Why does my horse eat mud?

time spent eating may reduce the drive to eat faecal material or bedding. 
Supplementation
It is advisable to provide a salt lick specifically formulated for horses and most will make use of one. A readily available salt lick may reduce the incidence of pica. However it should be noted that care must be taken with iron supplementation. Although iron has been shown to successfully treat pica in humans most horses dont benefit from iron supplementation. It may be detrimental to some as it is toxic to the liver in high doses. If in doubt please consult your veterinary surgeon. If you are adding a salt lick to the diet always ensure that fresh water is readily available at all times. 
In conclusion if your horse has recently started to consume soil or faecal material it is always advisable to discuss this with your vet. They will be able to carry out a full clinical exam of your horse as well as being able to offer advice on diet and housing if these are potential causes of the problem.