For the benefit of anyone reading this section who is not familiar with the topic, Epsom salt is an inorganic compound primarily used as a bath salt which is reputed to give relief to tired, aching muscles. It is named after the town of Epsom in England, where the substance was discovered naturally occurring in mineral springs. Gardeners have been using the substance for many years to supplement plants with magnesium, an important plant macronutrient required for growth.
First off, in response to the quote from Jeff Gillman, in the otherwise informative article in the NJ Star Ledger (http://www.nj.com/homegarden/garden/i...), in which he claims
“Epsom "salt" is a misnomer, since the principal ingredients are magnesium and sulfur”
Well, sorry, Dr. Gillman, Epsom salt IS, by chemical definition, a salt. It may not be something you want to season your food with, but it is a salt nonetheless. It is formed commercially either by the reaction of the mineral kieserite with water, or, less commonly, by the reaction of magnesium oxide with sulphuric acid (see Figure 1).
Both kieserite and the form of magnesium sulphate in Epsom salt (epsomite) are termed hydrated as they are associated with water molecules. Epsomite is more properly known as magnesium sulphate heptahydrate (from the Greek hepta meaning seven), denoted by the chemical formula MgSO47H2O; see Figure 1).
Magnesium sulphate can be used to treat magnesium deficiency in plants. It is often used in intensive cropping systems to meet the Mg and S requirements of the plants in magnesium depleted soils. This has led to many gardeners attempting to mimic the practice by using Epsom salts which are readily bought at most stores or pharmacies. However, salts can be easily misapplied and the underlying issues with plants in the garden should usually be treated using other means.
One of the most common causes of magnesium deficiency is the loss of nutrients through “leaching” in sandy or light soils. Leaching is the process by which nutrients are removed from soil in draining rain or irrigation water. Water drains more easily from light soil, making it more prone to leaching. This very fact is one of the reasons why the addition of Epsom salts is ill-advised. The salts dissolve in the water and are also leached away from the plant. This may not seem like a big deal but the dissolution of magnesium sulphate leads to draining water that is now contaminated with magnesium and sulphate ions. Not good.
Another cause of magnesium deficiency in some plants is an excessive amount of potassium in the soil. This interferes with absorption of magnesium through the plant roots, even if is is in plentiful supply. The answer here is clearly not to supplement magnesium. It is to dilute the potassium to less harmful levels for the plants. The recommended method is to supplement soil nitrogen.
In heavy clay a more long term solution would be to loosen and amend the soil to make it more permeable to ensure nutrients are easily available to the plant.
Epsomite (magnesium sulphate heptahydride)
Figure 1. Formation of magnesium sulphate