Magnesium is an important part of enzyme cofactors in plants, and secondarily, it is a component of chlorophyll. By the time the plant starts to yellow it is down to something like 25% of its optimal magnesium level. Among other things, magnesium containing cofactors help in the enzymatic process of sugar loading from the leaves into the plant's vascular system, especially in cool weather when enzyme activity is slowed. Magnesium deficiency is one reason why plants often turn reddish in cool weather. Ther red pigment is a sunscreen to slow sugar production/photosynthesis, because excess sugar in the leaves starts to cause problems.
Inlike most fertilizer salts, magnesium sulfate is tolerated by plants in extremely high concentrations, which is why you will commonly see recommendations as high as two tablespoons per gallon of water. Unused magnesium is stored within the cell vacuoles so you do not need to give the plant frequent doses. Orchid growers generally apply it four times a year. You do not want to mix it in with fertilizer because the sulfate binds to calcium and precipitates out as white crusty gypsum crystals on top of the soil, around the holes in pots, etc., depriving the plants of both the calcium and sulfur.
Epsoms salts can leach out of the soil pretty quickly, and spring soil should have plenty of magnesium for small plants, so adding it to the planting holes is not the most efficient method of application. It is when the plants are growing fast and producing alot of fruit that the plant may become magnesium limited. Epsoms salts can be applied as a foliar spray or to the roots. When the lower leaves of the plants start to look yellow with red/purple veins I generally just sprinkle Epsoms salts on the ground near the base of the plants and water it in.
Adding dolomitic lime to your soil, if the soil is acidic, should circumvent the magnesium problem long term, but if you have basic soil, Epsoms salts might be a better approach.