Ian Tolley’s Simple Soil Moisture Probe for Home Gardeners

Renowned citrus expert, Ian Tolley, shares his tips for making your own DIY Soil Probe at home.

"I first made this soil probe over 40 years ago, and still recommend it today," Ian Tolley OM

Construction

1. Obtain a piece of ½’’ (13mm) galvanized water pipe, 9” (230mm) long as a handle.

2. Ream out the pipe ends and smooth so children don’t cut their fingers.

3. The “probe” is a piece of 3/8” (9mm) rod 24” (61cm) long.
Be sure NOT to taper the end, but make it square with a soft edge. The flat end is the resistance pressure you need for measurement.

4. Drill a tight hole in the centre of the handle but only through one side.
Force the rod into the hole squarely, weld it around the join, and smooth the weld.

5. Starting from the bottom of the rod, measure and mark every 6’’ (15cm) and make a shallow groove around and into the rod.

6. Spray it with rust-proof paint (preferably in white, yellow or silver) to maintain its visibility in the field.

How to use

It will take force to push this rod into dry soil.

When it is at field capacity during watering, it can easily be pushed gently through the wetted area until it meets dry soil. You will learn to ‘feel’ the difference.

Most importantly, you need practice to gain sensitivity to the probe's soil resistance and you need to record the length of time you watered.

After one hour, push the probe down until you meet resistance (dry soil) using the rod markings to measure. Record the results. 

If you wanted to wet down to the full citrus rootzone of 18’’ (46cms). Then you need to water for three hours.

It is quick to learn to be sensitive when you push the rod into the soil.

Make it a practise to check in several places in the wetted pattern to obtain a reliable average.

As soon as you reach the wetted depth you wanted water, stop watering.

You can reliably use this probe to determine the efficiency of watering, how much water to apply and when you next need to water your soil and plants.