If your into Organic Gardening and a part of one of the many online communities you have no doubt come across claims surrounding the use of Epsom salt in the garden.
As you know I have been putting garden practices and claims to the test to see which ones are supported by science and which ones are not.
For example the science behind the use of coffee grounds in the garden is supported whereas the use of cold coffee to fertilize and lower the pH around your blueberries is not.
Today we are going to see if Epsum salt really good for your garden?
Epsom salt is made of magnesium sulfate (MgSO4) when dissolved in water it releases Magnesium and Sulphur.
Magnesium and Sulphur are both essential elements for plant growth and production. Magnesium playing a key role in chlorophyll and sulphur plays a key role in amino acids. 
It would seem logical that more Magnesium and Sulphur in the soil would help your plants and support these claims right?
Lets talk about a few of these claims and for the purpose of discussion focus on Magnesium.
Industrially produced crops such as tomatoes, watermelons and grapes are grown using taxing growth techniques including irrigation and synthetic fertilization. Large scale irrigation has been shown to cause Mg deficiencies.
Deficiencies can cause yellowing of leaves, reduced leaf number, and a number of other stress responses that can present differently in different plants. Epsom salt is used in commercial production to replace the Mg that has leached away and reverse the symptoms I just mentioned.
Home organic gardeners typically replace synthetic fertilizers and irrigation with compost and mulch to provide the nutrients and reduce watering needs. So its unlikely that we are causing magnesium leaching and the corresponding deficiencies in the soil.
To test this out let’s look at the concentrations of Mg in my soil. Last fall as a part of the ongoing Home Garden Field Trials testing Maxxam Analytics analyzed for both available and unavailable forms of magnesium in my typical soil. The results showed more than sufficient concentrations of Mg. Over time though as we harvest crops the levels of the Mg and S will lower over time right?
To see if the free and local resources I commonly use in my garden have Mg lets take a look at their lab results.
I had things like autumn leaves, used coffee grounds, and comfrey was analyzed as a part of our testing garden assumptions work. I selected these materials as they are common ingredients in my compost and mulch. 
It should be no surprise because all plants require sizeable amounts of Mg that we found good concentrations in all three materials. So by adding these materials to your compost and mulch layers your soils will be replenished as the material slowly breaks down releasing the elements to the nutrient cycle.
The other way you can have a magnesium deficient plant is through imposed deficiencies. An imposed deficiency is when the soil has sufficient concentrations but the plant cannot access them. Generally speaking this is due to nutrient imbalances where one element like potassium is high and it interferes with the uptake of other elements like magnesium.
Most imposed deficiencies in our home gardens are as a result of the over application of soil amendments. Although it is hard to do with things like homemade compost, store bough products are often concentrated and make over application much easier to do inadvertently.
In cases where there is an imposed deficiency adding more magnesium will likely not fix the problem and may result in the nutrients running off. The only way to identify this problem is a soil test.
Now that we have established that organic gardens are likely not going to have magnesium deficiencies what about the claims about fighting disease? Some sources claims that through the foliar feeding of Epsom Salt disease like blight can be controlled?
According to Dr. Linda Chalker-Scott a Master Gardener at Washington State University: There is absolutely no science behind the use of Epsom (Potassium Bicarbonate) salts for any kind of disease control.
A great resource summarizing the available research testing individual diseases is her information sheet on the matter. I will post a link to it in the description below.
Increased nutrient absorption
So if there are plenty of sources and it does not help fight disease what about increased yields and nutrient absorption claims?
The soils in our trials had a surplus of most nutrients. The levels of Mg and S were comfortably above deficiency levels. If there are deficiencies then there may be symptoms in the plants growth and production.
When nutrients are in surplus the plant functions as intended gaining little to no additional benefit from higher concentrations in fact large nutrient surpluses may cause imposed deficiencies.
As an example of principal this year’s home garden field trials Biochar Test bed had higher nutrient concentrations when compared to the rock dust bed however yields and elemental make ups of the produce were statistically identical.  Improved flavor
Improved flavor is most often related to higher sugar concentrations. Often when comparing home garden produce to store bought among other reasons, allowing the fruit or vegetable to ripen on the plant allows for higher sugar levels to build up while store bough produce is often harvested prior to being ripe in order to allow for shipping. 
If flavor is compared between produce grown in a magnesium deficient environment vs not you may detect a difference. However if you grow in two difference concentrations of magnesium both above the deficiency levels barring any imposed nutrient deficiency you likely will not be able to detect a change.
Common anecdotal observations people have told me is the year they started using Epsom salt the flavor improved of crops like tomatoes. Unfortunately in the absence of a side by side comparison there are more factors that can explain why you had a great tasting crop this year past just the addition of Epsom salt. Changes in environmental condition like sun, water, heat and soil can all have impacts on how your plant and the flavor of the crops.
Epsom salt as a method to alleviate magnesium deficiencies is a perfectly good short term soil amendment while you find a way to fix the issue long term. The only way to confirm that is with proper soil testing.
For other claims there is no additional value for disease control, increased yields, and improved flavor and as a preventative or precautionary soil amendment.
 Dr. Linda Chalker-Scotts paper on Epsom Salt
http://en.wikipedia.org/wiki/Magnesium_sulfate  Sulfur in life forms
http://en.wikipedia.org/wiki/Sulfur#Protein_and_organic_cofactors  Soil Lab Results
http://www.albertaurbangarden.ca/wp-content/uploads/2014/11/141120_AlbertaUrbanGardenCertificateofAnalysis.pdf  Free and Local Resources Lab Results
http://www.albertaurbangarden.ca/wp-content/uploads/2015/02/150214_AlbertaUrbanGardenCertificateofAnalysis-RevisedReport.pdf  Why do Garden Fresh Produce Taste Better then Store Bought?
 Does Rock Dust increase the Nutrient Density of Food
 Does Biochar Retain Nutrients in the Soil?
 Garden soil analysis