My question is can I mix all my nutrients all together and feed all at once? I use superthrive and recently found out that it doesn't have any npk so my plants are turning yellow and old leaves are starting to die any suggestions???
You don't mention what exactly it is that you're thinking of mixing, or what you're growing in, or how old they are, so it's hard to say. BUT... it sounds like a bad idea. If you've been starving them, then overfeeding them now will not turn back the clock, it will probably just finish them off. It sounds like you have young plants, so I'd take a basic plant food that's higher in N than P & K and give it a low dose of that. On the other hand, if you're growing in Miracle Gro with that time release crap, I wouldn't feed them at all since the nutrients in that soil might be hurting them, & I'd transplant. A rich, generic soil with a few amendments will get you right through the vegging phase without any need for bottled nutrients. And one final note, there are other reasons for yellowing and dying leaves besides N deficiency. Add a few pics and some more info and people will be able to help you out.