Gardening

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.

This meter is not for the application we are taking about, but it's close enough: https://www.hannainst.com/nitrate-photometer-hi97728.html

Totally agree. Send it to a lab if you need real results.