Good question!
The best "root booster?" - KELP extract for watering it in
AND
Adding Kelp meal to soil's/soil-builds.
Kelp extracts should be used at every up potting. That's about it. 5 ml per gallon of feed.
IF the product says to use more then that? It is a watered down kelp extract compound. It has other things in it..
It can be a helpful product IF you need to apply some micro nutrients to "depleted" soil. (You can get this from keeping a poorly made/balanced soil in a pot and plant for too long.
If you add the right source's of micro nutes in a soil build, or supplement a known "weak" soil. The micro's should last about 3 runs of that soil B4 you have to recharge the micro's.
Look for products that are high in S and Mn - It should also be close to or equal in Fe to the Mn.
Soils amended with kelp products can increase root growth by as much as 170%... Those results come to us by the University of South Carolina (if'n I's member right.
Good luck! Don't over use as you can get micro issues.