Calcium for Tomato Blossom End Rot


I live in Oakville Ont., lots of sun, loam soil which drains easily. I have trouble with a fungus which I think is Tomato Blossom End Rot. It’s a black spot on the bottom of the tomato that expands quickly inside & outside of the tomato. I’ve read that one of the causes of this is a lack of calcium. I rotate my Garlic garden(1300 plants) with the Tomato garden(35 plants) every 3 years. I heard that it takes 1 year for calcium to be in the soil before it can be taken up into the plants. Since I put 1 heaping teaspoon of Bone Meal into each hole for garlic & each garlic is only 6″ apart there should be lots of Calcium in the soil after 3 years for the tomato plants to use. The Bone Meal I have is high in calcium (25%) & it doesn’t seem to be working very well. Maybe the garlic used up all the calcium. But the last year’s crop couldn’t use the calcium because it hasn’t been the soil for 1 year. Do you know if there’s something that I can add to the soil so that it can be taken into the plant almost immediately? We just found an old partial bag of “Water Soluble Granular GYPSUM” (looks like coarse lawn fertilizer) which is 67% Calcium Sulfate, 18% Calcium(C2) & 13% Sulfur & 1% Magnesium. Do you think this is what I need? If not please advise what you think I should do.



Thank you for writing about this problem with tomatoes. This is something that many tomato gardeners often experience, and so hopefully your inquiry will be helpful. Blossom end rot is neither a fungus, nor a disease, but in fact a physiological problem within the plant. This is also seen in cucumbers, peppers and melons. The key issue is the inability of the plant to maintain consistent moisture levels throughout the growing period, from too much, or too little water. So the problem is generally not whether there is enough calcium in the soil, or growing medium, but, in fact, whether the root system has enough available moisture to deliver  the available calcium to the plant above ground. Yes, the soil should have a reasonable amount of calcium, with a PH of about 6,50, which you clearly have maintained. But beyond that, consider mulching your tomato plants to avoid moisture loss, and endeavour to maintain consistent moisture levels for the developing fruit. For example, blossom end rot will often appear after a wet spring that soon becomes a very dry summer. Container tomatoes are particularly susceptible, if the roots systems tend to become parched between waterings.

Here is a previously answered query by the TMG on this topic, which should also be helpful. Therein you will see two more links to helpful advice.

In summary, regular, vigilant watering is key, not so much the quantity, or type of fertilizer you might use. I hope this helps for your upcoming season for a wonderful tomato crop!