Soil Moisture Sensor Project – Keep your garden alive!

Soil Moisture Sensor Project – Keep your garden alive!

Soil Moisture Sensor Project

Welcome to the home page of the Soil Moisture Sensor Project.

This project came about because I want the garden to live. Even if I haven’t had time to water it that morning. We worked hard to pull out the gnarly and lifeless “low maintenance” sprawl that produced nothing but thorns, neglected long ago by owners who never went “out there”. Plants that did nothing didn’t need water and didn’t belong in our garden. A year later there were fruit trees, tomatoes, herbs, and birds. The presence of birds probably had more to do with the fact that we kept the neighbour’s cat out of our garden.

That first summer, I learnt that it is hard to water a garden when you are on crutches.

The second summer, I learnt to set my alarm half an hour earlier so that I could water the garden before I went to work.

What else could you do?

We are stuck when we go on holiday because we either have to:
a) get someone to water the garden regularly (everyday in summer), or
b) put a timer on the tap.

Neither of these solutions are optimal because I want the garden to be watered when it is needed. I don’t want the garden watered when the timer decides to open the valve after a downpour that has soaked the garden. I also don’t think that it is fair on friends or family to ask them to come around every morning to water the tomatoes. I’d like to be able to go away knowing that a soil moisture sensor will help keep the garden will be watered and – disaster notwithstanding – that the plants will be alive when we get back.

The aim

In order to do this I’d like to be able to measure the amount of moisture at the root zone of each fruit tree and the tomato patch, and turn on a dripper when the soil moisture level drops too low.

I don’t need to accurately measure how much moisture is in the soil. I normally just stick my finger in the soil; if my finger is dry, then the soil is dry; if my finger is damp, the soil is damp. What I want is a system that turns on a tap when the soil is dry and stops watering when the soil is damp.

How this project is organised

This project is organised by the stages I am going through to design the “Soil Moisture Sensor” (fancy acronyms are welcomed, but I can’t promise any royalties).

The first part of this project is a wish list of what I want from the final product

The soil moisture sensor


After setting out the project requirements, it was time to review similar projects to see if any of them could be used or adapted. All projects work one of two ways, soil will either resist the flow of electricity or will store an electrical charge. It is very easy to measure the resistance of the soil to the flow of electricity which is which makes it a very easy and common project. But this method requires you to leave uninsulated metal in the soil which will corrode over time. In contrast, methods that store an electrical charge in the soil let you insulate the metal so that it doesn’t corrode. But these methods are more complex. I initially rejected several ways of storing charge in the soil because the methods were either potentially inaccurate or used relatively expensive components.

After reading about other projects, I decided to try and use soil as a “tuning capacitor” in something called a variable frequency oscillator.

Plan A: Variable frequency oscillators

Write-up coming soon!

Sneak preview – I eventually gave up on this method and looked again at a method I had previously rejected; the low pass filter.

Plan B: Low pass filters

Currently in testing.

Water delivery system




Soil Moisture Sensor

The drip system itself