Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/54

What is the Niger Delta? #

Also known as the eluvial topsoil, dominated by eluviation - the downward movement of nutrients. Within this horizon there may be four further layers.

On the surface lies the litter of fallen plants; then partially decomposed litter, plant roots and dead animals; below this is a layer of fully decomposed organic matter, stained black or brown with the nutrient-rich end product of decomposition called humus; at the bottom is a lighter coloured layer from which nutrients have been leached, or carried down by drainage water, to the lower B horizon.

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The B horizon is the illuvial topsoil, dominated by illuviation - the acquisition of nutrients carried down by drainage water from higher layers. This is why it is generally darker in colour than the lowest layer of the 'A' horizon.

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This is the subsoil, made up mostly of broken-down parent material and some elements of the topsoil.

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This is the underlying parent material itself, which may for example be rock, gravel, or sand deposits.

This is a generalised description. In reality, some soils show very clear layering while others vary more gradually. An entire soil profile may be a few centimetres in depth, or extend for tens of metres.

4.5.2 TYPICAL TROPICAL MONSOON SOIL

Typical tropical monsoon soils, as found throughout the Delta, have a very shallow A horizon, because decomposition is so rapid. However, the underlying B horizon may be very deep, because the parent material is breaking down rapidly. The B horizon may vary in colour from deep red to white, depending on the proportion and chemical state of the iron in the soil.

Under natural conditions, leached nutrients in this B horizon are taken up very rapidly by tree roots so that the soil is often inherently poor. On anything steeper than a gentle slope, this soil is very easily eroded.

4.5.3 SOILS, VEGETATION AND OXYGEN

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