I bought a 3-way soil meter from a Chinese online store. The first thing that caught my attention in the soil meter was that there was no battery compartment to store a battery pack.
This three-way battery-free soil meter, with two soil probes and a photosensor, not only measures soil moisture level but also reads pH and ambient light levels. This simple to use soil meter typically gives you a reading within sixteen seconds, all of them through a little analog dial. There is a small slide switch to select test modes as needed. Be that warning if you put the slide switch in the wrong position during any test (an easy mistake to make), the results displayed will be inaccurate!
This cheap soil meter consists of a pair of test probes (Moisture & pH), and each probe is made of two electrodes of dissimilar materials. The soil moisture sensor measures the water content in soil. When the probe is inserted into soil, it generates a small amount of voltage (typically a few hundred millivolts to a couple of volts – the more water in the soil, the higher the generated voltage). This simple soil meter doesn’t even need a battery because it employs the Earth Battery principle.
Now to the crude schematic of my 3-way soil meter. I hope no further explanation is required.
The soil meter also holds a photosensor to measure sunlight levels from 0 to 2000 (dark — light). Note, the meter shows moisture values from 0 to 10 (dry — wet) while pH values from 8 to 3.5 (alkaline — acidic).
The photosensor component seems like the ubiquitous BPW34 silicon PIN photodiode (https://www.vishay.com/docs/81521/bpw34.pdf).
Let me open up my three-way soil meter for you to see what’s inside:
There is a three-way slide switch, a photosensor, a “VU” moving coil meter, and a bunch of thin wires from the photosensor and the galvanic test probes to the slide switch terminals. Pretty straightforward!
By checking the analog meter, it looks like a 1mA FSD (full scale deflection) type moving coil volume unit meter (https://en.wikipedia.org/wiki/VU_meter). The meter has a resistance close to 1KΩ when measured with my trusty benchtop DMM. See, most cheap VU moving coil meters are 1mA FSD types, 500uA and 200uA versions are more delicate and cost much more than 1mA meters.
I have my soil meter at various locations on my lawn and had no problem getting soil moisture and sunlight readings. I haven’t done much with pH value measurements as my little lawn is not a good test field or study site for that.
Testing the pH of your soil helps to determine what plants are best suited for a specific area.
Note, pH is a measurement of how acidic or how alkaline (basic) a substance is. When you test pH, you’re measuring the amount of hydrogen atoms that carry a positive charge. The higher the concentration of hydrogen ions, the more acidic the sample is, whereas the lower the concentration of hydrogen ions, the more alkaline a sample is (Pure water is completely neutral at pH7, it’s neither acidic nor alkaline). Correct soil pH is essential to ensure optimal plant growth and crop yield as it allows nutrients to be freely available for plants to take in.
You may browse these site to find more about pH measurements http://www.ph-meter.info/ , https://blog.hannainst.com/soil-ph-testing
Also read https://www.gardenhealth.com/combination-ph-and-moisture-meter
I suspect that most multi-purpose soil meters do not have a good track record of pH tests. As I noticed, such a soil meter is essentially tailored to be a disposable commodity (perhaps just a fancy device for your amusement than anything else – you cannot calibrate it, nor will it run with any accuracy). Aboveboard, I do not know much about precise pH measurements and I’m glad to receive useful suggestions.
Finally, one thing I really liked about buying more off-the-shelf soil meters is that they’re inexpensive and have rigid sensor probes (and compact casings). That when blended with an everyday microcontroller enables a lot of wired/wireless agro-electronics projects at low cost and little effort. One idea on the back burner is a handheld battery-powered soil meter with moisture, light, temperature, and pH probes along with a digital readout. So, stay tuned to take a deep dive into some do it yourself digital soil meter/tester projects.