Epsom salts (magnesium sulphate) applied to soil naturally lacking magnesium alleviates the yellowing leaf symptoms in a variety of crops, including tomatoes, according to Linda Chalker-Scott, Ph.D. at Washington State University. If your garden soil, based on a soil test, reveals a deficiency in magnesium, apply a solution of Epsom salt. Do not over-fertilise with it though, as an overabundance of magnesium ions prevents uptake of other nutrients, like iron or manganese according to the authors of "Your Florida Landscape." Water-soluble Epsom salts readily leach through soil that is frequently irrigated.
Fill a container, such as a gauged sprinkling can or bucket, with one gallon of water.
Add 1 tsp of Epsom salts to the water and mix it well. Placing the Epsom salt crystals in the empty container and then pouring in water is one way to agitate and evenly mix the solution. Or, use a stirring stick or long-handled spoon.
Pour the Epsom salt solution evenly around the root zone of the tomato plant, which extends 2 to 4 feet from the main plant stem base. Mix additional solution, always at a rate of 1 tsp per gallon, as needed to wet the garden soil around the tomato plant.
Repeat the application of Epsom salt solution and apply to the tomato plants' root zones every 4 to 6 weeks. Tomatoes grown in containers and hanging baskets need applications every 10 to 21 days because they leach nutrients quickly from frequent watering.
Water the tomato plants with the Epsom salt solutions once a month across the growing season, between the last spring frost and the first killing frost in autumn. Sandy soils leech nutrients more quickly than clay or loam soils. Adding organic matter to the garden soil before planting tomatoes improves soil fertility and drainage. Often adding compost and manure diminishes the risk of vegetable plants needing supplemental applications of magnesium to grow and perform well.
According to Chalker-Scott, garden soils overly rich in potassium or lacking in nitrogen lead to magnesium deficiencies, too. Don't arbitrarily shower tomato plants with Epsom salts for the sake of fertilising; use it when your garden soil truly lacks magnesium. A soil rich in organic matter and fertilised appropriately with well-balanced fertilisers do not create tomato plants deficient in magnesium. Remember, yellow leaves and dried up flowers on tomatoes occur from a variety of reasons. Don't blindly fertilise with Epsom salts if your soil is fertile. Learn what insect or disease is present that's causing the problem and address that issue.