I have purchased a few cheap hydrometers on Amazon only to find they do not work at all. Is there any decent instrument under $50 that will let me dip down at least two feet into a gritty mix soil to determine how much water is remaining at the base of the pot?

I need to water very sparsely during the Winter, but I do have a few plants that I cannot allow to completely dry out, so I need a reliable way to check the amount of moisture.

I also have a problem with rhododendrons where the symptoms of over and underwatering look identical. I want a way to get a water measurement underneath the roots to better understand whether the problem is overwatering.