Update 2011-09-05: I have created a new blog about electronics only – check it out here: Electronic measurements, NSLU2 and soil moisture sensor
My Watermark soil moisture sensor and additional circuit has been running 24/7 for about 4 months now, and it really is a stable system. Here are the data collected so far:
On the week graph you will notice an oscillation peaking high at 6 PM and low at 10 AM every day:
Compare the above graph to the weekly temperature graph and you will see the connection to the soil temperature:
When the soil temperature goes up, so does the soil moisture reading. It could be that the accuracy of the soil moisture measurement is highly temperature dependent. Another explanation could be, that air is able to contain more moisture at higher temperatures and this more humid air somehow spreads to the soil and raises the soil moisture level.
What do you think is the explanation? Please leave a comment.