Tomato Blossom End Rot (BER) seems to be a subject of ongoing discussion – most recently in articles published by Underwood Gardens: ( http://www.underwoodgardens.com/slide-gardening-tips-and-tricks/blossom-end-rot-what-to-do/ ) and Mother Earth News: ( http://www.motherearthnews.com/organic-gardening/blossom-end-rot-prevention-and-treatment-zbcz1502.aspx ), but we have seen many other articles in different publications about the same topic, over the years. While it is true enough that this classic nutritionally-precipitated disease is caused by Calcium deficiency in the plant, the better question – which seems to have not been discussed - is, "Why is there a deficiency?".
While both articles (and most of the others) accurately cite "plant-unavailable" Calcium issues (paradoxically, often in high-Calcium soils) as the problem, there can be other plant stresses inducing the disease.
By using today's current soil-testing methods, employed by most all labs, Calcium is "over-extracted" – into those Calcium compounds (especially, bicarbonate) which are not readily plant-available - thereby reporting to the grower an adequacy or over-adequacy of [plant-available] Calcium levels in the soil.
But it can get worse. Using irrigation water that is hard (high in bicarbonates), on a calcareous (high pH) soil, can over time, effectively turn your soil to Stone and decimate your plants, because, "What's In Your Water Becomes Part Of Your Soil"™.
But by far, the most frequent cause of BER we see is NOT plant-unavailable soil Calcium, but a deficiency of the micronutrient, BORON in the plants.
Boron is essential for the transport of Calcium (as well as other nutrients) within the plant. Your plants could be planted in pure lime but without Boron, they would soon die from Calcium deficiency. But Boron is VERY rapidly tied-up by Calcium (see Mulder's Chart). And don't overlook von Liebig's "Law of the Minimum"...
It used to be, in the olden days of soils with generally high levels of Organic Matter (OM), Boron toxicity was unremarkable, especially if the irrigation water came from deep sources, such as deep wells or lakes, as OM holds Boron (and other things). Accordingly, over time, repeated applications of Boron-laden water would result in toxic levels of Boron being built-up in high-OM soils.
Pretty much not so, any more. Now, we most often see soil samples containing less than one percent OM and Boron deficiency is endemic, most especially in, but certainly not restricted to soils with a pH of over 7 – not only in tomatoes, but in sugar beets (as Crown-Rot) and in other crops, as well.
Comprehensive plant-sap testing is generally too expensive for the small grower, so what is the small grower to do when BER is observed and the soil is known to have adequate plant-available Calcium – and the other conditions known to cause plant stress are unremarkable?
BORAX contains about 11% Boron and is readily-available at most grocery stores and is readily-dissolvable in water. The most efficient and rapid induction of many nutrients into the plant is by foliar spray. Of course, when applying a foliar spray, a surfactant or "spreader-sticker" must usually be a part of the spray mix.
Foliar sprays bypass the chemical interactions of soil chemistry. However, as with many foliar nutrient sprays – especially those containing micronutrients and trace elements, an extremely cautious, judicious and conservative approach must be used.
Unless you have gotten specific recommendations from your comprehensive plant analysis report and have precision spraying equipment, Always Titrate.
It does not take much of micros and/or traces to kill your plants. These things are called, "micros" and "traces" for a reason. It is better to be some short than a lot over…
BER is perhaps the most easily and inexpensively-cured disease there is – but it helps to know what the deeper underlying causes could be.
In general, adequate micronutrients and trace elements are essential for the plant's ability to protect itself from disease. Additionally, they can make a very substantial difference in the quality, storage longevity, nutrition and taste of the fruit.
"Nature always eats the weakest." With stressed plants, it is usually bugs and/or disease.
And remember that plants' nutritional demands can change frequently and dramatically throughout the growing season…
For more information about tomato cultivation, please see: http://asktheplant.com/?p=75