Could be Magnesium; but it looks more like a Manganese deficiency, which is common in the early Spring; it is caused most commonly by a combination of high pH soil and cool weather. If it is Manganese, it should improve as the weather warms. If it is Magnesium a little Epsom Salt will solve it; if it isn't the Epsom Salt won't hurt.

I have a bunch of epsum salt stocked up for tomatoes so I'll get right on it! Thank you! It is pushing out lots of new growth and forming little flower buds so maybe it just needs more right now. Hopefully that's all it is!!! Fingers crossed!

I put Magnesium Sulfate (Epsom Salt) on my garden Meyers every time I put it on my roses. If they don't need it, it is a very small cost.

Oh that's good advice! I'm starting double delight ( also called fire and Ice) roses this summer so I'll remember to add epsum salt to them too! I have bought a bunch of rose- specific growing mediums and I'm ready. Apparently I am drawn to try and grow the most difficult things haha.

Amanda, Magnesium Sulfate (Epsom Salts) will not do any good with your calamondin's deficiency, because your plant's deficiency is a manganese deficiency not a magnesium deficiency. Manganese sulfate, manganese nitrate or even a fertilizer formulation with manganese in the trace minerals will normally take care of the situation. . Or you can do nothing at this time and wait for warmer weather.

Thanks! I will get some tomorrow. Most places sell it, right?

my ML looks just like this too, i'm listening to these guys and just wait for the weather to warm up, just fed them well with some espoma citrus tone last weekend.

I added the epsom salt to the foliage pro and watered them. Then mixed up more foliage pro + Espoo salt and used it as a spray. The green is returning to the leaves and the yellow in diminishing! Even the new growth is getting dark green! Thanks everyone! I ordered som manganese and as soon as it's here I will give it a dose of that too!

Amanda, I was going to say what Silica mentioned (which I learned from him). Mg and Mn deficiencies look the same except Mn occurs on new leaves and Mg on older leaves (Silica pls correct me if I am mistaken). Question: why will warm temps cure it?

Stable warmer temps will help the roots to take up nutrients a lot easier...By the way, anyone using a very good well balanced fertilizer like Foliage Pro should not have to ad any additives...The only thing you should do is either go up on dosage, be more consistent, make sure the fertilizer solution is acidic enough to benefit your trees, make sure temps are warm consistently, above 55 or check their watering practices..)

The spraying the leaves method with Epsom Salts is can be an immediate quick relief just a coincidence or perceived that way, but getting to the source of the problem is always better in the long run..

Some of mine look like that for reasons stated above by the time it's finally time for them to go back out at the end of winter, but the main cause to blame is especially from a lack of longer sunny warm days..

Thanks for the explanation Silica! I remembered what you said, but did not know why.

Looks like manganese deficiency. Nitrogen deficiency can also play a part in exhibiting these signs.