[Quoted material in blue, excerpted from Gene Ovnicek's website]

Q-8: Why is there always dirt packed in the bottom of horse's feet,
and particularly in the back 2/3?

A-8: The dirt compaction in the caudal portion of the foot is a
natural function of the life cycle of the foot. The dirt's presence
maintains a constant environment in and around the frog and rear of
the foot. The dirt compacts in the foot to help absorb concussion and
help maintain heel expansion.

Unfortunately, Mr. Ovnicek's statement is based on conjecture, not
science.  While feral horses in arid, abrasive environments may survive
with the sulci of their frogs packed in dirt - or whatever else the horse
has stepped in - that packing creates an anaerobic environment in the
lateral and medial sulci of the frog.  Domestic horses in varied
environments, especially moist environments, are prone to infection by
various pathogenic organisms that grow and proliferate in anaerobic
environments.  The causative organism of thrush, Fusobacterium
necrophorum, is a anaerobe as is the causative organism(s) of hoof wall
disease (aka, white line disease, Fungal Onchonmycosis).

The contention that dirt packed into the lateral sulci helps "absorb
concussion" is ludicrous; in reality, such packing destroys the ability
of the frog to expand and keeps that structure from functioning in a
normal manner, as well as inhibiting the normal action of the digital
cushion.  Similarly, packing the sulci with dirt and whatnot has never
been demonstrated to be beneficial in terms of biomechanical efficiency.

In most environments, a properly trimmed foot in a domestic horse is
more-or-less self cleaning and functions most efficiently when it's clean.
If having the frog's sulci packed with dirt, fecal material, and goodness
knows what else had ever been shown to improve the biomechanical
efficiency of movement in horses, race trainers would be carrying
pails of mud, instead of hoofpicks, to the saddling paddock.