Actually, when grass is lush and has a very high water and sugar content, the Calcium (Ca) and Magnesium (Mg) content are often VERY low. A Mg deficiency on lush grasses and cereal grains is so common that many cattle die of Grass Tetany every spring because they are not supplemented with Ca and Mg. They also need an ample amounts of Vitamins A - D - E in order to assimilate the minerals they need. They can get more Phosphorus (P) than they need, so that means they need more Ca to balance it out.
The ratio of C:P should never be less than 2:1. The only way to get that ratio in the spring when (P) is greatly over-available is to make sure that the loose mineral supplement fed has 4:1 Ca:P up to 6:1 Ca:P. That will make sure the overall ratio is at least 2:1 Ca:P.
Most good loose minerals have less than 25% salt in them and have at least 150,000 to 200,000 Units of vitamin A per pound of loose mineral.
Horses being fed any grass or grass hay and any grain product are usually getting too much Phosphorus and too little Calcium. In the winter (on Feed like wheat pasture or rye grass) and in the early spring when all grasses are very lush, they are much more deficient in Ca and also very deficient in Magnesium and to a lesser degree in Zinc. Our horses eat a LOT more mineral in the late winter to early summer because we have a lot of rye grass.
We feed a loose mineral year 'round that is
150,000 Units of Vitamin A
Vitamins D & E
Plus traces of Zinc, Manganese, Copper, etc.
[Do not let sheep or goats have access to any mineral with Copper in it]
Adding Vitamin A to your horse's diet through a 'good' mineral or a Vitamin Supplement will probably fix your dry skin and itching. It will also prevent and/or cure Rain Rot, biting lice, runny and crusty eyes, and a multitude of other things.
Hope this helps. Cherie