Sometimes also referred to as beneficials, these additives cover stress relievers and enzyme based products that breaks down old root mass to allow for and stimulate new growth.
These are great products most effective when transplanting when plants are suffering.
Beneficial biological products, like Great White Mycorrhizae and Ecothrive Charge & Biosys are worth checking out. They colonise your rootzone with microorganisms that have a range of functions. Look out for one that contains Mycorrhizae – they feed roots nutrients and water!