^ I agree. If her hooves really are too dry or are more prone to cracking you can always create a mud patch for her in the field around the water trough. My farrier said to use that to give the hoof the right moisture and help it not crack as easily. It's worked for all of ours (that have had these problems that is) and we've never had to buy any supplements.
If your horse has 'shelly' or peeling or overly brittle feet, or 'microcracks', it may well be that her diet &/or nutrition isn't the best though. Could also mean that she's had topical treatments that have damaged the outer layer of wall, or she's been in a waterlogged environment recently.
While there are many nutrients specifically associated with hooves(eg copper, zinc, amino acids, etc), basically what is right for the horse is right for it's hooves. Most horses on a 'normal' diet are lacking/imbalanced in some nutrients and do benefit from *appropriate* suppliments. Assuming you think it's nutritional, finding whatever suppliments 'fill the gaps' in her particular diet is best, rather than just choosing any old 'ration balancer' or such. Obviously this means you need to know what she's getting in her current diet. Hay/pasture analysis is a good idea to be accurate, but you can get a pretty good idea, and of what prods are best, using programs such as feedxl.com.