I got those 20-gal tubs from a friend who raised cattle with her husband (they had to sell the cattle 2 years ago because of the draught) and they had obtained mineral nutrients of some sort in them. I drilled four 1/2” holes around the lower part of the sides, about an inch above the bottom. They drain any excess water.
I fill the tubs about half-full with loose tree leaves and then put the dirt in. That seems to allow the holes to remain open and work. The leaves pack down and begin deteriorating, but tend to allow the water to wick through and out. I haven’t had any holes plug up with dirt. Some plants grow a mat of roots in that bottom inch to suck up the excess water. We dump the tubs out each Spring and re-do the dirt to liven it up again. They have raised us excellent gardens for the past few years, even with the draught.
CD in Oklahoma