Soil – Dishing the Dirt

How to Analyse your Soil

Soil is special. It is far more complex than simply the dusty stuff we try to keep the children out of, wipe off our shoes, try to keep out of our homes and hide beneath lawns, paving and shrubs. It is not a sterile, stagnant medium but a wonderful interaction of big plants and small; of animals from large to microscopic; of chemicals in solution and in storage; of stagnant air and wind and of water, trapped, flowing and percolating, all within a wide range of temperatures.

Soil is not only the product of climate, topography, source material and time. It also needs a fifth component: life. And life needs space. A soil’s characteristics are determined by the relative amounts of fine, sticky clay, smooth-flowing silt and coarse sand. When these components are in more or less equal quantities the soil is ideal and is called loam. If the soil is very fine with too much clay, it packs closely and leaves little space for anything else. If the texture is too gravelly with too much sand it’ll be unable to retain anything long enough before the water, seeping through the matrix, carries everything away.

Soil texture determines, to a large degree, its water retaining characteristics which in turn determines what nutrients are likely to be present. Once the texture is known it is seldom necessary to proceed to a complete chemical analysis because remedying the texture will go a long way toward correcting any chemical imbalances.

Texture Testing

• Collect two scoops of soil from root level. If an analysis is required over a large area, collect smaller quantities from representative sites and mix them together. The surface soil is less suitable because rain and watering carry the finer components deeper.
• Spread the soil out in the sun, breaking up any large clumps.
• When it is dry, sieve about a cup full into a straight-sided jar with a screw-top lid.
• Add a tablespoon of dish washing liquid.
• Fill the jar with water, screw the cap on tightly, and shake it vigorously for three minutes, making sure nothing sticks to the sides.
• Once this mixture has been properly shaken up, set the bottle aside and let the contents settle. The bigger, heavier sand particles will quickly fall to the bottom to be followed, in succession, by the silt and the clay. The silt will have settled within the hour but the fine, light, clay particles may take a day or two.
• At the end of the settling process the clay, silt, and sand components of the soil will effectively have separated and will be lying in separate layers.

Doing the Maths

What we need to calculate is the percentage each of the constituents contributes to the whole.
• Measure the combined thickness of the three layers. This represents 100 percent of the soil in the bottle.
• Next measure the thickness of the bottom layer – the sand;
• Then the middle layer – the silt;
• And finally the top layer – the clay.
In the pictured sample the overall thickness of the sediment is approximately 71mm, with the sand layer measuring 57mm, the silt 13mm and the clay another 1mm.
• Divide each layer’s thickness by the overall total. Then multiply the answer by 100 to get the percentage.

From the Featured Example

Divide the sand thickness of 57 by 71 (the combined thickness). The answer is approximately 0,8 which, when multiplied by 100 gives a percentage of just over 80 percent. The silt calculation would be 13 divided by 71 multiplied by 100, giving just over 18 percent; and the clay would contribute 1,5 percent.

Soil Analysis Triangle

Analyse your soil by following these steps. (In this example the figures derived from the above exercise are used.)
Step 1 Our sample showed about 80 percent sand so make a mark at the 80 percent point on the soils triangles sand axis. Draw a line parallel to the silt axis, toward the clay axis.
Step 2 On the clay axis estimate the 1,5 percent (the clay content of our soil sample), and extend a horizontal line toward the silt axis.
Step 3 From the 18 percent point on the silt axis (sample’s silt content) extend a line, parallel to the clay axis, toward the sand axis.
Step 4 These three lines will merge in the sand area of the soil analysis triangle. This is typical of many soils in the south Western Cape, where this sample was taken.

The triangle has not only identified the soil texture, but also indicates a corrective course of action. The simplest way to move this sample intersection point towards the loam portions of the triangle would be to add silt or, if this proved impractical, a rich compost to assist in retaining moisture in the sand.

Another solution, adopted by many Western Cape gardeners, is to choose plants that like a sandy, well-drained soil. Any soil falling into the loam areas are suitable for gardening. Soils with too high a silt or clay content and too little sand, may need some organic material (or sand) to help them drain properly. Good compost helps prevent clay soil from clumping, and it adds the needed fertility to fine silty material.  

Regardless of whether the garden is a container or an estate, this simple, low-tech soil analysis method will provide every gardener with valuable information. The soil’s composition not only indicates the most suitable plants but also possible solutions to many problems.