I do what my hydro man tells me....and he tells me to use the Penetrator because it assists absorbtion.....and having used it both ways I can honestly say it DOES impove the absorbption rate in my opinion.

Day 1 and Day 10 according to the instructions but (again) according to my hydro guy you DEFINITELY want to get a dose on in the in the first few days of 12/12 even if you skip or are late with the second dose. Because the stuff is only good for 15 minutes once mixed (according to the bottle) a lot is wasted so we have developed some slightly relaxed rules with it that so far have worked for us (ie: see no hermies in what we've harvested so far). Get the first dose in within the first 3 days of 12/12 and give a second dose between 7-14 days later. Again......this is not what Dutch Master says to do but SO FAR it's worked for us.