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.
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.
