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Harvst smart control systems come provided with two soil sensors, which measure soil moisture and temperature. The moisture reading from the sensor can be used to give plants extra water when they are dry, and turn off timer watering if the soil is already wet. The temperature reading can be used to turn heaters on or off, and to regulate the temperature of our heated seed trays. All our greenhouses will measure temperatures, but only the mains powered models will be able to change the temperature via heating.

To get the most from your soil moisture readings, you will need to install the sensors carefully. This article focuses on moisture readings.

How does soil moisture sensing work?

Our sensors use the capacitive method of detecting the water content of soil: the probe has a weak electric field around it, which is affected by everything it comes close to. Wet soil (or your hand) will change the electric field more than dry soil. Any tiny changes in the field are measured by electronic components within the sensor. 

This non-invasive method of sensing soil moisture lets us keep all electrical contacts away from water, to provide a long-life probe which doesn’t corrode.

Getting to know your sensors

The important thing to know is that not all soil responds in the same way. It’s not just water content that affects the capacitance of soil; all kinds of other substances such as salts, fertiliser, even plant roots and wiggly worms can affect the readings. These variances mean that domestic soil moisture sensing isn’t a precise science; we can’t say that a given soil is 20% wet, for example, without lab based calibration of a sample of your actual soil.

Your soil sensors will output a reading from 0 to 100, but this should not be interpreted as a moisture percentage. A sensor in the open air will read near zero, and when immersed in water will read 100 or thereabouts. Some soils will read 40 when almost fully dry, while others might get down to 20 - there is still soil around the sensor which affects the capacitance. A good average is 30.

You’ll need to get a feel for how your soil responds to changes in moisture to set “guard rails” on either side of good growing conditions; so that watering is turned on when soil gets too dry, and self watering is disabled when the soil is already wet.

Here’s how to get your “too dry” and “wet enough” readings.

Prepare two pots of the soil or compost you are using in your greenhouse, 10cm ones work well. Soak one of them to the point where you decide it’s wet enough, and doesn’t need any more water. Dry the other one out to the point where you think “this guy really could do with some water now”. It might take a day or two on a warm windowsill to dry the soil.

Plug your sensors into the 4-pin connection on the bottom of the control unit. Two sensors connect via a Y-splitter into one connection port.

Turn on your control box, and log in to the Harvst control app. If you have a solar powered control box, press the yellow button on the side; you’ll hear a beep and the red light on top will start flashing.

Visit the dashboard for the control system you are working with, and click the blue “live mode” button. It will open in a new tab / screen. Now you’re looking at what the sensors are telling you - data is updated every five seconds. 

Use the tabs at the top of the screen (“Lower” and “Upper” zones) to see readings for each sensor. The sensor with the red band around it is for the upper zone.

If both sensors are in air, you should see a reading near zero. Put both sensors into a cup of water. After a short while, you should see the readings go up to near 100. Sometimes it can take up to ten seconds for the numbers to change.

Dry the sensors off, and put one in your wet soil, and the other in the dry soil. Wait a minute or so for the numbers to stabilise. Use the dry soil number in the control app to trigger dry-soil watering, and the wet soil number to prevent watering if the soil is already wet.

Typical values are 30 for dry, and 70 for wet - bearing in mind that “dry” and “wet” are subjective terms! If you have plants that like being wet, you can increase the “dry soil” number so that the self watering comes on sooner as the moisture content falls.

Leave your sensors in the soil, and see how the numbers on your app chart change over the next few days. Soil in a plastic pot will dry out a lot slower than soil in a clay pot, and soil will generally dry out faster in warmer temperatures.

Where to position the soil sensors

The sensors are designed to be inserted into the soil all the way up to the black outer casing (so that all the flat sensing board is hidden). A partially inserted sensor will indicate that the soil is dryer than it really is (since half the sensor is in the air). For a shallow seed tray, you’ll need to bury the soil sensor under the soil sideways so that all the flat part is working next to the soil being measured.

Soil sensor in sideways Soil sensor in vertically

When to turn on moisture based watering

We recommend that you don’t activate moisture based watering until you’re happy that you know the range of your sensors in the soil you are using. The risk of choosing the wrong number is that your plants will get over or under-watered. Use the timer based self watering to start with.

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