Sodium is needed for proper muscle contraction, including the heart muscle, as well as nerve impulses throughout the body and brain. If your horse is getting plenty of forage every day, they are getting many key electrolytes, except sodium. Hay and Pasture are very low in Sodium.
Chloride helps with balancing the blood pH and water flow in and out of the cells. It is needed for potassium regulation, allowing for proper muscle contraction as well as water balance. During exercise or hot & humid conditions, large amounts can be lost from perspiration. Fortunately, hay and pasture are high in chloride. Together with Sodium, you get Sodium Chloride. This is your normal table salt and it is also the major electrolyte in the body.
Chloride also plays a vital role in the production of stomach acid, known as hydrochloric acid. Hydrochloric Acid is needed to start protein digestion. Without enough Hydrochloric Acid, horses can become malnourished due to an inadequate pool of available amino acids with which to produce body proteins (such as those found in muscles, joints, skin, hooves, vital organs, and other tissues, as well as enzymes, antibodies, and some hormones).
An important aspect of Hydrochloric Acid is its role in the immune system. Horses normally eat off the ground, ingesting beneficial (but also harmful) microbes. Hydrochloric Acid acts as the first line of defense against potential infections. 
Salt levels in blood are monitored while it is filtered through the kidneys. Any excess salt is excreted through urine and feces. Salt levels also effects sweating. When a horse is dehydrated and the blood sodium level is more concentrated, the body will reduce sweat production or even stop it completely in order to keep the critical balance of salt and water. This can be very dangerous because the horse can overheat very quickly without sweat. 
Magnesium plays a part in the functions of over 300 enzyme reactions within the body. This means if magnesium is not present or is present in an insufficient amount, each of these 300 reactions will suffer. Magnesium is involved in the metabolism of fats and proteins as well. It is also essential for DNA to be able to send messages to the cells. Magnesium is required for normal muscle function, especially for relaxation of a muscle. 
Potassium’s major role is to keep the sensitivity of nerves and muscles at normal levels. We don't want the nerves to be under or over reactive to impulses telling them to contract. This includes both skeletal and heart muscles.
Even small changes in potassium levels can negatively affect athletic performance. Because the sensitivity of the nerves and muscles is based upon maintaining a fairly constant gradient between the potassium and sodium outside the cells, the body works hard to keep the body's potassium level within a narrow normal range. To do this, the blood sometimes ‘robs’ potassium from the inside of cells.
Calcium also plays a critical role in muscle contraction, blood clotting and enzyme regulation. As calcium is involved in so many bodily functions, the level of calcium in the blood is tightly regulated. Calcium moves into and out of the blood by being absorbed from the digestive tract, eliminated in the urine or feces and mobilized from or stored in the skeleton.  Also like potassium, the levels of calcium in the blood are closely regulated by the body to be kept in a narrow normal range. If needed, the body can “rob” itself from the bones and other body cells to maintain its calcium blood levels. 
As you can see from above, Electrolytes in horses play a very crucial role for a healthy horse. Sodium, Chloride and Potassium are the top three that are lost. This is why your horse should always have a readily available supply of salt for them at all times. We recommend that you feed them loose salt in their grain and always have a salt block available as well. Keep in mind, horses tend to consume loose salt more often than they will consume salt from a block. Electrolyte supplements are great tool for working horses that sweat a lot, but they should never replace regular, plain table salt. They should be used in addition to salt. Make sure you never put dissolvable electrolytes in your horse's only water supply. They should have plain fresh water available at all times. You may also want to err on the side of caution on how often you give your horse Electrolyte supplements. There was a research study conducted on Endurance Horses and it was found that the horses who received electrolyte supplements had an increase of number and severity of gastric ulcers. You can read more about that here.