Cherry tomatoes grew fine over the summer. Most things just needed low intensity morning sun then shade afterwards. I planted a couple trees to help out.
So heres the thing with these devices, they aren’t accurate but they are sort of precise.
Every soil is different, but the same soil at roughly the same wetness will fairly precisely read the same way each time.
So in conjunction with a much more accurate readout from lab equipment, you can get an initial handful of readings from your specific patch of land and record what this meter read, so you benchmark the actual spot you wanna hit with the dial for your very specific fertilizer comp you use. Say, mark it with a little marker.
Then for the rest of that season, this little meter should be pretty solid enough because you first took the time to calibrate it against more accurate tools
At which point its not a reader on its own, its merely a well calibrated gauge, calibrated against other tooling.
And that will work fairly well enough for most use cases. The point of the device isn’t to track precisely getting your mixes right, the point is it will instantly point out if something has gone very wrong before it gets worse.
Ok, this is a rough science and likely very inaccurate with the pictured device.
Different nutrients in soil can alter electrical signals passed between two or three different probes that are on these kinds of devices. That can be measured and approximate nutrient content can be calculated.
Since the pictured device is likely under $5, I would speculate that it has you wet the soil first before the probe is inserted. Nutrient salts can change the conductivity of water and it is trivial to amplify any change in resistance or capacitance on one of those types of needle meters. It only takes a tiny amount of current. The probes themselves could act as a battery of sorts, similar to how you can make a simple battery with a potato. (I would guess we are around 1mA or less with this setup.)
It’s not a great solution, it’s prone to false readings but its approximate enough for the home gardener. At the very least, you know the soil conductivity: More conductivity is sometimes better than less.
Edit: Looking at that picture, the soil doesn’t look very wet to me. It could explain the “lack of nutrients”.
Oh, that $5 piece of crap is absolutely telling you a number that has very little value. If you use it properly and the needle pegs in one direction or another, you probably want to invest in a decent test. (Even pegging that needle doesn’t mean much, TBH.)
ISM sensors are typically used in hydroponics for disolved O2 and are “decently” priced. The ones that are available for nitrates and such are generally considered lab equipment and are not cheap.
It’s not a great solution, it’s prone to false readings but its approximate enough for the home gardener.
I have a small handful of plants. It’s good enough for me. I completely agree that if it were my livelihood I would be sending it off to a lab. If something happens to my garden I get less salsa and have to buy tomatoes at the store. If something happens to a farm they’re fucked for the year unless they get on top of it.
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