Placement of soil sensors

Question

I am looking into soil sensors to connect to an arduino project I am doing. This is just for me playing around and trying to optimize my garden as much as possible with tech 😋 So far the sensors seem to be working great and I am making cool charts. BUT I am not sure the best placement for sensors. I have 30 sensors so far, and I have up until now, only placed them in the top soil. I have read that in large installations these sensors are placed 3 or 4 on top of each other like in this picture:

So! here is my question: I have some "Soil Moisture/temp/EC" sensors, they are really cheap. I also have some "soil PH" sensors, they are not so cheap.

Does the PH sensor work better at any specific level of the hole, that being 5cm,10cm, 20cm,.... so on. OR should I have a PH sensor at EVERY level as well as the soil moisture/temp/ec.

The soil that I have is primarily top soil, it is soil that I have turned and try to keep fresh (I add more top soil every year). Under the top soil there is clay (aprox. 1.5m down). I think water does collect at that area.

I am growing a few plants such as sweet potatos, and tomatos as well as cucumbers. All plants that don't particularly have deep roots.

I do have 5 orange trees and some tea bushes.

Thanks for the info! -Kevin

Answer 1

I've done sensor measurements with my hydroponics setups and haven't done them in soil, but I can give some insights especially regarding pH sensing.

To quickly answer your question: up to you how much you want to spend on how many sensors. Moisture/Temp/EC can be useful at all levels and the tiering detailed in the photos make sense. There are more sensors where there are more roots.

For the long answer, re: the pH sensor...

First for the sensor: keep in mind the electrode will wear down, and relatively quickly. The sensor has an electrode bulb, usually encased in glass with a liquid (almost always a 3 molar KCl solution). The solution expires, the metal contacts oxidize and the electrode will need to be replaced. Check your manual for how often. Putting it deep or on every level means you'll need to dig up the plot that often.

Next, the readings: pH will fluctuate, by a lot. Don't expect accurate or precise readings--and whatever spot you put it, will not be an average of your garden. I've seen pH readings drift by over 0.3 on the same day in the same bucket of nutrient solution 8 hours apart. Readings on soil may be more stable on the same spot, but the area half a meter away could be off by 0.5 or more depending on nutrients picked up by the roots there, or how much water it had, etc. I would expect readings 3cm above or below the same spot to be different.

Consider what you're using the pH sensor and readings for, while keeping the above in mind. Readings deep down could be useful (eg, as an indicator of possible phosphorus runoff) but is it useful for what you want?

If you want an average value over the whole garden, or to compare sections, more sensors at roughly the same level would be better than sensors at different levels in the same spot. You can at least have reasonable expectation that your readings can be averaged across the plot for the same surface layer. In this case, I would put it around the 20-30cm mark as that would be roughly where the topsoil [1] ends and where the majority of shallow roots will end up around (species dependent).

[1] using topsoil here in the horticultural sense where it's referring to the uppermost soil horizon layer.