Davey Jones got my plant ( black spot )

ok so i noticed maybe two to three days ago there was a little black edge on one of my lower fan leaves. i ignored it until today when i noticed that about 3 leaves had it. also they are slowly turning yellow. i cut them off before it could spread. it looked like how the tip of the leaf burns and turns really dark but a little bit darker. i havent had any other problems with the plant, temp it fine, humidity is fine, watering is good. rarely use nutrients and very very little if i do. any suggestions would be helpful. thanks and happy smoking!

What type of soil are you using? How old are they?

it is 48 days old from clone. and im using ace hardwares brand of potting soil, i've always used it with no problems. its the same thing as glacier gold.... from some of the threads here i think its a p deficiency

Nute Deficiency

The third picture shows P deficiency and ways to treat it. You may need to click the pic twice to enlarge it to full size.

P stands for??????

N-nitrogen P-phosphorus K-potassium

ok well i dont have any bat guano nor do i have the funds to really buy anything at the moment. i do have some nutrients with slightly higher p for the pre flowering and flowering stages. would it be ok to give it just a tiny bit of one of those? or is there anything else i can use? and thanks much SkunkDunks. that guide you linked is actually better than the other two i found

any home made remedies? will my nutrients be ok to use???