There are many reasons that your dog may soil in his dog crate. Some of these can be very easy to fix while others can be quite difficult. It often depends on your dog's age, health and history.
Medical Causes of Crate Soiling
If your dog starts suddenly soiling in his crate, consult your veterinarian. Your dog may have worms or a digestive problem that prevents him from holding it for long periods of time. If your dog has diarrhea, he may simply need a diet change, but it will still prevent him from being able to hold it for long periods of time. Before instituting a training program, you will have to solve the health issue.
Length of Time Spent in Crate
If your dog gets a clean bill of health, examine how long your dog is spending in the crate. Dogs can't spend more than 4 to 6 hours in their crate, and puppies can often spend even less time. A puppy can be expected to hold it an hour for their age in months. If you have a small breed dog, he might not even be able to hold it this long.
It's very stressful for dogs to have to relieve themselves in their crates because they are clean animals by nature. If you are leaving your puppy in his crate too long, find an alternative, such as an X-pen with a potty pad, when you are gone.
Your Dog's Crating History
Many dogs who are born in puppy mills or raised in pet stores learn to relieve themselves in their cages because it's the only option they have. In this case, they haven't learned to hold it and may even feel comfortable relieving themselves in that environment.
In this case, you may not be able to have your dog in a crate until you have properly potty trained him. Find an alternative, such as an X-pen or small room with a potty pad, until your dog is used to going outside.
Additional Solutions
If your puppy has had an accident in his crate, you must clean it very well to remove the smell. Clean it out more than once and let it air out for a couple of days outside. Wash all bedding in hot water more than once to really remove smell.
Properly potty train your dog by feeding him at the same time each day and keeping track of when and how often he relieves himself. If you feed him at the same time each day, he will need to go at similar times. Take him out around those times and reward him for going outside.
Don't leave him in his crate during times when you know he needs to relieve himself and don't leave him in his crate for longer than he can hold it. As time goes on, he can hold it longer, but don't push it until he's trained.
Don't punish for accidents. Your dog is already stressed from having to spend hours in his own waste, and unless you catch him in the act, he doesn't know why you are punishing him. Be positive with your dog and find a solution that works best with your schedule.