check this link out as it has pics that you can match up
Marijuana Plant Abuse
this is what i think it is Affected plants are usually tallest and appear to be most vigorous. Necrotic spots form on lower leaves. Red stems. Leaves appear pale or yellow. Add chemical fertilizer containing K. but if your ph is off it could cuase this aswell by locking up the k in the soil
i got my ph tester from a local pet shop for $8.00 enough for 100 tests