Phosphorus(P) is a macro element and one of the big 3 important mobile elements for your cannabis plant. Remember being mobile means it usually shows up on your older leaves first. It is the second number written on virtually every bag of fertilizer you will ever buy. It is used in large amounts all during your plants life but in particular the beginning and the end. Your plants root system in the beginning needs Phosphorous(P) to develop its root system. The very foundation and support for your entire plants system. During the middle of your plants life not enough Phosphorus(P) can cause slow growth for your plant leaves and they might even drop off. In the end is when large amounts of this element are needed for the plant to bloom and flower. Buds are where its at on the marijuana plant and big Phosphorus(P) can mean big buds.
A deficiency in Phosphorus(P) can of course be slow growth by your plant where it can become very weak. The edge around the leaf can be brownish and work its way inward cause the leaf to curl up slightly. The fan leaves will be a very dark green with dull purple and blue tints as well. Sometimes a good sign is the stems can be a reddish and purple color although you must remember that some plants are bred for purple and have it in their genes to show color. Cold air in the 50ºF can also bring on the purple color because of how it can block the Phosphorus(P) intake for your plants as well.
Incorrect PH can also lock out this element as well. In soil Phosphorous(P) lockout can occur below levels of 5.8 and above 7. In hydro and soiless this can occur below 5.5 and above 6.2. Always remember how important PH can be, this needs to be something constantly checked.
To much Phosphorus(P) can cause your plant to have problems taking in Iron(Fe), Potassium(K), and Zinc(Zn) which can cause a deficiency in those nutrients. Always that too much of something is just as bad as not enough.
A Phosphorus(P) deficiency can be taken care of with bone meal, fruit eating bat guano, and crab shells but remember we always recommend stuff like this being made into a tea rather than a direct add to the soil so it can always be flushed out in case you add a little too much.
Now it's onto San Luis Obispo to see the boys at Healthy Harvest. These guys swear by their Phos Bat Guano NPK 0-7-0 from their favorite company Aurora Innovations as well as Auroras Roots HP2 NPK 0-4-0. I think either one of these can kick your plants Phosphorus(P) butt just as hard as you want it to.
Well I hope we have helped out you and your garden with its Phosphorus(P) deficiency and remember you can always return to check out any problems with your plants.