What is the cause of soft spot on bottom of horse hoof?

Horses normally have a soft, spongy triangle of tissue in the middle of each hoof. This is called the frog and is part of why a horse is able to run and jump as well as it does.

Around the frog, however, the rest of the hoof should be fairly firm since it is made of keratin (similar to what your fingernails are made of, although thicker and more durable). A soft spot in the keratin of the hoof is generally caused by a fungal or bacterial infection that is eating away the keratin. Standing on wet or moist ground for long periods of time, getting impacted debris around the frog and having an injury to the hoof wall or sole are common starting points for these types of infections.