Soil Composition

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Soil Composition

Soils are classified according to the proportions of different sized particles they contain.
In the lab, special ultra fine sieves will be used to separate the particles, but an easy rule of thumb guide can easily be undertaken at home.
The technical term for this subject is "Soil Texture" but I think "Composition" is more expressive, so I will use it.
You will need a small sample of the soil that is typical of the area you want to test, some water (distilled or rainwater is best if your water comes from a source that is hard or produces limescale), and a tall flat bottomed straight sided glass container at least half an inch or so in diameter. At a pinch a large test tube will do, but something that shape with a flat bottom like a tall thin jam jar is ideal. The amount of soil you need depends on the size of the container. It needs to be about half full of soil.
Put the soil in the container and fill to three quarters full with water. Close the top with a lid or cork and shake it vigorously for a minute or two, so that all the soil particles are broken down into suspension in the water. Then put it somewhere to settle where it won’t be disturbed at all for a day or so, If you have a heavy soil, it might need a few days, and in this case put it somewhere dark or the algae and bacteria will start to turn it green!
What happens is that the laws of gravity take over. The heaviest (largest) particles sink to the bottom first, and the fine clay particles are the last to settle out of suspension. Organic matter which is not decomposed either floats or sinks to the surface after the clay particles. Occasionally it will settle as a band before the clay.
You will be surprised how easy it is to see the separate bands of particle sizes as they are laid down, and by using a ruler up the side if the tube as a measure and a bit of imagination, you can get a total column height and are often able to measure the individual depths of sand, silt, and clay sized grains, together with organic matter to give a proportion of each as a percentage of the whole sample.
A few soils cannot be assessed in this way. These are the peat moss or fen soils where the proportion of organic matter is perhaps more than 50% of the total sample. Another group are the chalk or limestone soils where the overriding factor is that they are almost exclusively composed of chalk or limestone. These are described in the section dealing with the characteristics of different soil types.
Knowing the composition of your soil gives a better understanding of what your soil type is and, if you are an enthusiast, it provides the baseline against which to measure the effect of future improvements. For example: how much organic matter is now left in the soil from that lot you put on two years ago?
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Assessing the composition mix of your soil
For those who want to go a stage further, you can plot your results on a three sided chart which is widely used to classify soil types. Unfortunately, although they are similar, international standards vary, so your country might be slightly different than the one shown which is my own, based on several years research and practical testing of a wide range of soils.
The diagram shows a three sided grid with each side representing the content of a particular particle on a scale from 0% to 100%. The bottom line is the sand content, starting at 0% at the bottom right hand corner, and rising to 100% in the bottom left hand corner.

texture triangle

Moving clockwise around the triangle, the line from the bottom left corner to the top point of the triangle is the clay content, starting at 0% in the bottom left corner and rising to 100% at the top.
Continuing clockwise, silt starts at 0% at the top of the triangle and increases to 100% at the bottom right hand corner. By following the percentage lines for sand silt and clay to wherever they meet inside the triangle you can determine which category your soil falls into.
To locate your exact point on the grid you should first calculate the percentage of sand, silt and clay in the total sample without including the organic matter in the total. Then take the percentage of sand as your starting point and, using the bottom of the triangle grid to find the corresponding percentage, begin to draw a line upwards and to the left (parallel with the right hand side of the triangle) at the exact percentage.
Next take the percentage of clay and, using the left hand slope of the triangle grid find your percentage point. Starting at that point, draw a line horizontally across (parallel with the bottom of the triangle) toward the right slope of the triangle. It will cross the sand line at some point.
The final confirmation that you have done it right is that the percentage of silt, located by the same process on the right hand slope of the triangle and following a line drawn downwards toward the bottom left will cross in the same point as the other two.
The intersection of these three lines will fall within one of the coloured areas on the grid, and this is your soil type.
Obviously if it is in the middle of the coloured area it will be absolutely typical of that classification, but if it is near to the edge of two areas, then it will behave with a mix of the characteristics of both soil types.

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