Blossom-End Rot on Tomatoes: Causes, Prevention, and Calcium Management

Black Bottoms on Tomatoes? It's Blossom-End Rot

Those dark, sunken lesions on tomato bottoms aren't a disease—they're a physiological disorder caused by calcium deficiency during fruit development. In EarthBox containers and similar systems, inconsistent watering is the primary trigger.

How Watering Affects Calcium Uptake

Calcium moves through plants via transpiration stream. Shallow, frequent watering encourages surface roots that can't access deeper calcium reserves. Overwatering displaces soil oxygen, impairing root function. Both scenarios halt calcium transport to developing fruit.

Effective Prevention Strategies

1. Deep, consistent watering: Apply 1.5 inches weekly in 1-2 deep soakings. In containers, water until excess drains freely.
2. Mulch heavily: 2-3 inches of straw or dried grass clippings stabilizes soil moisture and temperature.
3. Calcium supplementation: Crush eggshells into powder and incorporate at planting, or apply calcium nitrate (1 tbsp/gallon) as a foliar spray every 2 weeks after first fruit set.
4. Epsom salt role: Magnesium sulfate (1 tbsp/gallon) improves overall nutrient uptake efficiency, including calcium—but doesn't supply calcium itself.

Container-Specific Tips

EarthBoxes and similar sub-irrigated planters reduce blossom-end rot risk by maintaining consistent moisture. Ensure the reservoir never runs dry. Top-dress with compost mid-season to replenish nutrients.

Quick Diagnosis Checklist

• Water deeply 1-2×/week, not daily sprinkles
• Mulch to reduce evaporation
• Test soil pH (target 6.0-6.8 for calcium availability)
• Avoid high-nitrogen fertilizers during fruiting

Existing affected fruit won't recover—remove them to redirect energy. New fruit developing under corrected conditions should be healthy.