Help! What's wrong with my tomato plant?

The Garden Spot Ask a Gardener
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1

The watering is perfect with RainPoint, so I don’t think it’s that. Could it be a nutrient issue? Any tomato experts here?

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To help the experts here help you, could you tell us:

What do the yellow leaves look like exactly? (e.g., all over yellow, yellow with green veins, yellow spots?)

Which part of the plant is affected? (Old leaves at the bottom, or new growth at the top?)

Are you seeing any issues with the tomatoes themselves? (Blossoms dropping, fruit not forming, or spots on the fruit?)

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Great question! You’re right to suspect nutrients, especially for heavy feeders like tomatoes. The fact that your watering is dialed in with RainPoint is a huge advantage—it lets us rule out the most common issue.

Here’s a quick diagnostic checklist:

1. Observe leaf location and symptoms:

  • Are the older leaves at the bottom turning yellow? This is likely a nitrogen deficiency, the most classic symptom of a mobile nutrient shortage.

  • Are the newer leaves at the top turning yellow, but the veins remain green? This could indicate an iron deficiency or other micronutrient issue, common in alkaline soils.

  • Are the leaves showing a purplish tint? This may signal a phosphorus deficiency, especially during early growth or in cool temperatures.

2. Recommended next steps:

  • Take clear photos (of the whole plant and close-ups of affected leaves) and upload them here. The location and appearance of symptoms are key to diagnosis.

  • If possible, share your soil type and fertilization history (what fertilizer you’ve used and how often).

The community is full of tomato enthusiasts—let’s work together to figure this out!

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4

Good call ruling out watering first. For tomatoes that are setting fruit, a common nutrient need is potassium (K), which supports fruit quality and ripening.

Here’s a simple, low-risk action you can take today:
Water your plants with a diluted solution of liquid seaweed fertilizer or a tomato-specific liquid feed. These are fast-acting and can give the plant a quick boost while you assess.

Also, check the undersides of leaves for any tiny pests (like spider mites or aphids) that could be causing stress, as that can sometimes look like a deficiency.

Keep us posted!

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5

Ah, the classic tomato mystery! Been there. Since your watering is perfect with RainPoint (great choice, by the way!), nutrients are indeed the prime suspect.

From my experience, blossom end rot (dark, leathery spot on the bottom of the fruit) is often mistaken for a watering issue, but it’s actually a calcium uptake problem, frequently caused by inconsistent watering in the past or soil pH being off. Since you’ve now got consistent watering locked down, the next step is to check your soil.

My go-to move in this situation is to:

  1. Get a simple soil test kit – they’re inexpensive and will tell you pH and key nutrient levels.

  2. Side-dress with a balanced, slow-release organic fertilizer (like a 5-5-5 or one specifically for tomatoes) if it’s been more than a month since you last fed them.

Let us know how it goes!

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Looks like it could be a nitrogen deficiency. Have you tried a balanced fertilizer?

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Check the underside of the leaves for tiny bugs! It might be an early pest issue.