Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/56

What is the Niger Delta? catenas. For example, even the very slightly undulating landscapes at Akassa and Ogoni show at least three noticeable soil conditions as follows:
 * on top of a ridge or hill are the eluvial soils, which tend to be comparatively coarse and well oxidated because material is always being moved downhill
 * soils on the hill slopes are colluvial; they both gain and lose material. Further down the slope they tend to be both deeper and less well oxidated
 * material accumulates at the bottom of slopes to form illuvial soils; even less well oxidated, they are still potentially fertile if kept from continuous waterlogging.

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This is generally a direct function of topography. In the Delta, the natural soils form in an environment of flooding and high water-tables that is subject to both freshwater and brackish water regimes. These conditions severely limit the downward movement of dissolved nutrients. Soils are also flooded with water for long periods, so that chemical reactions occur in low-oxygen or 'reduced' conditions.

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The tropical climate of the Niger Delta naturally has a direct bearing on the soils; it ensures that the climax vegetation is forest, that biological activity and biomass is high, and that nutrient uptake and recycling are both rapid.

4.5.5 COMMON SOIL TYPES OF THE NIGER DELTA

Two general tendencies are to be remembered in understanding soil types in the Delta.


 * Firstly, that waterlogging tends to lead to reduction (loss of oxygen); and
 * Secondly, that the better-drained the soil, the greater the leaching of nutrients.

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Inceptisol Aquept soils cover the greater part of the Niger Delta. They are young, poorly drained and shallow, and form only one layer or 'horizon'. (This is why they are also sometimes called 'Camibisols'.) The terms 'Inceptisol' and 'Aquept' are used by the U.S. taxonomic system; 'Inceptisol' is a general term for the young soils of the humid regions of the earth. The term 'Aquept' refers to soils that have been waterlogged for long enough to become reduced.

They are characterised by a blue, grey and/or green colouration, coming from the reduced iron in the soil. Where these soils are drained and exposed to the air through seasonal drying, or by human activity, this reduced or ferrous form of iron is oxidised into the ferric form. This shows in the soil as streaks of red. Soils that have been waterlogged and then oxidised in this way may then be known as 'Gley' or 'Hydromorphic' soils.

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Sulphaquepts, or Acid Sulphate Soils, typically occur in the brackish conditions of Delta mangrove areas ''.....where sulphate ions, carried by inundating seawater, are reduced to hydrogen sulphide under anaerobic conditions in sediments high in organic matter. Hydrogen sulphide then reacts with iron compounds present in the soil-forming pyrite (FeS2). When these deposits are exposed to the air and the soil is low in calcium carbonate, FeS2 is oxidised into ferric sulphate and free sulphuric acid, producing pH values in the '' 54