Humic acid really is a "wonder product" for all plants and I can't recommend its use highly enough!
I have been using it for 25+ years and consider it an essential part of growing cannabis.
Fulvic acid and amino acids (especially) are also "wonder products" that I recommend to all growers interested in maximum plant health.
A quick google search revealed a dilution rate of "one ounce per gallon" so roughly 30ml per 5 litres.
I would suggest applying at a rate as what you would give your plants when just giving a general watering.
There is no danger of giving too much, it will not burn your plants.
Hope this helps,............
Organoman.