In a recent press release, one particular tillage equipment company mentioned that use of its latest product would encourage soil capillary action, although it did not expand on quite what soil capillary is or why this should be done.

Promoting soil capillary action is indeed an important facet of soil management, and this was one of the very few occasions when it has actually been acknowledged, let alone sold as being of benefit, so a closer look is called for.

We all know that rain falls on the ground and seeps its way down through the soil and into the water table below.

Hopefully, some of it will be caught by plants on this passage and return to the atmosphere by transpiration, bearing nutrients to the leaves as it does so.

Yet nature does not work in such a simple manner. There is always a twist or two to the plot and one of the major interruptions to this neat little scheme is the phenomenon known as capillary action.

And it is no minor occurrence either, for the capillarity of soils has a significant effect upon their properties. In agriculture, it is a feature that is frequently ignored when discussing soil structure and degradation.

Drainage is priority

The reason for this oversight is that the major concern of farmers is drainage, and where the overriding concern is water removal it is not always appreciated that moisture retention is also important.

This is where some understanding of the forces within the soil at the microscopic level comes into play.

The physics behind capillary cation can be quite involved, yet there are two main forces, cohesion and adhesion.

Drainage ditch — In the rush to ensure fields are well-drained, moisture retention is often overlooked

Clay and organic particles will add a third, electrolytic force, but that can be put to one side for the moment.

Basically, cohesion refers to water molecules sticking together while adhesion is the attraction between water molecules and a solid surface, such as a grain of sand.

It is the cohesive forces that form internal bonding, causing falling water to form drops rather than just dissipating in the air – as we can see when taking a shower. This internal bonding is also visible as surface tension on pools of water.

Adhesion, on the other hand, is responsible for water clinging to solids, something we experience every day, and it is what forms the meniscus around its edge when in a container such as a cup.

Physics of the teacup

The meniscus is created by the water molecules sticking to the solid cup side. The adhesive forces of water try to cover the whole surface of the cup, but they are held back by the internal cohesion and gravity, so the effect is very limited.

However, reduce the diameter of the cup to a very thin tube and the adhesion is better able to overcome cohesion. In this case, the water will rise up the tube, which is known as the capillary action.

Surface tension is the force that forms this droplet and holds it in shape. Source: Michael Apel

This is capillary action at its most basic, but the effect is real enough in a complex soil matrix where the pores between the soil particles can act much like a tube and draw water up, and the same principle applies.

The smaller the pores, the greater the capillary effect.

Yet soils are a mixture of particles of various sizes, which for the purposes of this discussion can fall into one of three types, sand, silt, and clay.

Sand has a very low capillary action and will therefore drain easily. Silt is made of smaller particles and so has moderate capillarity, which will tend to resist drainage, while clay has very fine particles and holds on to the water, forming pools on the surface.

Naming the soil

A mixture of the three is what we know as ‘loam’, which is usually prefixed with the dominant constituent, such as a sandy loam.

This is not an ideal method of nomenclature, for clay can have a distinct effect at a low concentration, allowing clay loams to have a wide spectrum of properties whilst still sharing the same name.

The ratio of inclusion of each particle will dictate the nature and property of the soil, which may then be modified by the amount and type of organic matter that is also present.

Soils vary considerably in countries such as Ireland, with mixed geology on a local scale, something that is becoming more apparent as yield maps and drone footage highlight the differences in productivity within fields, let alone across whole farms.

The magic of muck

Despite this, there is one measure that will universally improve the soil properties whatever its make-up, and that is the addition of organic matter.

This has the affect of aggregation, which is the formation of larger particles through the clumping together of smaller ones.

Slurry tanker — The addition of organic matter will encourage soil aggregation and so aid in the retention of moisture during dry periods

This may sound counter-productive, because it appears to be mimicking the action of sand grains.

Yet in reality the peds – as these aggregations are known – are porous and can draw water into themselves while allowing the larger spaces between them to contain air.

Creating a soil that aggregates through microbial action would be the ideal for cropping and grassland, as it displays the virtuous properties of all three soil types and its formation through careful management can only benefit yields.

The devil of compaction

A large part of that management is the avoidance of compaction, for this will force the soil pores to close up and in doing so create a homogenous mass rather than an aerated matrix.

In such a situation, the lack of porosity will deter root growth while anaerobic conditions will discourage the micro flora and fauna that maintains the health and productivity of soil through the creation of its structure and the release of nutrients.

Smearing of the soil by tractor wheels will create a barrier to to the capillary action along with capping the soil, which prevents the circulation of air.

The avoidance of damage by either suggests that minimising the amount of fieldwork done by tractors, or reducing the ground pressure applied when carrying it out, is of major importance to soil health.

Tractors have been getting bigger and heavier, which may increase efficiency but will cause more harm to the soil. As always in agriculture, it is a question of finding a compromise that works.

Drainage is priority

Physics of the teacup

Naming the soil

The magic of muck

The devil of compaction

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Loam

Capillary action: The forgotten factor of soil dynamics