I believe what want to know is when do you NEED these.While your plant is growing(vegetative the plants need n) when budding begins(the plant needs p) and fruiting (the plant needs k).Please keep in mind that all previous responses to question are spot on and correct.There are always variables involved.WICKED MIKE'S answer is a treasure chest of highly valuable information no matter what you're growing.Remember keep it simple and you'll do fine.