npk is npk. nh3 is nitrogen amonia aka piss. in fox farm they use slow release nh2, basicaly bugs eat the nh2 and then piss out drops of nh3... which the plant eats(mg uses nh3)
I do use MG, However i mix it with other fertalizers to get the npk ratio i desire.
*piss also has an npk of 50-0-0...