Determining the soil’s suitability for mole drainage
Simple tests can be carried out with the soil samples collected at mole draining depth. The tests will determine whether the clay content and its type is suitable for sustaining mole drains (several years life) or whether gravel mole drains are required. These are the ring, ribbon and ball tests. If the soil sample has dried somewhat, add some water and knead the sample until it has the consistency of plasticine or putty.
Ring Test
The ring test will indicate the clay content and possible presence of sand or small stones. Roll the soil sample between your palms into a rod-like shape of approximately 10 mm diameter and long enough to form a ring of about 50 mm diameter when the ends are brought together. If the ring is hard to form (continually cracks or falls apart), it probably will not hold a mole drain as its clay content will not be great enough to sustain a mole channel. If easily formed, clay content is high and possibly suitable for moling. Now check this soil for slaking or dispersion with the ball test.
Ribbon Test
Soil aggregates in distilled water (left) and distilled water plus gypsum solution (right)
Alternatively, the rod formed above could be squeezed between the thumb and forefinger to form a ribbon with a width of about 10 to 13 mm and thickness of about 4 to 5 mm. If a long ribbon can be formed (greater than 40 mm in length), then the soil has a high clay content. If it continually breaks, clay content is probably too low for mole draining. Check for slaking or dispersion using the ball test.
Ball test
The ball test indicates whether a soil is prone to slaking and/or dispersion which will indicate its ability to hold the mole channel shape.
Slaking is the breakdown of the soil aggregates into much smaller aggregates when wet and indicates low organic matter in the soil.
Dispersion is an indicator of sodic soils and occurs when excessive sodium (sometimes magnesium) is present. When water is added the sodium attaches to the negatively charged clay particles. As this is only a weak bond, clay particles are forced apart. This results in a cloud of clay colloids forming around the aggregate pictured above. These soils are very prone to tunnel and gully erosion if mismanaged and often difficult to drain safely.
In Figure 1, the petri dish on the left has three surface soil aggregates at the top and three subsoil aggregates at the bottom. Note that no dispersion has occurred for the surface soil but that strong dispersion has occurred with subsoil aggregates. The petri dish on the right has surface and subsoil aggregates placed the same way in a gypsum solution. No dispersion has taken place due to the immediate electrolyte effect of gypsum.
Aggregates remoulded, gypsum in right dish
The same aggregates as above have also been remoulded (representing ploughing) and then placed into distilled water in the left petri dish. Note that strong dispersion has now occurred with the surface soil as well as the subsoil. Again, no dispersion has taken place in the gypsum solution in the right petri dish.
Although gypsum is very efficient at reducing dispersion in the upper layers of the soil profile, it is unlikely to penetrate to the depths where mole drainage occurs. Expert guidance is needed when designing a drainage system for dispersive soils.
To test for dispersion knead a sample of the soil into 10 to 20 mm diameter balls and place them gently into distilled water. Observe them regularly for half an hour and if not actively showing any signs of slaking or dispersion, inspect them after two hours and again after 24 and 48 hours.
A soil which slakes or collapses quickly (within hours) is likely to be prone to slaking and, if wetted up after mole drains are installed, will reduce their life substantially. Gravel mole drains may be an option in these soils. A soil may disperse quickly or slowly and is recognised by a cloud of clay colloids around the clay sample which moves away into the body of the water when disturbed. Many soils may slake first and then disperse. Dispersive soils are a problem for any drainage system so seek expert advice.
A whole farm plan for any likely future extension of the drainage system is crucial to allow for suitably sized main drains and outfall drains. Land topography must be considered also such as water moving down slope to the flats and occurrence and frequency of flood events.
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