Soil & Methods Of Soil Sampling (Water Retention In Soil)


Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. (Definition from Wikipedia)

Main characteristics

Sandy soils
These occur in many parts of West Africa and are characteristic of many areas in the savanna zone of northern Nigeria. These soils are free-draining and do not retain any appreciable amount of soil-water, which rapidly drains down to the lower horizons, leaching any soluble minerals which may be present in the upper layers. Sandy soils contain gravel, coarse sand, fine sand and limited amounts of silt. 

They may be termed sandy clay loams (approximately 20-35% clay, 40-50% sand and 30-40% silt), sandy loams (approximately 15-20% clay, 50-60% sand and 35-40% silt), loamy sands (10% clay, 85-90% sand and 5-15% silt). The approximate identification of any particular type of soil can be obtained. These soils are only suitable for crop growth when
they contain adequate amounts of organic matter and minerals and can be supplied with water during dry periods. They are suitable, when these requirements are met, for short-lived annual crops such as groundnuts and also tree crops such as the cashew, which are relatively drought-resistant.

Clay soils
These soils are suitable for crop production if the clay content is not high enough to cause excessive cracking and hardening of the soils during dry periods. During wet weather, heavy clay soils can retain a great deal of water which The capacity of clay soils to retain soluble minerals which are essential for plant growth makes them potentially fertile, the addition of organic material improves the structure of clay soils, making them less retentive of water.

Applications of materials which contain calcium, such as lime (calcium hydroxide) and dolomitic limestone (calcium carbonate and magnesium carbonate) also assist in the improvement of the structure of clay soils, but their use in West Africa is very limited due to the fact that they generally have to be imported may also make them difficult to cultivate.

Humic soils
This term is used to describe soils which contain a high level of organic material which has been broken down by the action of bacteria and other soil organisms. Humus is often the final product of the decom position of woody material which has been buried in the soil and it is most commonly found in the upper layers of the soil. In West Africa, as in many other tropical areas, the high soil temperatures promote rapid bacterial activity and therefore any material which is either placed on the soil surface as a mulch or buried by hoeing or ploughing will rapidly decay. The level of organic material in West African soils is therefore relatively low and requires constant replacement if crops are grown on an intensive scale.

Humus particles have the property of attracting to their surface soil minerals, in the form of cations, in a similar manner to clay particles and therefore soils with a relatively high content of humus are considered to be potentially fertile.

Methods Of Soil Sampling

The chemical analysis of soils can only be carried out properly in an adequately equipped laboratory, but it is important to provide the most suitable soil samples for analysis, if the results are to be useful and if the farmer is to be able to obtain sound advice on the application of any chemical fertilisers which will aid him in improving the growth of his crops.

Soil samples should be taken to a depth of 15 cm from the surface and sometimes from 15-30 cm if an examination of the deeper layers is required. Samples may be about 50-100 g in weight, and, for small areas, 5-10 samples should be collected from each 0.1 hectare. Where there is an obvious difference in soil colour or texture, samples from each area should be kept separate for analysis, but samples from one soil type should be mixed together to obtain an average or representative sample for analysis.
Table also gives the U.S. Department of Agriculture and the Massachusets Institute of Technology systems, for comparison. The main objective of mechanical analysis is to separate a sample of soil into its constituent parts or fractions, but this does not take into account the chemical content of the sample. An outline of the procedure which can be followed in the classroom is as follows.

The sample is first crushed and then passed through a sieve with a 2 mm mesh. The particles which remain on the sieve are classified as gravel. The sieved sample is then placed in a large glass cylinder which is filled with water. The water is then stirred vigorously so that the contents are thoroughly mixed. After stirring for about five minutes, the contents are allowed to settle. The sand and silt fraction will settle first, followed by the clay fraction. This fraction may take several days to separate from the water and some of the finer colloidal clay particles may remain in suspension for a considerable time. The organic material which is insoluble, will float on the surface of the water.

The pH value of soils, which measures the excess of hydrogen ions (H+) over hydroxyl ions (OH-) in a soil structure, is a good indication of their acidity or alkalinity and the most accurate method of determining soil pH is by means of an electrically operated pH meter. Portable battery-operated types are available, which give a direct reading of the pH value when glass electrodes are inserted in a suspension of the soil water, obtained by mixing one part of the soil sample with two parts of distilled water.
In the classroom, litmus paper, which turns red in acid conditions and blue in alkaline solutions, can be used as a general indication of the pH value of a soil water suspension. A number of colour indicators, included in soiltesting outfits, are also available. These normally involve mixing a sample of soil with barium sulphate and distilled water. The colour reaction
which results from adding a few drops of a colour indicator to the solution can be compared with a chart supplied with the equipment.

Machine Used In Soil Preparation

Machinery used in soil preparation
Tillage machinery The term 'tillage' is widely used to describe operations which prepare the soil for crop establishment, but the term 'cultivations' has also a similar meaning.

There are two main stages in cultivationsPRIMARY & SECONDARY 

Primary Cultivations: The main objectives of this first stage in soil disturbance are to invert or turn over a slice of the topsoil, burying weeds, surface trash or manures spread on the surface and to break up the soil to a depth of up to 20 cm in order to encourage aeration y, water penetration and to generally prepare the soil for the secondary operations.

Secondary cultivations: These are mainly directed towards the preparation Of a tilth, a term used to describe the surface layer of a soil which has been subjected to movement or pressure so that it is well broken up and free from large lumps or clods of soil. Such a surface is most suitable for the uniform sowing and covering of seeds, transplanting seedlings or
planting tubers and is favourable to the rapid establishment of crop root systems. In tropical areas, it is generally considered unwise to cultivate soils unnecessarily. This is due to the fact that, particularly on silty soils, heavy rainfall breaks down the soil surface, causing 'capping' or the formation of a crust of hard soil on drying. This can seal the soil surface, preventing the exchange of gases such as oxygen and carbon dioxide between the atmosphere and the soil air. Roots require oxygen for respiration and growth and an excess of carbon dioxide, produced by soil bacteria,
can be harmful to plant roots. It has been found that many tropical crops can be successfully established by sowing the seeds in fairly rough seed beds.

Primary and secondary cultivation equipment may be grouped into mouldboard, disc and tined implements, which are either trailed or direct mountcd by a three-point linkage on the tractor, plus operations

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 reaction so 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.

                 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 shompty into drains the side walls of which should be protected from erosion by establishing them with a grass cover.

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