Dog pens can develop unpleasant odors over time. The more time your dog spends in his pen, the more likely it is the area will take on the unpleasant aroma of urine and feces. Gravel is a good substrate for a dog pen because it allows urine to drain into the ground. Unfortunately, even gravel dog pens can begin to smell like urine and you will need to take corrective measures to prevent the smell from becoming overwhelming.
Wash out your dog's kennel with water and diluted bleach on a daily basis; you can spray diluted bleach throughout the kennel and then rinse the kennel out with a hose to disinfect it and remove residual odors. Remove your dogs from the kennel while you wash it out. Allow the kennel to dry completely before placing your dogs back inside.
Sprinkle lime or another canine-friendly deodorizing product throughout the kennel to eliminate urine odors. Completely cover areas of the kennel where your dog urinates. Follow all instructions on the product packaging when using a specific deodorizing product to ensure the product is being used appropriately.
Remove old gravel and dirt underneath the kennel and replace it with new, fresh dirt and gravel to completely remove odors you have been unable to neutralize. Old gravel and dirt can be removed with a shovel and disposed of as trash, and then new dirt and gravel can be brought in to replace it.