Irrigating your home garden

If you apply too much water, you’re in danger of leaching nitrogen out of reach of the plant roots, says vegetable expert Bill Kerr.

If using sprinklers, install them at the ends of the bed as well, so that the plants here receive water from both sides.
Photo: Torange.Biz

In my experience, very few home gardeners irrigate their vegetable patch correctly. This is often true of the rest of the garden as well.

A good friend of mine once asked me to visit her vegetable garden to see why she was having such poor results. Suspecting that the area was overwatered, I asked her how often she irrigated it. “Daily,” she replied, “and I use a hand-held hose.”

She was doing almost everything wrong. To irrigate successfully, you need to know the following:

  • How much water you are applying. To measure this, collect empty food cans and place them in various positions before irrigation. Then time how long it takes to apply about 30mm of water.
  • How deeply the water you apply penetrates the soil. As a rule of thumb, 1mm of water will wet 1cm of soil. With a high clay content, less is wetted; conversely, more is wetted if the soil is sandy.
  • How deep to irrigate for the plant’s particular root system. Carrot roots, for example, penetrate very deep; cabbages and lettuce roots are much shallower; beetroot roots are in-between.

Results of overwatering
If you water excessively, the nitrogen in the soil is likely to drain away (leach) so that it cannot be accessed by the roots of the plant. This will result in poor plant growth.

In addition, by irrigating too often, you fill the pores in the soil with water, displacing air. Roots need to breathe, and a lack of oxygen restricts or even prevents root growth.

Insufficient oxygen in the soil also creates a semi-anaerobic environment that restricts beneficial soil organisms and favours those that cause root disease.

Yet another negative effect is that anaerobic bacteria remove oxygen from the nitrogen molecules, converting the nitrogen to dinitrogen. This is the form of nitrogen that leaves the soil and enters the air.

Precise control
Bearing all this in mind, it is far preferable to set up an irrigation system where you can control the amount of water applied.

If you use sprinklers, ensure that the edges of the wetted areas touch each other, as this overlap will ensure that all parts are reached and water is applied more uniformly.

Install sprinklers at the ends of the bed as well, so that the plants here, like those in the rest of the bed, receive water from both sides.

Checking moisture
You should also take into account the moisture content of the soil before irrigation, and irrigate accordingly. Check the moisture level of the soil by digging into it with a garden trowel or thin, round bar.

When soil is moist, it has less resistance to pressure, and vice versa. You can also use this method to discover how deep each irrigation penetrates.

Repeat all over the garden to determine how long sprinklers need to work in order to supply enough water without wasting it.