I have a black Diesel going now and it seems happy with a general purpose feed. I know the term "If they look hungry" doesn't help you much, but it is something that kind of comes with experience and is difficult to explain, since plants react differently to fertilizers. I recommend using half strength nutrients... even less (if any) when using a fortified soil, like Fox Farms supplies. It is easy to correct a deficiency, but almost impossible to fix overdoses. A LOT of fertilizer issues arise from following the directions. As an example, one of my 7 ft tall Sativas is going to use a LOT more nutrients than a plant the same age but 1/4 it's size. If you start with a light application of fertilizer once it gets growing , you will get an idea if you need to increase the dose or not. Meters are made for checking the concentration of nutrients, although you probably won't need one after a while as you get more familiar with growing weed. If you are at 8 weeks of flowering, you aren't going to need too much more, if any, to finish your grow. Don't be surprised if you start seeing yellowing leaves, starting at the bottom large fan leaves. This is the plant pulling reserves out of it's storage lockers...the large fan leaves.
As a final comment. I don't measure the TDS/PPM of my plants. There are quite a few growers that swear that you need these, and perhaps they are correct for their setup. The advice given is what has worked for me...your result may be different. The one thing that I've learned is that the less you interact with plants, the happier they are. Much like cats... they like you when you are feeding them, but after that, they have no use for you! :D
