Guide to soil moisture sensing
Using soil moisture to drive an automatic watering system is a great way to save water, and to make sure that your plants don’t get too little or too much water.
However, soil moisture sensing is not a precise science, even at a commercial level where operators are trained and installation is precisely controlled. With domestic level equipment, you’ll need to understand how it works to get the most from your system.
Various things will affect the reading you get from your sensor; not only the moisture in the soil. Things affecting readings can include soil salts, fertilisers, the type of soil, root density around the sensor, even how tightly the soil is packed around the sensor.
Be patient and make controlled changes, and over a few days you will get a feel for how your system works.
Connect to the sensor to get live readings
In order to do the tests later in this guide, you’ll need to connect to the device with your phone to see the readings.
- Open the sensor
- Press and hold the button until the light stays on continuously, then let go (it will flash for about 3 seconds first)
- Log into the sensor WiFi network; it will be “sensor-XXXX” with the X being unique to your device.
- If you’re not automatically taken to the dashboard, open a browser and go to http://192.168.4.1
- You’ll see the reading on the dashboard. This will update every second or so.
If you’re struggling to connect to the dashboard (your phone keeps dropping the WiFi network), turn off your 4G. Sometimes Safari on an iPhone causes trouble; Google Chrome is more reliable. The sensor WiFi network does not have internet access; this is normal.
Learn how the soil sensors work
- When the sensor is in the air, you’ll see a reading near 0%
- Hold it tightly with your hand; you’ll see the value go up. This is the sensor detecting moisture in your skin.
- Place the sensor in dry soil. Note the reading.
- Compress the soil around the sensor and take another reading; it should go up. This is an important thing to get a feel for.
- Place the sensor in moist soil.
- Compress the soil again to see the reading change.
Dry point
This is the moisture level below which you would like to turn watering on, if you have an automatic system connected. Experiment with different soil moistures to establish the dry point for your soil.
Wet point
This is the moisture level above which you don’t want the timer system to be watering, because the soil is moist enough.
How to place the soil sensors
The sensor needs to be fully inserted into the soil up to the casing. To measure moisture levels deeper in the soil, you can bury the casing – but the range will be reduced.
In paired mode where the control box is in the same greenhouse or polytunnel this won’t be a problem, but in WiFi mode where the router is in the house further away, you might find that burying the sensor takes it “out of range”. The sensor will beep 5 times if it can’t connect to WiFi.
Put your sensor in an area which represents the general dryness of the area you are watering. If you put it under a dripping nozzle, it will read wetter than the rest of the area and your plants may be under watered. If you place it in an area that is not reached by the irrigation, it will read dryer, requesting more water, and your plants may be over-watered.
Don’t keep moving the sensors
Once you have placed your sensors in the soil, wait a few days to see a moisture trend line on the chart on your app dashboard (this works far better on a computer screen than a small phone). If you remove and replace the sensor, you’ll probably see spikes and steps which confuse things.
Don’t disturb the sensor
If you wiggle the sensor it will create and air space around the probe and won’t measure the actual moisture of the soil properly.
Soil moisture sensor FAQs
My soil moisture goes UP when I have not done any watering.
If moisture condenses from the soil around the sensor, the moisture reading will go up. This can happen at the end of a hot day when the temperature drops and the soil “breathes”. The reading should go back down again when the condensed moisture returns to the soil or evaporates – usually the next day when things warm up again.
Can I use readings from multiple soil sensors to trigger watering?
Not at the moment. We are considering an approach which averages the reading across multiple sensors to trigger watering. This would be rolled out as part of an automatic firmware update.
How many sensors can I pair with a control unit?
Up to 4 wireless sensors of any type.
