Tensiometers provide an easily interpreted guide to soil moisture levels.

A tensiometer is a closed tube filled with water, with a ceramic tip at one end and a vacuum gauge at the other. As the soil dries out, water is drawn out through the ceramic tip, creating a vacuum in the tube. When irrigation or rainfall occurs, water is drawn back into the tube, decreasing the vacuum. A high vacuum reading on the gauge indicates that the soil is dry, and a low reading shows that the soil is moist.

Preparation

1. Fill the tensiometer with clean water, leave the cap off and allow to drain through overnight. This saturates the tip and confirms that it is working. Do not handle the tip.
2. Fill the tensiometer with a mixture of rainwater (l litre), methylated spirits (50 mL), and a few drops of food dye. Use a vacuum pump to remove any trapped air.
3. To test the tensiometer, replace the cap and leave the tensiometer out of the water for a couple of hours. The reading on the gauge should rise.
4. Place the tensiometer into the bucket of water. The reading on the gauge should drop within half an hour. The tensiometer is now ready for installation.

Installation

1. Make a 25 mm diameter hole that is 100 mm shorter than the tensiometer to be installed. Make a 19 mm diameter hole for the remaining 100 mm
2. Push the tensiometer firmly into the hole. Fill the hole with loose soil and pack it down. Heap the soil up around the tensiometer.
3. The number and length of tensiometers installed at each site depends on crop type and root depth. Tensiometers should monitor the moisture levels in the rootzone and just below this.

Covering tensiometers helps prevent frost and physical damage and reduces algal growth in them.

Maintenance

With regular maintenance tensiometers will provide years of reliable service. Every couple of weeks, loosen or remove the cap to allow any air bubbles to escape and to allow water to fill the tensiometer tube. A length of 3 mm tube can be used to dislodge any air bubbles. Top up the reservoir if necessary. Replace the cap. Do it up until firm, then tighten it half a turn. Do not over-tighten! Perished stoppers cause problems. Replace the stoppers annually.

Interpretation

Reading	Interpretation
0–8	Soil is saturated (0) to near field capacity (8). Continued low readings indicate waterlogging.
8	Field capacity.
8–25	The best conditions of soil moisture and aeration.
25–35	Consider irrigation at critical stages of crop cycle.
35–50	Mild stress on well-drained soils.
50+	Soil is very dry: this will affect crop yield and possibly fruit quality.

Recording

Tensiometers should be read and recorded regularly, 2–3 times per week in summer. Ideally the figures should be graphed to provide a picture of soil moisture movement.