Plants need a number of essential nutrients. The major nutrients (macronutrients) are nitrogen, phosphorous and potassium. There are other macronutrients, often called secondary macronutrients, which are calcium, magnesium and sulfur. In addition to these, plants also require some nutrients in very small amounts, and these are called micronutrients or trace elements. These include zinc, manganese, boron, copper, iron and molybdenum.
Epsom salts cover two of the secondary macronutrients, magnesium and sulfur, as Elaine mentioned. Magnesium is often not included in general fertilizers. If magnesium is deficient in your soil you will likely see what is called interveinal chlorosis on the plants' leaves. This is where the leaf gets pale but the leaf veins stay darker green. This can be confused with a micronutrient deficiency (iron and/or manganese) but magnesium deficiency is more likely to start on the older leaves. If your soil has a high pH then these symptoms would more likely indicate iron or manganese deficiency than magnesium, magnesium deficiency being more likely on an acidic soil.
If you have an acidic soil (and I believe that at least parts of Texas are the opposite and tend towards higher pH) there are better ways of providing magnesium than Epsom salts.
If you're adding Epsom salts for a reason other than because there is a magnesium deficiency, say because it may enhance flower colour, I agree with Elaine it probably won't do any harm even if it is unnecessary based on the amount of magnesium naturally present in the soil. It is supposed to have little or no effect on soil pH. If you have an acidic soil and aren't growing plants that like a low pH, you might want to look at dolomitic limestone instead if you need to supply magnesium.
