Thursday 3 September 2020

So, What is Soil Moisture


I recently heard a gardening segment on a NZ radio station. The gardening commentator was answering questions and providing advice on ‘how to irrigate your garden’. Her advice was: “Deep watering will encourage the roots to grow into the water table below. This is desirable as it allows the plants to be self-sufficient in accessing water.”

We all have our own perception of water and how it is stored in the soil, but the gardening commentator’s description isn’t an accurate description of what actually happens within the soil or what we are aiming to achieve through irrigation.

There are several processes at play when water is “stored” in the soil: 

·        cohesion - the attraction of water molecules (H2O) to one another it causes water molecules to stick to one another and form water droplets;

·        adhesion – the attraction between water molecules and solid surfaces, in this case soil particles;

·        surface tension – as a result of the cohesive properties of water molecules and their attraction to other water molecules, a water surface behaves like an expandable film; and

·        capillarity – is a combination of cohesion/adhesion and surface tension forces and is the primary force that enables the soil to retain water and to regulate its movement.

In this article we will take a closer look at these terms and and apply the concepts to soil moisture storage.

To demonstrate or understand adhesion and cohesion, pick up a rock or stone, dip it into a pool of water, pull it out again. The water dripping off the rock is free water (lost to gravity, same as free water will be lost to drainage when soil is at saturation point). If you give the rock a shake you will free it of more water - this is the water “stored” by cohesion. The rock is still wet even after the shaking - the water left on the rock is “stored” by adhesion (Figure 1). Water is stored in this way on all soil particle surfaces, whether it be a clay, silt, sand or gravel particle.

Figure 1 Soil moisture is stored on soil particles like a film via adhesion. On this stone adhesion is demonstrated by dipping it into water solution containing blue dye.

Figure 1: Soil moisture is stored on soil particles like a film via adhesion. On this stone adhesion is demonstrated by dipping it into water solution containing blue dye.

Capillarity is the key to storage of water in the soil. It allows water to move upward (and through) soil pores against the force of gravity. The finer-textured the soil (silts and clays) the greater the ability to hold and retain water in the soil in the spaces between particles. The pores between small silt (less than 0.02mm diameter) and tiny clay (less than 0.002mm diameter) particles are known as micropores. Compare these to the larger pore spacing between larger particles, such as sand (0.2-2mm) and stones (larger than 2mm) which are called macropores. Micropores enable greater capillarity rise.

Capillarity can also be simply demonstrated by placing a dry sponge into water – it will progressively wet upwards through the sponge (Figure 2). The finer the sponge material the higher the water will wet the sponge.

Figure 2: Fine sponge placed into a dish with water solution containing blue dye demonstrating capillarity.

When we irrigate, we want the water to have the opportunity for adhesion and capillarity to take place; i.e. “coat” the soil particle surfaces with water and be retained in the micro pores by capillarity this is best achieved through low application rates and by matching the applied depth to soil moisture deficit.

Back to the garden commentator’s recommendation to practice deep watering and aim to push roots into a water table. Very few farmers/growers/irrigators will have a water table shallow enough for roots to reach the water table. When roots explore the soil profile, they form perfect contact with soil particles, via this contact they can extract the moisture stored on particle surfaces. Deep watering is accurate to an extent. We want roots to explore as much soil as possible as this allows them to access more water and nutrients. Roots will only grow in moist soil, so they’ll only explore the soil profile if it’s been wetted. However, it is unusual for the subsoil not to be moist enough for root growth as the plant advances through its growth stages. Irrigation should therefore only be aimed at wetting the soil within the active root zone.

Aquacheck sensors measure soil moisture at several depths. This depth profile is a very useful tool in managing your irrigation. It allows you to see if you are wetting the active root zone and whether the subsoil is wet enough to allow for root growth.

Jane Robb 

Vantage NZ Customer Support Specialist