I bought a FD off ebay in early spring of this year. Looked very nice when I got it. I repotted it up by about 2 inches in a mix of miracle grow citrus, ½ inch CHC and some small rockwool cubes. I added a little STEM.
About 6 weeks ago I noticed some leaf yellowing and I did a foliar spray of a citrus fertilizer.
Problem worsened and I began to worry about alkalotic chlorosis. I check my pH and it was around 7. I repotted with similar mix and added aluminum sulfate to get pH close to 6. Added a slow release fertilizer then. That was about 10 days ago. Soil did not seem too wet.
Problem continues to worsen. These are the photos. I just now began to realize this may be magnesium deficiency and have made a solution from Epsom salts to foliar spray and water with.
Am I doing the right thing?
If it is magnesium def., is the pH now too low for Epsom- would I be better off adding dolomite?
Is it too late?
I have other citrus in the same mix that seem okay. Should I change things for them or did my plant arrive with this baggage?

Kyle

A pH of 7 is a little high and that may be due to the cactus mix as they like an alkaline soil. That is not so high that I would expect it to cause mineral deficiencies, but high pH does reduce the plants ability to take up minerals.

You can also get deficiencies when some minerals are high--they interfer with the plants ability to take up other minerals.

You might be better off to repot in a different media--I have not used cactus soil for citrus, so I do not know if that is your problem--maybe other have used it and had good results.

Looks not like Mg deficiency, more like to much salts...
So repotting is a very good advice:

Correcting the soil pH and bring down harmfull slat content in one act.

I must have mis-read citrus for cactus when I first read your post, I also missed the rock wool cubes (I have never used them and do not know what their effect might be).

The inverted V green section of the leaves does agree with this site's description of magnesium deficiency:
http://www.agnet.org/library/bc/52006/

As I mentioned earlier, excess minerals (salts of various minerals) can interfere with with uptake of other minerals. As an example, last year I applied calcium sulfate to my blueberry bushes--they almost immediately began to show magnesium deficiencies (red leaves in blueberries)--it took repeated applications of Epsom salts to get them back to normal.

pH is also a major factor in uptake of minerals--pH for citrus should be 6-6.5

I believe Malcolm Manners said that Trifoliata liked the PH around 5 - 5.5 the lowest for all citrus.

Thanks for the comments. I am now doing every 2 week applications of high nitrogen fertilizer (because 2 of my smaller trifoliates have since developed yellow older leaves) with epsom salts. I avoided dolomite for fear of raising pH.
I was about to give up on it but this week noticed 2 small flushes of growth.