Tomato Leaves Turning Yellow
"Why are my tomato leaves turning yellow" almost always needs a follow-up question: where on the plant, and where on the leaf? Yellowing that starts at the bottom of the plant points somewhere very different from yellowing that shows up on new growth, or right after transplanting — the position is often more informative than the color itself.
Read the pattern first
Where the yellowing starts is the biggest clue. Uniform yellowing of the older, lower leaves while new growth stays greener usually points to nitrogen deficiency — the plant often looks pale overall and grows slowly. This is especially common in containers, where frequent watering washes nutrients out.
Yellow tissue between green veins on the lower leaves points to magnesium deficiency, which is more common in sandy soils that leach easily. In advanced cases the affected areas can turn purple.
Yellow leaves with brown spots, especially target-like rings, point to early blight rather than a nutrient problem. It starts on the lower leaves and often shows a yellow halo around each lesion.
Water and roots
Too much water starves roots of oxygen, so the plant cannot take up water even though the soil is wet. The main symptom is yellowing or wilting of the lower and inner leaves. Overwatering can look like drought stress, so check soil moisture a few centimeters down before assuming a nutrient problem.
When it is just normal
Yellowing limited to the oldest, lowest leaves as the plant matures is usually normal aging and no reason to act. A few yellow leaves in the first days after transplanting are also common while the roots recover.
What to do, step by step
- Note exactly where the yellowing is and whether the veins stay green.
- Check soil moisture a few centimeters down, and drainage. Let the top layer dry slightly between waterings.
- A soil test can confirm a nutrient shortage before you fertilize.
- If spots with rings appear, consider possible early blight — see the brown spots guide.
- Causes can overlap: for example, early blight is often worse when nitrogen runs low in mid to late summer.
Possible Causes
- Natural lower-leaf yellowing as the plant matures and redirects energy upward — normal if it stays limited to the oldest, lowest leaves
- Nutrient deficiency (commonly nitrogen or iron) — nitrogen tends to show on older leaves first, iron on newer growth
- Transplant shock or root disturbance in the days right after moving the plant
- Overwatering or poor drainage limiting how much the roots can take up
- A fungal or bacterial disease affecting the roots or stem, especially if yellowing comes with wilting or spots
What to Check
- Where the yellowing is — bottom of the plant, top, or all over
- Whether it's between the veins or across the whole leaf
- Timing — did it start after a recent transplant or a change in watering?
- Any other signs alongside it — spots, curling, wilting
Sources
This guide was checked against the public references below. It is general background, not a diagnosis of a specific plant.
- University of Maryland Extension — Key to Common Problems of Tomatoes
- University of Wisconsin Horticulture — Troubleshooting Tomato Problems
- UC IPM — Nitrogen Deficiency
- South Dakota State University Extension — Monitoring and Correcting Magnesium Deficiency
- Colorado State University Extension — Tomato Early Blight (GardenNotes 718)
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