Plants need all 3--N (nitrogen) P (phosphorus) and K (potash i.e. potassium) thoughout their entire life, but for N & P in varying amt.'s depending on what stage the plants are in.
Potassium isn't needed in varying amt's as much as N and P but there always has to be some.
There are many sources of K, both organic and chemical.
Unless your very experienced, mixing your own up is tricky. It's much better imo, to get a full-spectrum nutrient like for instance Fox Farms.
P.S. if your backyard growing (in the ground), you can get good results by with adding fish emulision for veg and high-P bat guano for flowering.
so i shoudnt even bother with the potash? which is kelp by the way its only (0-0-1) and do you know of any other good organic fertilizers besides fox farm, the nearest store that sells it is almost 2 hours away?