If meet sodium needs are met, so will chlorine needs. Feed as block or loose salt. Depends on amount needed to be ingested. Maintenance horses do well on block salt. Deficiency could occur on low grass roughage and high grain diets when horses sweat a lot such as endurance horses do.
Legumes are higher than grasses in potassium.
Only toxicity is seen in HYPP disease in quarter horses. Magnesium and Sulfur - not a problem in normal diets of horses. Early grass is high in potassium and low in magnesium and may rarely cause a grass tetany problem in lactating mares. Magnesium oxide is often added to grains as a safeguard. Sulfur is adequate in diets of horses.