Plant expands about 40% of its photosynthetically fixed carbon to synthesize chemicals that are transferred to the soil through the root and are called root exudates. It contains wide variety of compound (low Mw and High Mw) regulating the microbial community at the immediate vicinity of root (rhizosphere). Low Mw compounds include amino acids, organic acids, sugars, phenolics, and various other secondary metabolites which comprise the bulk root exudates while High Mw exudates are polysaccharides and proteins. Here is some review articles for your help to understand the topic. Hope this may help u.