I think I need to P

[f-e]

I thought this collage (of other peoples examples) might be of use

 

[f-e]

More stolen then (slightly) edited work

 

[Creeperpark]

Most of the time you see this only because of a imbalance of chemistry in the growing media. By using a complete fertilizer correctly, one should not have a Phosphorous Deficiency Ever. Most of the time this problem is caused by pH or over exposure to nutrients. So check the pH and EC when you see this come up in a grow, and to fix the problem. Don't Add Extra Fertilizer!!