Soil moisture sensing – for smarter irrigation.

Soil moisture sensors

Harvst soil moisture sensors are used in conjunction with our smart garden control system to provide a better watering regime for your plants.

Water less when the soil is already moist.
Water more when the soil is dry.

You can choose the wet point and dry point in your Harvst control app, depending on your soil type, your sensor position, and the plants you want to manage.

How do Harvst soil moisture sensors work?

There are a number of ways to measure soil moisture. The most basic is to measure the conductivity of the soil; water conducts electricity, so the wetter the soil, the lower the resistance. However, various things such as salts, acidity and even plants roots can affect the conductivity of the soil, so this method doesn’t always give good results.

An industrial method is to use tensiometers which measure suction pressure at the tip of the sensor, replicating how hard a root would have to work to get water from the soil around it. These work well where soil is fairly moist, but less so when the soil gets dry, and are very high maintenance. They are also outside the budget of the domestic smart gardener.

We use a method called capacitive soil moisture sensing, where an electric field is set up around the sensor to measure how the soil reacts to the field. It’s the same method that’s used by other leading smart watering products. While better than most methods, it’s still not perfect (none are) … getting the installation right is critical to getting good results.

How to install Harvst soil moisture sensors

You need to make sure that as much soil as possible is in contact with the active side of the sensor; if you wiggle a sensor around in the soil then it will have an air gap around it, causing it to under-read the moisture. Imagine you’re transplanting a seedling; insert the sensor and push gently around the base to firm up the soil.

Soil moisture sensing readings
A typical period of soil moisture readings coupled with watering events from our data logging service.

Interpreting soil moisture sensor readings

Our sensors output a number between 0 and 100. This does not correlate directly to the percentage of water in the soil, but it’s a useful approximation for the purposes of domestic watering. Each installation will give slightly different readings, due to soil compression, salts, fertilisers, and other impurities in the soil … so we can’t definitively say that “xx% is too dry” or “xx% is too wet”. However we have seen some pretty good ball park figures for determining the ‘dry point’ and ‘wet point’ for your watering.

When installed correctly, a reading of 30% would suggest that plants need more water. At 20% it’s almost certain they need water. The top end is more variable, and will need you to test your particular soil to determine the ‘wet point’ – the point above which you don’t want to give the plant any more water.

Harvst soil sensor technical specifications

  • 20 x 120mm sensor dimensions.
  • Supplied on 1.2m 4-pin cable with waterproof plug.
  • RS485 (Modbus) communication.
  • Programmable with a slave ID from 1-247.
  • Multiple sensors can be run on the same communications bus.
  • 12v power supply with on board regulator.

If you’re interested in commercial and farming soil moisture sensing, this is a good article to start with.