Blossom end rot or the cracking and splitting of the blossom end of a tomato is caused by sudden changes in moisture in the soil and also by a lack of calcium. Mulch the soil to seal in moisture and regulate temperature especially if summers are above 90 degrees. A fertilizer like Osmocote that contains micro nutrients like calcium should be applied in the spring. Adding crushed egg shells to the soil at planting time is an old tradition that farmers once used to prevent the cracking of the tomato because egg shells contain calcium. Keep growing, Marianne Binetti