We usually talk about soil drainage in quality categories. But I was wondering if there a quantity categories we can refer to? This is what I mean. You make your soil mix. Dry it out and weigh. Now, soak it completely(completely is a key word here) any way you prefer, then let it drain until no more water draining. Now weigh it again. Difference would be amount of water your soil can hold in weight. Knowing specific gravity of the water calculate volume of the water in your pot. Now you can calculate a volume proportion of water in your pot knowing it size. This is simple. But now the question is - what to do with this number - is there any chart of “preferred” numbers for different cultures, or at least some general guidelines for it? I changed my veggie starting mix this year by replacing coco coir with pine bark fines for cost reason. The mix seems to be drying out in my onion starters too fast, but looks like onion doesn’t mind. I am about to start my main batch of peppers and eggplants and want to make sure I am not about to ruin it.
I found this article, it gives me some understanding on numbers. One thing I missed in the method above - drainage depends also on the height of the pot, not only on the properties of the media.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwiaxoburLTZAhVGq1MKHdi7AFwQFghXMAQ&url=https%3A%2F%2Foregonstate.edu%2Fdept%2Fnursery-weeds%2Fpresentations%2Fnw_ag_phys.pdf&usg=AOvVaw0K2lOpvCrJVgWbzkoCJf38