Soil Moisture Probe Position

Soil moisture probes can be fantastic tools, enabling growers to monitor hourly fluctuations in soil moisture at various depths in the soil. However, to obtain accurate and representative data, representative soil moisture probe positions are essential.
A probe must be installed in the root zone of a representative tree and in a typical wetted area. Probes monitor a few centimetres of surrounding soil, and the data captured needs to give readings accurate enough to irrigate several hectares of orchard.

If you find yourself doubting the accuracy of your probe’s data, there’s a good chance that it isn’t in a representative position.

In this image, I have highlighted some placement errors that can lead to untrustworthy readings, along with fixes for each issue:

1. Dripper lines pulled together.
– Issue: This creates an overly wet area around the probe, not representative of a single “pot’s” soil moisture.
– Fix: Position dripper lines correctly. Place the probe about 10-15 cm away from a single dripper to monitor a single “pot”.

2. Dripper spliced in (red arrow) near the probe.
– Issue: Adjusting dripper spacing near the probe creates a wetter than usual area. Blue dots indicate the drippers.
– Fix: Cut out a section of pipe (~5 drippers long) and splice it in to maintain consistent dripper spacing.

3. One spaghetti pipe redirects water towards probe.
– Issue: As with example 2, dripper spacing is adjusted and the soil at the probe will be wetter than the rest of the orchard.
– Fix: Also place spaghetti pipes on the adjacent drippers to retain the dripper spacing.

4. Probe placed in a ‘water shadow’ & near an undersized tree.
– Issue: Obstacles like poles or low hanging branches can create dry zones. Smaller trees also take up less water and will result in soil’s being wetter.
– Fix: Move the probe to monitor a representative tree. If just the pole was the concern, move the micro to the other side of the pole. Never place micros on each side of a pole.

5. Probes proximity to poles.
– Issue: Poles too close to the probe can disrupt the distribution of water and effect the soil moisture readings.
– Fix: Move the probe to a representative position.

6. Too many emitters near the probe.
– Issue: Additional micros at a probe will result in the soil being wetter. Additionally, placing the probe in a ‘young tree’s’ position can misrepresent the mature root zone.
– Fix: Adjust the emitter count and plan to reinstall the probe post-harvest. Ideally, there should be a triangle between the probe, stem and micro.

Takeaway:
If you are questioning the data from your soil moisture probe, inspect the placement in the orchard, check the spread of water from the emitters and the operating pressures at the emitter.

When in doubt, dig a hole (but not right next to the probe).