Measuring soil moisture and controlling irrigation

The OpenSprinkler has 2 digital inputs, which are basically nothing more than an on/off switch. This makes it easy to switch off the irrigation with a rain sensor when it rains.

However, the situation is different when it comes to soil moisture. This is not a value that can be defined as 0/1. Different soils have different soil density for different plants with different requirements. Therefore the value is measured in 0-100%, where 0% is bone dry and 100% is pure water (or some air). Already 50% is the same ratio of water and soil.

Now what is the correct value?

There are various tables from experts who have compiled these values, for example here:

https://www.acurite.com/blogs/acurite-in-your-home/soil-moisture-guide-for-plants-and-vegetables
or use one of the well-known plant apps, e.g. FYTA or PlantNet, to find out the care instructions for your plants.

In order for this to work, you have to be able to determine a value that is not digital, but analog. And this is where our Analog Sensor Board with SMT50 comes into play. The SMT50 can measure 0-50% moisture and is therefore sufficient for most plants. However, if you want to measure up to 80%, for example for cranberries, then you need the SMT100, which can measure moisture from 0-100%.

The Analog Sensor Board has 8 connections for analog sensors. All data is measured in a time series and is available for download.

And how can you use it to control irrigation? With the help of the “Program Adjustments” program function, the watering time can now be dynamically adjusted. They specify a humidity range, e.g. 3-20% and OpenSprinkler then scales the watering duration according to the humidity! So if 20% or more is reached, irrigation is deactivated; if it is 3% or less, maximum irrigation is carried out. In between there is linear scaling (or various options can be selected).

This example shows an irrigation adjustment to 28.8% due to soil moisture of 14%

For further information, it is best to take a look at the product pages Analog Sensor Board and that SMT50 to.

For professional users

For the professional Use, i.e. if a large number of sensors are to be used, then we recommend using ours Truebner RS485 adapter, with which up to 64 SMT100 can be connected via Modbus.

The SMT100 in RS485 Modbus mode can deliver 3 values:

– Soil moisture

– Soil temperature

– Dielectric permittivity