#The 'A' horizon
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.
#The 'B' horizon
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.
#The 'C' horizon
This is the subsoil, made up mostly of broken-down parent material and some elements of the topsoil.
#The 'D' horizon
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
Photosynthesis: photosynthesis is vital to both plant and animal life. It is a chemical reaction in the green chlorophyll of plants that uses the energy of sunlight. In this reaction, carbon dioxide from the air is combined with water from the soil to make glucose (a sugar), releasing molecular oxygen as a by-product. This oxygen is vital to aerobic respiration in both plants and animals.
The glucose is then a source of energy for the plant, or a precursor to more complex molecules (such as the starch in cassava). These may be used by the plant itself for further growth, and/or then eaten by humans and other animals, because they can be broken back down into simpler molecules again to release the stored energy - this process is called respiration.
