Question Nitrogen deficiency

My older leafs on my girls are somewhat turning to yellow i think its a nitrogen deficiency i know people add organic fertilizer like 14-0-0 but i dont have any for the moment. I have done some reading and i saw people add nitrate of soda.

My question here is i curently have pure potassium nitrate (salpeter) i use in my rocket model can i use this to pump it up a little bit or it's too strong for my plant.

Also how much i should add ex: 1 tbs for 1 litre of water.

I curently grow my plants in soil with cfl.

Tanks,
FreshMaker

Potassium nitrate has been used as fertilizer for thousands of years, so as long as it doesn't have other chemicals in it, it should be fine to use. As for how much to use, I don't know. You're best bet would be to start with just a little and work your way up. If it starts burning your leaves cut back the dose.