This year I set up my container citrus on an automatic watering system, which has saved me a ton of stress. Most are in gravel mix, but the oroblanco and two gardenias are in coconut husks. The gardenias are doing fine, as is the Kaffir lime (the dark green leaf in the photo). Any ideas why the Oroblanco is turning yellow? Another citrus has yellow veins as well.

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N is for Nitrogen.

I have a auto water system too, they are awesome! however, the first nutrient to leach out will be nitrogen. you may add some extra granular based N or a fertilizer with more N. but the problem is N for sure

I think the medium got too basic.

Ad a drop of vinegar at 10 liter water.
The leafes will become green again.

Thanks for the advice! I'll do both....treat with vinegar as well as fertilizing. Since it's time for them to come indoors, the cure-implementation should be straightforward.

No !
They recover !

See here:
link

If you bring them indoors, you should low down your fertilizer.
Besides I think, it is NOT a lack of Nitrogene, but just the roots cannot get the minerals and trace minerals for the PH-value is wrong.
That is called Chlorosis.

I agree with RyanL that it is a deficiency of nitrogen, especially shown in the second photo.. Unlike some of the other nutrients, nitrogen is still available to the tree whether the medium is basic or acidic. Add vinegar if you wish, as it will probably do no harm, but I think the problem is not vinegar, but rather because nitrogen is so water soluble the automatic watering system has leached much of the nitrogen out. - Millet 99