Gledre -Please tell us how you fertilize. With what? How often? Do you flush the soil when you water?

1) Deficiency symptoms often confound experts, even when they have the affected plant material in hand. 2) In containers, I never use an element or compound aimed at relieving what I THINK is a symptom of a deficiency. I'll explain below.

Also consider that a deficiency of any given nutrient does not necessarily mean there is a physical shortage of that nutrient in the soil. An excess of nutrient A can cause a deficiency of nutrient B, even if B is represented in the soil at levels that would otherwise be more than adequate. Cultural conditions also have a significant impact on the plant's ability to take up some nutrients. Too cool - phosphorous is difficult to assimilate. Too much Ca and Mg can be tough to assimilate. Too much Fe and Mn has a hard time making it to the nutrient stream. Soil saturation makes assimilation of ALL nutrients plants normally derive from the soil difficult. In saturated soils low oxygen levels cause Fe and Mn to become electron acceptors. This leads to the precipitation of both, and even though
these elements might be present in soils at normally adequate levels, the plant’s inability to assimilate them. Soggy soils also make uptake of Ca difficult or impossible.

Combine all of that info with the fact that a deficiency of A often masquerades as a deficiency of B; toxicities (excessive amounts of a nutrient) often cause symptoms that mimic deficiencies of other nutrients; it's common for more than one deficiency and/or toxicity to be in play at the same time; and finally, pH VERY often makes nutrients inaccessible, even when they are represented in the soil at levels that would be adequate under conditions of favorable pH.

Let's return to this for a moment: Plants perform best when the RATIO of nutrients in the soil closely mimics the ratio at which plants actually use the nutrients, and when the o/a concentration of nutrients is high enough to ensure there are no deficiencies, yet still low enough that uptake of water and and nutrients therein dissolved isn't impeded by a level of EC/TDS (salinity) that's too high. If we take this at face value (the alternative would be to pose something more plausible), we can see that (in containers) strategies aimed at correcting deficiencies of a single nutrient are going to be a tough fit. A Tbsp of MgSO4 is a LOT of salt in a gallon of water, so we can be absolutely sure that it is going to significantly increase the EC/TDS (salt) level of the soil solution; and, it's going to throw the nutrient ratio way out of balance. You might wish to give consideration to what that much salt can do to a plant's ability to take up water during summer's dog days.

What if it's not an Mg deficiency? Did you know that massive doses (several times more than the plant needs or can use) of Fe or Mg can green up a sick plant to the degree it masks the symptoms of other problems and deficiency? So you add a heavy dose of iron chelate or Epsom salts, the plant turns green, and what ails it might continue to limit the plant while remaining off the radar.

These strategies would be much better choices. A) If you have good reason to suspect a bonified physical deficiency of Mg, mix 1/2 tsp Epsom salts in a gallon of water and spritz 1 branch or 1 cluster of leaves. If you were right, add 1/4 tsp of Epsom salts to each gallon of fertilizer solution you make, next time you fertilize. If the leaves green up, you were right and can add that 1/4 tsp each time you fertilize. B) Is what I would recommend. Simply get on a regular supplementation program using Foliage-Pro 9-3-6. This meshes perfectly with what's in bold above, you won't be skewing the ratio of nutrients in the soil solution, AND, if you follow directions, you won't see the EC/TDS surge you would if you fertilized with a Tbsp of Epsom salts in a gallon of water.

PS - Mg is highly mobile in the plant, so unless any leaf that has turned a lighter shade of green is in the process of being shed, there is no reason it won't green right back up if you are able to correct a deficiency.