Greywater and capillary irrigation

Not many people think about how they apply water to the garden. We have been conditioned to apply water deeply only when the soil or a plant is thirsty / dry.

The reality is that watering everyday, with a little water, provides a much better outcome (both in saving water and providing better growth) than watering heavily twice or less per week.

Capillary effects are the key. Before reviewing different methods of using graywater, we will spend a few pages on what Capillary Irrigation is, and why it is the most important concept of all.

Capillary Action

Capillary action occurs when the forces binding a liquid together (cohesion and surface tension) and the forces attracting that bound liquid to another surface (adhesion) are greater than the force of gravity.

A simple way of observing this is to take a teaspoon of water and gently pour it in a pool on a countertop. You'll notice that the water stays together in the pool, rather than flattening out across the countertop. This happens because of cohesion and surface tension.

Cohesion is the attractive force that pulls similar substances together. In this case, the individual water droplets are being pulled together. The force of the pull is strongest at the edge of the pool. The water droplets at the edge have fewer neighboring water droplets, so they cling more tightly to those around them; this is known as surface tension. Now gently dip the corner of a paper towel in the pool of water. The water is attracted to the paper and "climbs" up the paper towel - this is capillary action.

Water behaves almost exactly the same way in soil.

If the soil is dry, the surface tension of the edge of the water pool prevents the water spreading outwards quickly.