Take a good look and make sure there's no foreign object inside. Also put on some antibiotic cream or ointment ( after soaking with epsom salts or weak betadine bath ). Some creams would include polysporin, neosporin, fucidin, Nu-Stock.
Are there any other signs of allergies such as dirty ears and shaking head, licking paws? If so, consider treating for food and environmental allergies, eg. cetirizine (Zyrtec) 0.5 mg/pound once daily for environmental allergies and switching to a hypoallergenic brand of dog kibble with a different protein, eg. Zignature rabbit , Earthborn Holistics Venture line eg. duck meal and pumpkin, or Acana singles line eg. duck and pear. If your dog does have food allergies and a limited ingredient dog kibble doesn't help, try switching to frozen raw.
Also if your dog has allergies , as mentioned by 2Bullymama, give a wipe down after being outside, especially paws and underside of belly.