P and AP do have a white salt block that they seem to lick at a fairly normal rate. Sometimes I'm even able to break off pieces and crush them so that they get a bit of loose salt. I've heard that horses may eat dirt due to a lack of minerals - maybe P is missing something now that she usually gets from the summer pasture. I could try a mineral block or some type of mineral mix - I always try not to overdo the supplements, but I do feel that they have their place and can be helpful.
I'm really interested in equine nutrition - mostly because of P. She's typically an easy keeper, but she's lived in a couple of barns that didn't feed her very well. Unfortunately, I think it was from lack of caring by the feeders - they fed poor quality grain and hay and didn't always make sure that P got her food! P lost a lot of weight, and that's when I started learning a lot more about equine nutrition. I think the most important lesson is to feed high quality feed and make sure the horse gets it!