So, during my little soil moisture sensor project, I noticed that, when it comes to the actual sensors, people usually use one of two types of sensors: Capacitive and Resistive.
Resistive sensors usually have 2 legs, or a copper path acting as legs. I wouldn't use a resistive one, as their copper path is exposed to the soil, which would most definitely have water in it. Due to electrolysis (something I actually did a project on in 7th grade), the copper path would get oxidized, and would not only render the sensor useless, but the oxidized copper would pose a threat to your plants' well being.
Capacitive sensors opt instead for one legs, and no copper paths that are exposed. These, overall, last longer, however there are a few precautions to take when ordering one, because while they all might look the same in the pictures, there are a few details that you simply must look out for when doing this project.
- Ignore the v1.2 and v2.0 markings. There's no update, it's just a marketing ploy
- Look for the existence of a 662K voltage regulator. Some will have two parts soldered and bridged to make the illusion of a voltage regulator.
- Make sure the thing uses a TLC555C or TLC555I timer chip. I looked around a bit, and ones with the NE555 timer chip are older and have a minimum supply voltage of 4.5 volts, instead of the other two's minimum of 2 or 3 volts.
- The two resistors at the bottom should have a hole between them. Not to the side, but between them. If it's to the side, then the connection of the 1 megohm resistor to ground will get interrupted, and you'll run into some problems
These are just all of the details I've noticed while using some sensors that one of my teachers had, I'm not an expert by any means. Before buying a capacitive sensor, check the product's photo to look for any of these faults. And if you can't properly look at the photo cause it got potato image quality, its prolly better to not buy it
Thanks for taking the time to read my yapping lol 
