What is the connection between magnesium and blossom end rot?
Written by
Paul Reynolds
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Magnesium imbalance promotes blossom end rot in tomatoes. Excess magnesium inhibits root calcium transporters, resulting in reduced fruit quality. This calcium deficiency produces black leathery spots, ultimately destroying the tomatoes. I have rescued many crops by correcting this specific mineral ratio imbalance.
Root Transport Interference
- Magnesium ions overload calcium-specific carrier proteins
- Blocks calcium uptake even when soil calcium is abundant
- Creates false calcium deficiency in developing fruits
- Requires immediate reduction of magnesium inputs
Fruit Cell Development
- Calcium deficiency prevents pectin formation in cell walls
- Fruit tissue collapses at the blossom end first
- Leathery brown patches expand making tomatoes inedible
- Secondary mold infections often follow tissue damage
Prevention Ratios
- Maintain calcium-to-magnesium ratios between 3:1 and 6:1
- Sandy soils need lower ratios (3:1) than clay soils (6:1)
- Test soil monthly during fruit development phase
- Apply gypsum pre-planting in high-magnesium soils
Identify early symptoms. Small water-soaked spots appear ten days after flowering. These spots darken rapidly and cover one-third of the fruit. I check tomatoes daily during periods of critical growth. Timely discovery allows me to apply corrections quickly, thereby saving the crop.
Apply gypsum for rapid calcium correction, using 3-5 pounds per 100 square feet, as recommended by the soil test. Water well to stimulate the release of calcium. Gypsum is incorporated before planting in badly affected areas. This procedure eliminates the danger of blossom end rot later on.
Watering carefully. Inconsistent watering can exacerbate issues with calcium transportation. I work with drip systems to maintain a soil moisture level of 70-80% (by feel, as a general guideline). I check with soil probes to avoid drought stress. Your goal is to maintain constant moisture to regulate calcium levels in developing fruit.
Choose disease-resistant tomato varieties. 'Mountain Merit', for example, is tolerant of wide mineral fluctuations. These should be planted in the most dangerous spots. The yield is more dependable when the ratios are not 100% perfect, provided the referred varieties are used. This, combined with good fertilization, yields the best results.
Read the full article: Magnesium Deficiency Tomatoes: Complete Grower Guide
