Epsom salt can have a purpose in the garden, but if not used properly it can also be detrimental to plants. Here is some of the science behind the use of Epsom salt. Epsom salt is magnesium sulfate (10% Mg). Mg is an element that plants need, but it is rarely deficient in Oklahoma soils. Mg is a micronutrient most responsible for the absorption of other nutrients, especially phosphorus. It is an integral nutrient of chlorophyll synthesis as well as the formation of proteins, oils, and sugars within a plant. There is no research that shows applying Mg helps in flower formation of tomato. We'll get to flower formation of tomato in a minute. Excess amounts of Mg can interfere with calcium (Ca) absorption. Calcium is a nutrient component that helps prevent blossom end rot and other fruit-related ailments and disorders like cracking in tomatoes. Likewise, if calcium is in excess it interferes with Mg absorption along with the absorption of other micronutrients.

Sulfate, the other ingredient in Epsom salt, can influence pH of the soil by lowering it, so it is best to have a soil test before applying Epsom salt to determine the pH. You can also have the soil tested for the presence of magnesium, but if the pH is at the right level then the magnesium availability is probably good too. It is always a good idea to have soil tested regularly because it does no good or can be bad for the soil, plants, and environment if you are adding things the soil or plants do not need. The Logan County OSU Extension Office can assist you with soil testing.

The use of Epsom salt can have a benefit in intensely managed crops and sandy soils where MG deficiencies can sometimes occur and in soils with a high pH.

Here is a great link to an article that discusses the myths behind the use of Epsom salt - https://s3.wp.wsu.edu/uploads/sites/403/2015/03/epsom-salts.pdf.

Regarding flower formation or lack thereof on tomato, usually referred to as Blossom Drop. Symptoms of Blossom Drop are blossoms dry, turn brown, fail to produce fruit, and may fall from their small stems (pedicels). Blossom drop has been associated with environmental stresses and improper soil fertility that inhibit flower pollination. Prolonged periods of high temperatures (above 90 F) and wind during the day are associated with blossom drop. However, night temperatures may be most important for effective flower pollination. Night temperatures outside the range of 55 F to 70 F inhibit effective pollination and promote blossom drop. Given our current temperatures I would expect this to be the case for lack of fruit set right now, which is very common this time of year. When the weather cools off this fall you can expect flowers to pollinate and begin to develop new fruit. Excess nitrogen can also affect flower formation. Excessive nitrogen fertilization encourages lush vegetative growth and inhibits flower production and/or pollination. Unless you have been putting a lot of nitrogen on the plants I would not guess this to be a problem. I think it is most likely the high temperatures we are experiencing right now.

Control: Following recommended fertilizer rates for tomatoes reduces blossom drop, but if excessive heat and wind develop during flowering, little can be done. Heat-tolerant cultivars have recently been developed which may reduce blossom drop caused by high temperatures. Shading and overhead watering can reduce temperature, but their value decreases under prolonged periods of high temperature.