Hello. I have some calamondin or kumquat plants. I mixed up the plants so I’m unsure which one this is. The leaves are yellowing. I did some google investigation. I think it’s magnesium deficiency or boron poisoning. New growth sometimes shrivels and dies. I tried putting some epsom salt on the soil. What do you guys think is the problem?
Cold weather and/or cold soil can make nutrient deficiencies appear that actually aren't issues. If you've had them in a chilly place, and if it is about to warm up in the next few weeks, you might want to consider waiting and watching. (After checking for pests, as mentioned above). The crinkled up leaf looks like damage from aphids to me but I'm sure it could be many other possibilities.
If you do decide to add fertility, personally my permie approach is to add just a little compost or spent coffee grounds under some mulch on top of the soil (yes, even in pots I keep mulch) every few days and see if the plant perks up, and then stop when it does. I add only a little each time so I don't overdo it. Seems to work for me, but I garden on a small scale.
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As others have said, the pot is too small for the size of the tree, and, soil temperature affects the uptake of nutrients.
However, it looks like a lack of micronutrients e.g. magnesium, manganese, boron deficiency. These are easily washed out of potting soil and their uptake is affected by temperature.
So, suggest a much bigger pot, revitalise the potting soil with new stuff, and add an all-round citrus food, typically, poultry manure and/or a bit of blood & bone.
Also, it seems the young leaves have leaf-miner insect larvae in them - easily fixed by squashing the leaf between two finger or removing the leaves, getting a fruit fly trap, or simply accepting them - mature trees that have good nutrition aren't usually damaged that much anyway.
'Every time I learn something new, it pushes some old stuff out of my brain.'