Today 
Blossom end rot is caused by calcium deficiency. Without calcium, a plant's cell walls collapse and die, resulting in the bruised appearance of the fruits. Plants take up calcium from the soil through their roots, so if it's dry, the calcium stays locked in the soil and the plant suffers. Acid soils always have low levels of calcium. If treated early, later fruits will go on to ripen successfully. The problem can also be triggered by applying fertiliser to dry soil.
Solution
Organic
Don't allow the soil around the plant's roots to dry out. Water plants regularly, as even a short period of drought can encourage the problem. Compost sold specifically for tomatoes, such as grow bags, contains sufficient calcium for a good crop of fruits. Never apply fertiliser to dry soil, always give plants plenty of water first.
