Soil Moisture Value Bouncing

Hi Guys,

My soil moisture value bounces all over the range (from 200 - 1000) even when the readings are taken 5 seconds apart and the probe has not moved.

My understanding is that the UTM contacts the screws on top of the soil moisture tool head and the signal is delivered that way. I would think that would be problematic but I could be wrong.

Does anybody know of a way to prevent the values from bouncing around? Has anybody made a change that has given them reliable values?

I am looking at replacing the spark fun soil moisture sensor with a capacitive sensor from DF Robot. That will require some modification of the tool as they don’t mount the same. The electronics at the top of the sensor will also need to be waterproofed so I will have to develop a design that accounts for that.

Please let me know if you have experienced the same issue and whether you solved the problem.


For now, due to lack of free time, i have given up ambition to get anything useful from stock moisture sensor. I experience the same - unreliable and inconsistent readings. I haven t had time to troubleshoot this too much, but among first things i would look into the quality of electrical contact between the utm and the soil moisture tool.

In my sequences i use “tool on check” that reads whether tool was mounted properly. I get a lot of false alarms - i.e. based on pin reading, tool is not mounted, while in reality it is on and working well…

Hi eide,

I’m thinking that the design of the utm will never allow good electrical connection. In my experience, having that many electrical gaps in an analog signal is a recipe for disaster.

I have had an idea though. I am wondering if it is possible to wire a soil moisture sensor directly to the farmduino board. I am not near the bot but from looking at it on the net, there are analog pins that it could be wired to.

I would like to use it to determine if it has been raining so if I put the sensor in a location that doesn’t get watered and just leave it in the soil, it should tell me when the soil is wet from rain. That way I can prevent watering sequences after rain.

Does anybody have reasons why this approach wouldn’t work? It seems logical to me but I’m not really the sharpest tool in the shed!


Obviously the path and length of the wires to the electronics box would be an issue if I left it in a stationary position.

Given that the utm cable itself runs through the y axis cable carrier, I could mount a soil moisture probe to the z axis. The wires would be pretty much the same length as the utm cable. I would think wires that long could limit the signal being produced but it’s worth a test. The farmbot guys obviously think it could work or they wouldn’t have designed the soil moisture probe to attach to the utm.