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.
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
