There are several types of instruments that can be used to measure changes in soil water. Two types are most commonly used:
- Tensiometers which are considered more accurate
- Moisture Probes which are mainly useful as a rough guide.
A Tensiometer is comprised of a hollow plastic tube, tipped with a porous ceramic cup, and connected at the other end to a dial gauge or Manometer.
The tube and porous cup are filled with water and sealed. Tensiometers must be installed into soils at field capacity. To install, drill an oversized hole to the required depth and insert the instrument carefully. Pour back some of the excavated soil and tamp gently around the tensiometer until the soil is back at the ground surface. In all cases, read the instructions provided with the Tensiometer before installing.
Tensiometers respond to changes in the tightness (tension) that the water is held in the soil. If the soil is saturated, the gauge reads zero. Most plants should be watered when the tension is no more than 50-60 kPa.
Watering according to tensiometer readings is relatively simple. It’s just a matter of turning on your irrigation system when the tensiometer reaches a chosen maximum tension figure.
You should water shallow rooted vegetables in sandy soils when the tensiometer reads 25 - 35 kPa. For plants growing in loams, use a trigger point of 35 - 50 kPa and for heavier soils use a higher trigger point of 50 - 60 kPa.
Leaving the irrigation system on for the time you have calculated using the RAW values or know from experience will bring the plant root zones to field capacity.
Moisture probes are a cheaper option for putting numbers on the level of soil moisture but are notoriously unreliable in lots of situations. For example, if a soil has a very low level of soluble salts, the meter will read near zero (or dry), when the soil is saturated with water. Readings are generally unreliable in sand and higher than they actually are with clays.
In spite of this, I have used moisture meters to good effect during my career to identify failings in irrigation systems so their use should not be entirely be discounted. Also in the ARDC Orchards, we regularly take moisture meter readings at set spots around the orchard to help us to objectively measure the need to irrigate the trees.
I can recall a classic example of the value of using a moisture meter instrument when I was inspecting David Gregory’s lavender crop in the Blue Mountains foothills just outside Sydney. See the article Adding Value to Lavender.
David pointed out some dead lavender plants in otherwise blooming rows of lavender and asked me if I could work out why they died. I happened to have a moisture probe in my truck and when I pushed it into the soil under the blooming plants, the reading was 85%. However, when I pushed the probe into the soil under the dead plants, the reading was 5%. It was a simple as that - David’s irrigation system involved mini-sprinklers and the sprinklers in the vicinity of the dead plants had clogged up and were inoperative.
Tensiometers and Moisture Probes
