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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.
Department of Agriculture and the Massachusetts 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 2mm 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 sample is first crushed and then passed through a sieve with a 2mm 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 hydroxyions (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 soil testing 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.