To diagnose a tomato nutrient deficiency, check which leaves show problems first and note the pattern you see. Old leaves at the bottom turn yellow from nitrogen shortage. New leaves at the top curl or yellow from calcium or iron issues. The location tells you what nutrient is running low.
I learned to read tomato leaf deficiency symptoms by keeping notes on what I saw each year. When I first started gardening, I thought every yellow leaf meant the same thing. Now I know that uniform yellow on old leaves points to nitrogen while yellow between green veins means magnesium. The pattern matters more than the color.
Some nutrients can move around inside the plant when supplies run short. Nitrogen, magnesium, and potassium flow from old leaves to feed new growth at the top. This makes old leaves yellow first while new growth stays green. Other nutrients like calcium and iron stay put once they land in a leaf.
A nitrogen deficiency tomato shows the most common symptom you'll see in home gardens. The whole lower leaf turns pale green then yellow in an even pattern across the surface. The plant looks light colored overall instead of dark and healthy. Growth slows down and flowers may drop before setting fruit.
Not every yellow leaf means you need more fertilizer though. Tomato fertilizer problems often come from adding too much rather than too little. Extra nitrogen pushes leafy growth but cuts fruit production. High salt from over-feeding burns roots and blocks nutrient uptake even when plenty sits in the soil.
Get a soil test before you add anything to fix what looks like a tomato nutrient deficiency. Many problems trace back to pH issues that lock up nutrients already in your soil. When pH drops below 6.0 or climbs above 7.0, plants can't take up what they need. A pH fix often clears up symptoms better than more fertilizer.
Water issues cause many symptoms that look like nutrient problems too. Roots that stay too wet rot and can't feed the plant above. Roots in dry soil shut down and stop moving nutrients to the leaves. Check your watering habits before you blame a deficiency for what you see.
When you do need to add nutrients, start with a half-strength dose of balanced tomato fertilizer. Watch how your plants respond over two weeks. Add more only if symptoms stay the same or get worse. This careful approach avoids the trap of over-feeding that makes problems worse in many home gardens.
Keep a simple log of symptoms you spot and what you did to fix them. This record helps you learn your garden's patterns over time. You'll start to see which deficiencies hit your soil most often. Then you can adjust your feeding plan each year to prevent problems before they show up on your plants.
Read the full article: 8 Common Problems With Tomato Plants and Solutions
