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I had a lemon tree that turned yellow out of nowhere, and it turned out it was in a pot without drainage holes - basically it was drowning. I moved it to one with decent drainage, stopped watering so much, and in two months it got its color back. But it's also worth checking if it's getting enough direct light, because those trees are pretty picky about that; if it's in partial shade or against a wall, that'll yellow the leaves fast too.
Most likely it's lacking nitrogen or you're watering it too much. The lemon needs well-draining soil because excess water rots the roots and causes those yellow leaves; if the pot doesn't have drainage holes, that's the first thing to fix. Try letting the soil dry out between waterings and, if the plant has looked like this for a while, use a citrus-specific fertilizer following the package instructions. If it's in a pot that's too small, consider repotting it to a bigger one in spring.
Yellow leaves on lemon trees can come from several things, but before you dump fertilizer on it, it's worth checking where the plant is. Citrus trees are pretty fussy about light - if it's in a spot without much direct sunlight, the leaves turn yellow even if everything else is fine. Try moving it to a sunnier spot, because without that, no nutrient is going to fix the problem.
Drainage matters, yeah, but also check if it has pests. Lemon trees love spider mites, and when they show up you get that yellowing that looks like a nutrient deficiency but it's actually because the mites are sucking the sap. Get a magnifying glass or just hold the leaf up to the light and look for tiny red dots or fine webbing on the underside. If you find them, that does need specific treatment, not just adjusting your watering.
When a lemon tree turns yellow, people always jump to "it needs nitrogen" or "you're watering too much," but there's something a lot of folks miss: the substrate pH. Citrus trees are pretty picky about this, and if the soil is too alkaline (or the other way around, too acidic), the plant won't absorb nutrients properly even if they're there. It happens a lot with really hard water or if you've been using the same fertilizer for years without checking.
What I'd do first is look at which leaves are turning yellow. If it starts with the older ones (the ones at the bottom) and the new growth comes in green, it's probably nitrogen or potassium deficiency. But if it's affecting the whole plant equally or you're seeing a weird pattern, it's more likely a drainage or pH problem. Before you add any fertilizer, make sure the pot has decent drainage holes that drain in less than a minute after watering, and try switching out the substrate if it's been in there a long time.
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