A simple way of judging the depth of wetting after irrigation is to use a soil probe (sometimes called a push probe). You push the probe into the soil at points in the field and assess the soil's resistance.

The probe can be made from 10–12 mm diameter steel rod, about 1.2 metres long, forming a T-handle at one end and a sharpened tip at the other.

Make the base of the tip piece slightly wider than the rod by building up some steel weld and then sharpening the tip, or by welding a sharpened bolt to the end of the rod. The bulbous tip means the diameter of the hole in the soil is slightly larger than the rod: this reduces friction on the side of the rod, so that the only resistance when pushing through the soil is at the tip of the rod. Mark 20-cm intervals along the probe.

Using the probe

To determine the depth of water penetration during or shortly after irrigation, simply push the probe into the wetted soil. It easily penetrates the wetted profile but finds resistance to penetration when it reaches dry soil.

The probe works very well during irrigation when the water has penetrated 0.2–1 m and is still in fairly dry soil. It is not sensitive when the soil is already quite wet throughout the profile, because there is very little difference in resistance.

The probe does not work well in fine-textured or dense subsoils.

Measure the penetrated depth by reading the marks on the probe. If you repeat this procedure systematically, you will get a good idea of water penetration throughout the field and can then plan irrigation set times. A probe can also be useful to measure sideways spread into the beds when you are looking at furrow irrigation.

When using the probe to determine when to stop irrigating, remember that after irrigation the wetting front will continue to move downward for several days. Minimise drainage below the root zone by ceasing to apply water before the wetting front has penetrated the full depth of dry soil in this zone.