How to find out soil types

How to find out soil types by Maureen Odendaal

Good soil is not a given, soils need to be managed to remain productive. Soils which are not managed will gradually lose fertility and structure, and this will lead to reduced yield and increased problems with pests and diseases (in the same way that humans are more likely to catch a bug if they have a poor diet, so plants and animals likewise become prone to attack if they are deficient in important nutrients). 

Soils are usually a balance of clays (small particle size), loams (medium particle size) and sand (large particle size). Sandy soils tend to be infertile because nutrients cannot bond on to the particle surface and get washed through the soil quickly. Clays have very small particle size, are usually fertile, but tend to have drainage problems. Loams are good soils overall, with a balance between good drainage and good levels of plant nutrients. 

Binding all these particles together and creating the essential soil good structure is organic matter, derived from living organisms (e.g. roots and micro-organisms) and dead (i.e. decomposing biological matter). Decent soil organic matter (SOM) is absolutely essential. A good soil has between 3 and 6 percent SOM. 

Once you know more about your soils you can plan soil management with a greater degree of success. Here are some simple field and kitchen experiments which allow you to explore your soils. Simple kitchen equipment is all that is needed. Use the links below to jump to the different experiment. We’ll explore what the results mean in the next article.

 For each of these experiments you need to collect soil samples from the fields which you wish to test. Ideally several samples taken from random places. If you have a soil probe, use the method shown here in 1. A trowel or a dibble may be used instead (2.). Each sample should be 50-100g.

1. Clay, Loam or Sand?

a. The ribbon test

This simple field experiment will show you your soil type.

Firstly grab a handful of soil

Then add a small amount of water and roll the soil into a ball (as much as possible, depending upon the soil type).

Finally squeeze the ball between your fingers and thumb and push it out to form ribbon. See the diagram below which illustrated the process:

You can then judge the soil type using this table:

 b. Jam jar test

  1. Put some of the soil in a jar or glass bottle up to about 5 cm.
  2. Fill the jar or bottle with water.
  3. Stir the contents well with a stick.
  4. Let the mixture settle, watching it carefully.

2. Soil Organic Matter (SOM)

1

2 (Kelly & Rengasamy, 2006, p. 51)

3 (Kelly & Rengasamy, 2006, p. 52)

The method for this is simple. You will need an old tin can, and kitchen scales that can weigh to an accuracy of +/- 1 gram. Heating the tin on your kitchen stove can produce quite a bit of smoke, so it may be better to do this on a camping gas ring outside.

  1. Weigh the tin can, write down the mass of the empty can
  2. Half fill the can and then weigh it, write down the mass of the can + unburnt soil.
  3. Place the can on a gas (or electric) ring, on high.
  4. Heat the soil until no more smoke is produced. 
  5. Leave to cool.
  6. Weigh the can again and write down the mass of the can + burnt soil
  7. Calculate the mass of unburnt soil
    = mass of the can + unburnt soil minus   mass of the empty can
  8. Calculate the mass loss
    = mass of the can + unburnt soil minus   mass of the can + burnt soil
  9. Then calculate the % SOM using the formula below.

Percentage organic matter = Mass loss                    x 100

          Mass of soil before heating

What do your results mean?

Less than 3% means that your soils are deficient in SOM and action should be taken to improve it (we’ll explore this in the next article).

Between 3-4% SOM is good, but good management practices should be implemented to ensure that this continues.

Higher than 4% – excellent, looks like you already have great soil management practices

3. Soil pH

Knowing your soil pH (a measure of soil acidity or alkalinity) is an essential part of good soil management. Soils which are too acid or too alkaline have reduced availability of nutrients, and this leads to reduced yield.

These tests may give an indication of a potential problem, if you have any concerns professional soils analysis is highly recommended.

If you have a soil pH meter, great stuff. If not here is a simple kitchen assessment of soil pH.

Acid soil

To about half a cupful of soil, add an equal volume of water (preferably rainwater or distilled water), and leave the soil to settle (you’ll see some bubbles as air is removed from the soil). Then add a teaspoon of baking soda. If the soil fizzes this means that your soil is acidic. If the bubbling is very slight, this is no cause for concern. If the bubbles are very obvious the soil probably is too acidic and professional testing is recommended.

Alkaline soil

To about half a cupful of soil add an equal volume of vinegar. If the soil fizzes then your soil is alkaline. Very slow bubbles are no cause for concern. My soils are quite alkaline since my bedrock is dolomitic limestone. The fizzing was noticeable, so I then tested the pH with my cheap pH meter and was a bit concerned that the soil from my garden was off-the-scale alkaline. I’ve always suspected that this bed was old, misused agricultural soil, since anything I plant there struggles, so this reading gives an indication I may be correct. It is unusual for soils to be too alkaline, but this can happen if there is a build-up of salts as a result of poor farming practices (these soils are called saline or sodic soils).

We’ll explore how to control soil pH in the next article.

4. Soil Air Content.

Ideally a plastic measuring cylinder should be used for this experiment, but in the absence of this, here is a work-around.

  1. Fill an old tin can with soil from your field, taking care not to compact the soil or loosen it from the field structure. A normal 400g baked bean tin has a volume of 450 cm3(ml).
  2. Use a measuring cup with cm3/ml graduations and fill it to 200cm3.
  3. Then slowly add the water to the soil in the can, with shaking, to remove air bubbles. 
  4. Measure the volume of water left over, then work out volume of water added to the soil (200-vol remaining.
  5. Calculate the percentage air in your soil:

% air in soil = volume of water added   x  100

Volume of the soil (450)

References

Kelly, J., & Rengasamy, P. (2006). Diagnosis and management of soil constraints: transient salinty, sodicity and alkalinity. Adelaide, S. Aust.: University of Adelaide.

Warncke, D. D. (2000). Sampling Soils for Fertilizer and Lime Recommendations. Michigan State University, Extension Bulletin E498.

My next article “Maximising yield potential through careful soil management” will explore how to use the results of these experiments.


By Maureen Odendaal:

Maureen’s field of expertise is Integrated Pest Management (IPM), with BSc (Hons) in Agricultural Zoology from the Newcastle University and M.Phil from the University of Zimbabwe, where she researched systems of monitoring insect populations  and the development of cultural and biological means to reduce insect populations. She has experience working with IPM in Zimbabwe in both farming and forestry and has a special interest in working with farmers to develop bespoke, cost-effective, IPM systems which dovetail with each unique farm management system.

AGREN have a strong team of advisors, consultants and technical experts who are able to achieve appropriate solutions to complex and challenging business problems within sustainability and land management. If you would like more information, please get in touch.

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