For sure a nitrogen deficiency, Also as Dice stated, It looks like it needs some water and it wouldn't hurt anything to transplant into a bigger pot before flowering, I recommend it! As for your question, There are plenty of sourse of nitrogen so take your pick :
Sources of nitrogen are dried blood, Cotton seed meal which is slow acting, Insect eating bat guano which is fast acting. Bone meal which is a gradual absorption when not made into a tea.( also excellent source of phosphorus). Fish Meal Or Fish Emulsion is a good source of nitrogen and is medium acting. Worm castings, which is gradual absorption. Seabird guano, All purpose Millennia Seabird guano, Orginal Seabird guano All Purpose, Crabshell ,which is slow absorption. Fox Farm Grow Big, which is fast acting. ( can bring down your ph as well) I would use fish emulsions or Earth worm castings thats just me though.
