Yet this is a very important factor: overdoing irrigation causes nutrient loss and wastes money on pumping.
As previously discussed, depth of wetting depends on the soil type, humus content and how dry the soil was initially. There are several ways to ascertain how far the water has penetrated:
- If the crop has shallow roots, dig into the soil with a long garden trowel. (I am never without such a trowel in my hand – it’s invaluable for examining the soil.)
- Wet soil has a much lower resistance to physical penetration than dry soil. This can be felt by pushing a soil probe down into the profile. Every farmer should have a soil probe; it can also be used to check for hard and compacted layers in the soil profile.
- Tensiometers can be useful in some cases. These are set into the soil at various depths depending on the crop being produced. As plants and evaporation extract water from the soil, the vacuum inside the tube of the tensiometer increases. So the tip of the tensiometer acts like a root, with the soil pulling on it with greater tension when dry. The force is indicated on a gauge above the soil surface. You can use a tensiometer to show you when to irrigate. This tool is probably even more useful with a perennial crop.
Study the leaves
The best way for a vegetable farmer to evaluate water stress is to inspect the condition of the leaves. This requires some practice, but is certainly worth the effort.
Under water stress, leaves usually take on a duller appearance and a darker colour. In contrast, plants that have received enough water will have a sheen and a greener, brighter colour.
In addition to the condition of the plants, which can vary, examine stony patches and the edges of the land, which tend to dry out sooner. Also check the leeward (downwind) side of your land.
If the wind was blowing the last time you irrigated, less water might have fallen on the crop and stress symptoms will appear sooner. Note the differences in the foliage between the stressed and unstressed areas and look out for this when observing crops in future.
Blocked sprinklers or sprinklers that fail to rotate are also useful indicators to help one compare differences in foliage between stressed and unstressed areas.
Reading the early signs of water stress
Once you have learnt to identify the early warning signs of water stress effectively, you will be able to look at a crop and immediately determine whether it requires irrigation.
When the crop becomes stressed, there are physiological changes that induce the plant to slow growth towards a survival mode and recover into rapid growth again only later.
This results in a loss of production, especially for leafy crops.
Before you get to the stage of being able to ‘read’ foliage accurately, don’t take a chance.
Check the wetting depth a day after irrigating the land by using a trowel or a soil probe.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.