Soil water, soil drainage and plant growth

The way soil water moves and how it affects the soil and plant growth are important principles to understand.

When rain falls on the soil or you water the garden, several important processes begin. The first is infiltration, which is the movement of water down into the soil. The infiltration of soil water is strongly influenced by the structure and friability of the soil.

However, if the rainfall is very heavy, or you water the garden too much, water will begin to run off the surface. If you have ever walked across wet grass and heard a sloshing sound beneath your feet - this is an indication that water is beginning to flow across the surface.

As moisture penetrates the soil profile, it can 'leach' nutrients downwards - eventually taking them beyond the reach of plant roots.

The downwards percolation of soil water could be halted at some point by an impervious layer (if present) and water will accumulate above this layer to form a perched water table. However, water can also move upwards in the soil (above the water table) by "Capillary action". This is the process by which soil water is drawn through small spaces by its own surface tension.

You can see this by getting several straws or tubes of different thicknesses and dipping one end of each of them in water. Water will be drawn into the tube a certain distance above the level of the water that the tube is in. The narrower the tube, the higher the water will go.

Since soil water dissolves salt, water that moves up into the root zone by capillary action will bring salts with it. At some point, plant roots and surface evaporation will remove water from the soil, so a shallow water table with a capillary zone above it that extends into the root zone will lead to salts being concentrated near the roots of your plants.

This may restrict their growth and even kill plants that are sensitive to salt. To prevent this, it may be necessary to increase the depth of the water table by installing subsurface drains. This usually takes the form of a series of porous pipes running parallel to each other, down a gradient to a point where the water can be discharged.

It is important to ensure that if your water table is 1.2 metres (about 4ft) or less from the surface, that subsurface drains are installed before any other soil improvement is undertaken. This is because the improvements in soil structure can increase the capillary zone above the water table. The result is that soil improvement measures may (under these circumstance) increase the likelihood of salting within the root zone.