Although a phosphorus deficiency is rare, it can cause major problems with your marijuana plants. They will grow very slowly and produce small leaves (older ones will be targeted first). They will become dark green and weak, eventually producing purple or blue discolorations. At the leaf's edge, you'll see tan or brown colors and the leaves will turn downward. Fan leaves will change to a dark green color with the same purplish hue. The bottom leaves will yellow and die entirely.
Petioles and stems will start to change to a purple or red color, but some strains have naturally red or purple petioles and stems.
Phosphorus is vital during the flowering stage, meaning that it's important to keep its concentration up to produce higher yields.
The main goal of phosphorus is to help in the growth of the roots and stems while also promoting the plant's overall vigor and helping the germination process. Again, phosphorus is vital during the flowering and reproductive stages. Download my free marijuana grow bible for more tips about nutrients and marijuana plants.
You'll notice that phosphorus represents the "P" in the N-P-K ratio on most fertilizer packages. You can fix many phosphorus deficiencies by finding a water-soluble fertilizer (e.g. a bloom fertilizer) that's high in phosphorus. Guano that's high in phosphorus is available almost anywhere. Greensand and rock phosphate both provide phosphorus in a gradual sense.
You can also use a product like Marijuana Booster, which is meant to increase yields. It ensures that you have the right combination of nutrients (including phosphorus) so that you don't have to worry about deficiencies.
Phosphorus deficiencies during the flowering period often produce lower yields. By contrast, an excess of phosphorus can cause "chemical buds" or it can "burn" the plant. Phosphorus absorption in cold temperatures (sub-50*F and sub 10*C) is difficult. Soluble phosphorus can work to provide greater yield in colder weather.
If you want to start growing, download my free grow guide and order some marijuana seeds. All top quality marijuana seeds are available in my marijuana seed shop (buy 5 get 5 free). We ship seeds to the US, CA and many other countries. For any growing related question please visit the marijuana support page.