Hi Gio,
That looks like a Calcium deficiency to me.
A phosphorus deficiency usually presents differently - typically larger splotches of leaf discoloration and not as uniform as what displays in your pic.
You are probably OK on magnesium (hard to say without a soil test), and if that is the case, then a CalMag treatment could potentially cause other issues if it takes your Mg levels too high..
If all of your plants are in the same mix, then you may want to treat them all for a Ca deficiency so that you can get on top of the problem before it becomes a problem for the rest of your girls.
Despite what the previous poster mentions about over-doing Ca, it's very hard to do. Typically organic growers have a hard time getting enough Ca in the soil mix without pushing PH up too high, since plant available sources of Ca are in a carbonate form (oyster, lime, etc..) which raises soil PH.
If it were me, I'd make a WCA (water soluble calcium) supplement out of Oyster Shell Flour. Most of the internet stuff will show you how to make it out of egg shells, but I prefer to use Oyster Shell Flour as it's less scummy (no egg shell membrane) and it won't stink your house up with that egg/sulfur smell.
By using WCA, you will be providing your plant roots an instantly available source of Ca without negatively effecting the soil PH.
You will need oyster shell flour and distilled white vinegar - Mix 1 part oyster shell flour with 10 parts vinegar. The acetic acid in the vinegar will react with the calcium carbonate (CaCO3) in the oyster shell flour. The reaction will break down the carbonate, releasing the Ca into the vinegar and releasing CO2 and O in the process.
You would typically let this mix sit for 3-5 days (stirring once or twice a day) until the reaction is complete (you stop noticing bubbles).
Once the mix is prepared, you may use this at a rate of 1 teaspoon per gallon (1:1000 ratio) for minor deficiencies or 2 teaspoons per gallon for major deficiencies (1:500 ratio) EVERY watering.
If you don't want to wait a few days to a week for your solution to be ready, you can use 1 part oyster shell and 5 parts vinegar and give it 3 or 4 hours before using the solution in an emergency situation.
I've had calcium deficiencies in the past from using Gypsum as my sole source of calcium in soil because I didn't want to change PH.. I believed everyone on the internet who said Gypsum is a plant available source of Ca in organic soil. <--- Total BS by the way. Gypsum does not supply a plant available source of Ca.
Anyhow - I've successfully used WCA at 2 teaspoons per gallon (every watering) to ward off major calcium deficiencies in the past and get my plants to harvest.
One thing to keep in mind about WCA is that the solution will be acidic, so as you add it to your gallon of water, it may drop the PH a little bit. I use well water (high PH) missed with RO water to offset the little bit of acidity.
Good Luck Gio - hope this helps - let me know if any questions!
