The most common nutrient deficiency yellow leaves point to is nitrogen, but it is not the only one. Iron, magnesium, potassium, and manganese all turn leaves yellow too. The missing nutrient depends on the pattern you see, so the real answer is one of those five.
You do not have to guess. The plant shows you which nutrient is short by where the yellow starts and whether the veins stay green. Look at two things first. Is the lowest old growth fading, or the fresh new growth at the tips? And does the yellow fill the whole leaf, or sit between green veins?
Those two clues come from a rule that extension specialists lean on. Some nutrients move freely inside the plant and some do not. UConn Extension calls these mobile and immobile nutrients. When a mobile nutrient runs low, the plant pulls it out of old lower leaves and sends it to new growth, so the old leaves fade first. When an immobile nutrient runs low, the plant cannot move it at all, so the young upper leaves fade first while the old ones stay green.
Nitrogen, phosphorus, potassium, and magnesium are mobile, so their shortfalls hit the bottom of the plant. Iron, zinc, manganese, sulfur, calcium, and boron are immobile, so their shortfalls hit the top. That single split sends you to the right half of the answer before you check anything else.
Now read the pattern on the leaves. Nitrogen deficiency leaves turn an even, pale green to yellow across the whole old leaf. The yellow often starts at the tip and runs down the rib. The plant looks light and a bit stunted. This is the look most people picture when a plant needs feeding.
Iron deficiency leaves look different from that. The yellow shows up between the veins on the newest growth. The veins stay sharp and green. This between-the-veins look is called interveinal chlorosis. On fresh top growth, it almost always points to iron.
Two patterns trip people up because they look alike. Magnesium also gives you yellow between the veins. But magnesium is mobile, so it shows on the older leaves, not the new ones. Same pattern as iron, different part of the plant. That one clue keeps you from feeding iron when the plant wants magnesium.
Potassium is the odd one out. It turns the edges and tips yellow and scorched on older leaves. The center of the leaf stays greener for a while. It looks like the leaf is burning in from the rim. Crispy brown edges on old growth are the tell here, not yellow between the veins.
Here is the step that saves your plant and your money. Confirm the suspected nutrient with a soil or tissue test before you feed anything. A soil test reads what is in the ground. A tissue test reads what the plant has taken up. Together they tell you if the nutrient is truly short or just stuck.
That gap matters more than people expect. High soil pH, usually above 7.0, can lock iron out even when the soil holds plenty of it. The roots cannot pull it in, so the new leaves go yellow and look starved. But adding more iron will not fix a pH problem. You have to lower the pH to free up the iron the soil already holds.
Feed a deficiency the plant does not actually have and you can scorch the roots and make things worse. So work in order. Read where the yellow starts, read the pattern, then test to confirm. Match the fix to the real cause and watch the fresh growth come in green. The old yellow leaves usually stay yellow, so judge your fix by the new leaves, not the damaged ones.
Read the full article: Yellow Leaves on Plants: Causes and Fixes
