This could be an environmental allergy or a food allergy, or both; once it starts, it can escalate into an obsession if not controlled. My allergy dog has both environmental (grass)and food allergies (all grains, potatoes, alfalfa, chicken) and will sometimes get into a licking habit that I have to stop her from doing.
I've controlled most of it by feeding her a food free from all of the food triggers (Acana Singles Lamb and Apple or Acana Singles Pork and Butternut Squash works well for many food allergy dogs).
When the grass is growing, a foot rinse after coming inside, comprised of 1/2 Braggs Organic Unfiltered Apple Cider Vinegar and 1/2 water, then pat (not rub) dry is helpful. I put the mixture in a plastic container, deep enough to cover her foot, and I dip each foot in for up to a minute each. Maddie is small, so I use a large container and can fit two of her feet in at the same time. I put a lid on the container after use, and I use it for a day before replacing the mixture. It helps rinse off the allergens and is healing to the skin. You can also dab it on hot spots.