical of the trees that grow in these soils.
The fresh-water swamp soils are permanently wet and sometimes submerged. These soils have a higher clay content than the flood plain soils because they are the final sink for the alluvial deposits and because there is no downward leaching at all. Also because of reduced conditions and the accumulation of organic matter in depressions there are deposits of peaty soil.
20.3.2 THE BRACKISH-WATER (MANGROVE) SOILS
The Brackish-water soils are acid sulphate soils because sulphate ions, carried by the inundating seawater, are reduced to Hydrogen sulphide (H₂S) under anaerobic
223
