causes

im growing in cocoa and having a serious yellowing in leaves at bottom to middle of plant. What causes this and how do I correct it ? PLEASE HELP

Hi guys Been growing in organic super soil for many years and a few years ago my roots changed. The plants are perfectly happy and it causes no ill effects. All my strains roots now are covered in nodules and all I can really find is that nitrogen fixing bacteria causes root nodules but I was...