Thanks for the advice man! I had a little feeling it was magnesium. I have a bag of dolmite lime which says on the bag it's a good source of calcium and mg. Do you reckon it could be good to use instead of Epsom salt? I'll probably grab some anyway.
Cheers.
I would use the Epsom salt. Lime is something that has to be worked into the soil and break down to become available. Maybe not a horrible idea to work a little into the top few inches of the soil to give her a source for later, just make sure not to disturb the roots too much. But at this point it'll probably be more effective to just put about a tablespoon or so in a gallon of water every week or 2. Epsom salt is 100% water soluble magnesium sulfate, so too much or too often will cause issues.
Here are a couple of charts. I suggest saving them to a file or on you phone for later reference if you don't already have them. These charts are true for all plants, not just cannabis. The nutrient ions react with the pH of a solution, solubility in water changes at different pH levels. With the top chart you'll notice why we say plants grow best in a given pH range.
In the second chart you'll notice that too much magnesium will increase the solubility of phosphorus and boron, while decreasing the solubility of calcium and potassium. Mulder's chart depicts the strength of the bond of each ion. Some, like calcium, have a very strong bond. Aluminum, which is not depicted on the chart, but is still considered a micro nutrient for plant growth, has the strongest bond of all the ions if my memory serves me.
Also, check out the cation exchange capacity (CEC) of different soil particles if you haven't already. This relates to the second chart, and explains how each soil particle, whether negatively or positively charged, act as a magnet holding onto nutrient ions of the opposite charge.