Building a soil humidity sensor for pot plant


This section provides some basics for humidity soil sensor. Click on the section 2 for how-to-do steps.

Watering plant is a fun and relax after a workday, but as a geek, having a system to monitor the soil humidity and turning on the pump when necessary is intruging.

I am a lazy person, and if I need to something repeatly, eventually, I will figure out to automate or semi-automate that process. This project is about that.

Measuring soil humidity could be convoluated for the beginners. I will focus on two type of sensors using resistance and capacitance. The resistance type calculates the humidity content in soil by the resistance change when applying a constant voltage between two probes. One problem with this type of sensors is dissolving anode (the positive pole) into the soil. Copper, as many other metal ions, is toxic to the plant with a certain threadhole. Frequent sampling and the water content between two probes of the sensors, and so the current determines the lifetime of the resistance sensor. Resistance sensor is lowcost, but I would recommend against using this type because to having metal ions dissolved into the soil could harm your plant.


The second type is capatative, which measures the frequency change in charging a capacitor as a function of soil humidity. Instead of conducting a current between two probes of a resistance sensor, the capacitance sensors forms a dielectric fields similar to a capacitor. This type is more complex because of a wave generator with timing to calculate the charging time of pole-soil capacitor.

For the resistance sensor, a higher humidity content leads to a stronger current across the probes. For the capacitance one, a higher humidity or a higher fertilizers reduces isolation between the probes. For a capacitor, this means a more leak and takes a longer time to charge up.

Two versions of capacitance sensor is a 3-wire built with analog output, and a standalone for as a I2C slave called Chirp. Visit the link and read its creator post. For a low cost sensor, that is a well-written post. If you want to take a deep dive into the measurement technique, check this post out.


Chirp is a bit more expensive, but more fun to play with. Next, we will buy stuff and put things together.