Got ya. It seems the tap root is just that. It's taps it's way on down to the bottom quickly then roots start branching out. As far as roots being more efficient in a particular part I'm not really sure. Myself I try and keep my mix as equal as possible when it comes to watering. In theory they all should perform the same as long as they're healthy. The tops of my buckets always get thick with roots, I always end up adding soil to the top because I don't want the sun to damage them. I like what your trying to do though, I've always wondered myself. I'm about to try and find a 5 gallon bucket that's clear, that will also fit inside a standard 5 gallon. That way next time I grow in a 5 gallon bucket I can track the roots. Lol now I want to know exactly how, what and when. As far as your question about efficiency , I think they should be equal in all parts as long as the conditions are equal as far as wet dry cycle, nutrients etc.. I honestly question my ability to answer you correctly about this. Lol it's bothering me some. You definitely have my wheels a turning.