Epsom salts (magnesium sulphate), dug into the soil around plants or dissolved in water and applied as a spray onto the leaves, provides magnesium. Magnesium deficiency in a plant can result from soil that lacks the mineral, or from an overabundance potassium in the soil, which interferes with plant absorption of magnesium. Magnesium-deficient tomato plants develop interveinal chlorosis; the leaves turn yellow between the veins. This inhibits plant growth and interferes with optimal fruiting.
When applied to a tomato plant exhibiting magnesium-deficiency symptoms, Epsom salts will counter the deficiency, and the leaf discolouration will disappear. However, no increase in tomato production will result.
As much as 49 per cent of Epsom salts applied to the soil leaches out of the soil before it is absorbed by the plant. To increase the delivery of Epsom salts magnesium to the plant, dissolve them in water and then spray the solution onto the foliage. Do this early in the day, and use care to prevent oversaturating the leaves so that the solution doesn't drip off onto the soil.
Epsom salts are highly soluble. However, although the fertiliser is quick-acting and disperses rapidly from garden soil, some gardeners mistakenly conclude that it is easier on the environment than other fertilisers. However, the dissolved Epsom salts don't disappear. Rather, they can wash into and pollute groundwater, streams and lakes.
Test your garden soil for nutrient deficiencies before you plant, and don't supplement magnesium unless the test indicates there is a magnesium shortage. Unless your soil has been heavily cropped or is very sandy, it's unlikely that it will be low in magnesium.
According to Dr. Linda Chalker-Scott, associate professor and extension urban horticulturist, "An automatic application of Epsom salts to plants or soils that are not magnesium deficient is a poor management strategy that can injure the plants and contaminate the soil."