Ok I understand better now, thanks for the explanation. You have a good advice from taxonomist, Plant Phys. from Taiz and Zeiger is a very good reference book. Another good one is from Ross and Salisbury, same title. It will respond to many questions you may have concerning root function and nutrient availability, pH, etc.
One experiment I know of and that works well is one on observation of sudation. I will not go into details (unless you want me to). It consists of saturating the soil of the plant with water (you let sit your plant in water) and putting a dome over it. The soil and the air usually are water saturated overnight. Due to air saturation there will no longer be any transpiration (evaporation of water from leaves that is one process that drives water and nutrients up the plant). What you will observe in the morning, are little droplets around the leaves. These droplets show that although transpiration has stopped, there is another force driving up water up the plant. It's called root pressure. It occurs mainly in the tropics and generally when air is saturated with water.

You can easily adapt this experiment to what you want to do with pH. You can then play around with different levels of pH and see at what pH the formation of drops is either slowed or stopped.

For nutrient availability, it depends a lot on your plant. Most plants like it around 6 (4.8-6.2 roughly). You can start doing a search on the net to see if for your plant there are any papers or articles.

You might just find someone else has already found the answers to your questions. Someone on another thread mentioned Google Scholar. If a regular search using "Google" hasn't yielded the results you're looking for, try entering ... root permeability pH ... and see what you get.

The link below is the best I've seen on root structure and anatomy.