- MECHANICAL ANALYSIS
The basis of soil classification throughout the world is the Mechanical Analysis of the soil, and this simply measures the size of the particles in the soil sample.
Table 1: International Classification of Soil Particle Size or Fractions
|Gravel and stones
|Over 2mm in diameter
|2mm – 0.2mm
|0.2mm – 0.02mm
|0.02mm – 0.002mm
|Suspended: (of solid particles) be dispersed throughout the bulk of a fluid. Vigorously: in a way that involves physical strength, effort, or energy; strenuously.
From the above table it can be seen that the larger particles make up the sand fraction and the smallest particles make up the clay fraction, with the silt
fraction being between the other two.
The actual mechanical analysis of a soil is a complicated process and is done in a laboratory. Farmers can send a sample of their soil to any soil laboratory for analysis.
Although the method of mechanical analysis is difficult, the principle upon which it is based is simple and depends on the fact that if a small amount of soil is put into a jar, water is added and the liquid shaken up vigorously and then left to stand, the large particles (the Sand Fraction) will fall to the bottom of the jar first followed by the medium‐sized particles. The very small particle (the Clay Fraction) will not fall to the bottom but will remain suspended in the liquid.
Try this for yourself. Take an empty jam jar, add enough soil to fill the bottom 5cm of the jar add clean water until the jar is full, cover the top with a lid and shake‐vigorously for 3 minutes. Put the jar down on a flat surface and allow the liquid to settle. After 15 minutes you should observe the following:
Figure 1: Mechanical analysis of soil fraction sizes
The mechanical analysis consists of measuring the amount of the Sand Fraction, the Silt Fraction and the Clay Fraction in a sample and expressing the result as a percentage.
Table 2: Here are 3 examples of the kind of result you might get from analysing 3 different soils:
|Total Sand Fraction
Having obtained these results, we now have to look at a Soil Texture Diagram in order to put our soils into one of the 8 textural classes of soil found in South Africa.
Figure 2: Soil Texture Diagram
Take sample No 1: It contains 61% Sand, so look along the bottom line of the diagram (marked sand) until you come to 61%. Draw a pencil line straight up the paper. Look up the left hand line (marked Clay) until you come to 31% and draw a pencil line straight across the paper. Where these 2 lines cross is in the area marked Sandy Clay loam, so this sample is a Sandy Clay loam. Repeat with the 2nd sample and the 2 lines cross in the area marked Sandy Clay. Try the 3rd sample for yourself. (Answer at the end of the lecture).
- SOIL TEXTURE
THE TEXTURAL CLASSES OF CENTRAL AFRICA
|Over 50 % Clay
|30 – 50 % Clay; under 50 % Sand
|20 – 30 % Clay; under 50 % Sand
|30 – 50 % Clay; over 50 % Sand
|Sandy Clay Loam
|20 – 30 % Clay; over 50 % Sand
|Over 20 % Silt & Clay; over 50 % Sand
|15 – 20 % Silt & Clay; 80 % ‐ 85 % Sand
|Under 15 – 20 % Silt; over 85 % Sand
Other textural classes are Silty Clay Loam, Silt Loam and Loam, but are not found here.
It is possible to get a rough idea of a soil’s texture by taking a small piece of soil, wet it and rub it between the fingers. If it feels gritty it is a Sandy Soil. If it feels soapy it is a silty soil and if it feels sticky it is a Clay soil.
3. SOIL SAMPLING
In order to get a proper soil sample either for mechanical analysis or for fertilizer recommendations, the procedure to follow is:
- Use a spade or an auger.
Figure 3: A soil auger
- The object is to get an even sample of the land.
Follow the plan shown on the diagram below:
- Sample from the full ploughing depth of the top soil e.g. 45 – 60cm. Samples are best taken after the rains have finished and the soil has dried out. When sampling for fertilizer recommendations, sample to the approximate root depth of the crop to be grown.
- Ignore minor variations and avoid abnormal items like ant heaps, drains etc. Major soil changes, e.g. areas of vlei, should be sampled separately.
- Once all the samples have been taken, spread them on a suitable area (cement), and allow them to dry and then mix thoroughly.
- Use 1kg from the mixed samples, place them in the cardboard box available from the local soil laboratory, fill in the information sheet and dispatch to the Soil Laboratory
These are samples of common soils; they are Air Dried samples and the soils have been passed through a sieve with a 2mm mesh to remove the stones and gravel. What remains are the sand, silt and clay particles.