"Mykos" from Xtreme Gardening works well.
However, most products need to be placed in the planting hole at transplant time so that it contacts the roots.
Amino acids, aloe extract, humic acid, fulvic acid and kelp products can be watered in though....all these boost plant vigor.
Go easy on small plants, they are still very fragile and do not like heavy doses of anything.....many weak doses are better than heavy doses infrequently.