Mild deficiency — does anyone have an idea what they're lacking?

interveinal chlorosis can be many things... there's also a blue hue predominantly around the veins. It's happening in the middle of the plant, so it's a somewhat mobile nutrient. Too little information to be confident. If you get some spots accompanying it, i'd lean Mg deficiency, but if you get copper or dark blotches, i'd go p-deficiency. Migth be worth it to see how it progresses so that you don't compound the issue trying to correct the wrong thing. I can't tell how you are fertilizing from the diary.. if treating it like a soilless medium, should be easy to rule out possibilities simply looking at the formula you are providing.

Based on the visual cues in the picture, it looks like a classic case of early Magnesium (Mg) deficiency or a slight Potassium (K) deficiency starting to show. If you look closely at the lower and middle fan leaves, you can see some distinct interveinal chlorosis, where the tissue between the veins is turning a lighter, yellowish-green while the veins themselves stay relatively dark. Since the plant is well into its flowering stage, its demand for both phosphorus and magnesium skyrockets. Before adding more nutrients, the first thing to check is definitely the pH of the water/runoff, because a lockout will cause this even if there are enough nutrients in the medium. If the pH is on point, a light dose of Cal-Mag or a gentle Epsom salt foliar spray/drench should help clear that right up and keep those buds swelling nicely. Happy Growing Growmie🌱

Magnesium is what's presenting, clear interveinal chlorosis. I don't see any nitrogen shortage whatsoever, nitrogen would start from the tip of leaves and work in on bottom growth if it was nitrogen. Doesn't necessarily mean its magnesium deficiency though it could be antagonistic (lockout) from I excess of calcium or K. The red or purplish stems are almost always a distress signal, and at their root, they usually trace back to how the plant's roots are breathing. People will yell P deficiency, or N this or that, but once you run the symptoms to root cause not just visible symptoms it all stems from a lack of oxygen. Whether it be soil compaction, poor porosity, high RH, over watering, take your pick. Soon as oxygen goes every plant process grinds to a halt, glucose conversion goes from 40 atp per glucose molecule to 2 and the plant releases alot of acids that will skew pH, by time symptoms present there could be a myriad of visual symptoms, nutrients stop uptaking, symptoms appear. Soon as a plant goes aneroboc you lose all bulk flow too, which often explains cal and mag since a large majority of cal and mag would under normal circumstances uptake for "free" (without atp cost) Soon as bulk flow stops every single atom requires a atp cost to uptake, but side the plant went from 40 to 2, very very little is possible now. Good luck.

Looks like nitrogen deficiency. The older leaves are yellowing while the veins stay green, and new growth looks okay that’s classic N deficiency. Give her a nitrogen boost either bump your veg nutrients or add some extra N if you’re already in flower. New growth should green up in a week once you feed.