Horses should have access to salt at all times. I provide each of my horses with two salt blocks. One is a plain white salt block that is simply table salt; sodium chloride. The other is a calcium/phosphorus trace mineral salt block. It is sometimes called a 12:12 block because it contains 12% calcium and 12% phosphorus or an equal ratio of calcium to phosphorus, which is good for most adult horses.
Cherry Hill's Horsekeeping 
Could you ex[lain magnesium as an additive to horses diet? I have heard that deficiency can result in spookiness and nervousness, especially after stressful situations- like showing, moving, etc.. Why isn’t it one of the minerals in the red salt licks?
Thanks!
Very interesting.
Are trace mineral blocks avail for young horses as well ?
Calcium-Phosphorus mineral blocks for young horses generally will have a ratio more like 2:1 Calcium to Phosphorus.
With that said, it is important, especially with young horses that are growing bones, to be sure the Ca:P ratio is appropriate for the type of hay being fed. If feeding alfalfa, which is a high calcium feed, you would offer blocks with a lower Ca:P ration than you would if you fed grass hay. But the type and amount of grain that is fed must be taken into consideration too. Grain is generally high in phosphorus. So the bottom line is, analyze the type and amount of hay and the grain being fed and then choose the appropriate block. Your state extension agent is usually a good resource as they often have the analysis of representative samples of hays and grains that you can use to make your determination.
Magnesium is marketed as a calming aid (paste or feed additive). I am not aware of any studies that prove or disprove its effectiveness. Anecdotal accounts say that it does calm horses.
Magnesium is not usually included in a Trace Mineral salt block which is what I think you are referring to as a “red salt lick”.. Although each feed mill will make up their own formula, here is a link to the Trace Mineral salt blocks we purchase http://www.ranch-way.com/products/minerals-salt/ranch-way-trace-mineralized-salt-50-lb-block-or-50-lb-bag
The blocks I mentioned in my post are 12:12 Calcium to Phosphorus mineral blocks and they do contain magnesium. You can read their content here
http://www.ranch-way.com/products/minerals-salt/ranch-way-ranch-o-min-12-12-all-stock1
The National Research Council establishes the nutrient requirements for horses. http://nrc88.nas.edu/nrh/
Thanks for the sires! I found out one really important fact from them! I have a sheep. I know there is copper toxicity in sheep, but rarely are the blocks that i have purchased ( from horse supply places) listed as having Copper in them! I will need to be MUCH more careful now!