Structure Of Soil & Soil Profile (Water Retention In Soil)



Structure of soils

The arrangement of soil particles into natural clumps or aggregates, due to the mixing of the various constituents of clay, sand and silt, is an indication of the soil structure. The sizes of these units are described as: very fine, medium, coarse and very coarse but these are also related to the shape and arrangement of the aggregates, also their durability. In West Africa, many soils have little or no structure, they easily break into fragments when cultivated.

Soil profiles

A soil profile is a vertical section through an undisturbed soil which exposes the various layers which have been formed as a result of weathering, biological reactions and soil movement by wind and water. The profile therefore indicates the past history of the formation of any particular type of soil, each layer of material which can be distinguished from those below and above it is termed a soil 'horizon'. Under ideal
circumstances, it would be possible to  excavate a soil pit to expose all the horizons down to the parent rock layer, but this is rarely possible in West Africa since most of the soils have been considerably weathered and decomposed to a very great depth and the parent rock may be more than 30 m below the soil surface.

Each of the separate horizon layers are labelled alphabetically, the topsoil horizon being 'A'. This is the area from which many of the more soluble elements are leached and there may be more than one 'A' horizon, if distinct layers of material can be distinguished to a depth of about 30-40 cm. These upper layers are often dark in colour, due to the presence of organic material. The 'B' horizon or horizons may extend from 30-40 to 130 cm in depth, this is commonly known as the subsoil layer where many of the soluble elements and organic compounds leached down from the topsoil accumulate. There is only one 'C' horizon which includes the materials produced From the parent rock by weathering; this layer is relatively infertile since it is a transition stage in the formation of soil.

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                 Water Retention In Soil 

The degree to which soils retain water is important in crop growth since waterlogged soils restrict the amount of oxygen available for plant root respiration.

Clay soils often have impeded drainage and this also limits the extent to which they can be cultivated by either hand tools or mechanical implements.
The permeability or porosity of a given soil sample can be examined by preparing two large glass measuring cylinders with funnels and filter papers. A small plug of cotton wool may be placed below each filter paper. Equal quantities of two different types of soil are then placed within each filter paper which are then filled with water. The rate at which the water passes through the filter paper will be an indication of the porosity of the soil samples, sandy soils will be found to be very much more permeable than clay
soils. In practical situations, poor soil drainage may be a serious limitation to crop production and various measures can be used to improve poorly drained soils.

One means of ensuring that the roots of plants obtain sufficient aeration is to grow crops on ridges, so that surplus surface water collects in the furrows. Raised beds, such as are commonly used for vegetable cultivation, also achieve this. The use of bunds or channels to prevent surplus surface water draining from higher areas on to the land to be cultivated will also prevent waterlogging of the soil during the wet season.

Where it is practicable, the construction of drainage channels or ditches should be undertaken, these may be up to 30-40 cm deep and the sides should be retained with stones or a grass cover to protect them from being eroded during heavy rainfall. These lateral drainage channels should have a slight slope, leading the water away to a lower level where a main drain or stream will carry it away from the cultivated area. On sloping land, bench terraces should be constructed with a shallow drainage channel to collect surface water from the reverse slope of the terrace.
These should empty into drains the side walls of which should be protected from erosion by establishing them with a grass cover.


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