Do coffee grounds help prevent blossom end rot?
Written by
Benjamin Miller
Reviewed by
Prof. Martin Thorne, Ph.D.Coffee grounds offer minimal calcium benefits yet contain excessive levels of nitrogen, which is a cause for concern. Very high nitrogen sources encourage foliage production and also limit calcium from entering developing tomatoes. I have observed significant amounts of blossom end rot in tomato plants grown with heavy applications of coffee grounds.
Nutrient Imbalance
- High nitrogen content (2% by volume) promotes foliage over fruit development
- Acidity temporarily lowers soil pH locking away existing calcium
- Caffeine residues inhibit root growth reducing nutrient absorption capacity
Calcium Deficiency
- Only 0.06% calcium content - insufficient for tomato requirements
- Slow decomposition delays calcium release missing critical fruit set timing
- Competes with existing soil calcium through cation imbalance
Practical Constraints
- Mold growth risk when applied thickly creating anaerobic conditions
- Attracts pests like fungus gnats and pill bugs damaging roots
- Inconsistent particle size causes uneven nutrient distribution
Instead, use effective calcium sources. When cracked and finely ground, eggshells can provide up to 38% calcium if pre-composted. Gypsum provides an immediate soluble calcium source without affecting pH. I use both in my garden with consistent results.
Apply amendments properly. Incorporate eggshells or gypsum into topsoil before planting at a rate of 1 cup per plant. For plants already growing, side-dress them once a month while they are fruiting. Be sure to soak the soil after application to help release the nutrients. Keep it from touching the stem.
Prioritize balancing soil chemistry. Test the pH level every year to maintain a range of 6.5-7.0 for optimal calcium availability. Coffee grounds decrease soil pH, which exacerbates the problem. I always normalize the pH before adding any calcium supplements so that they can work to their full potential.
Calcium sources and consistent watering must be combined for optimal results. Calcium travels with water through plants. Using a drip irrigation line provides consistent moisture. Check the soil daily, especially on hot days. When you combine the two, it prevents blossom end rot more effectively than either one alone.
Read the full article: Tomato Blossom End Rot Explained Simply
