What dog food are you using? Look at the first five ingredients... if you see "corn" or any kind of meat "biproduct" it's time to find a new dog food. These ingredients are fillers and provide no nutritional value to your dog. Look for dog food that has real meat. Any brand of dog food you see advertised on TV is not good... even science diet. Blue is a good brand, but it is very pricey. I feed my dog Nature's Recipe... it's a lot less expensive than Blue and has similar ingredients. It works well on his sensitive stomach.
When you switch over your dog's food, do it slowly. Their tummies can't handle changes in diet the way humans can... Give your dog 75% of the old food and 25% of the new food for two days, then do 50/50 for two days, then do 25% old food and 75% new food for two days, then all new food... the whole process should take about a week to transition your dog to the new food.
If the soft stool persists, consult your vet. As long as it's just soft and not liquid the vet might not be too concerned but it's good to get the pup checked out anyway. Soft stool can also be caused by stress, so watch for signs of anxiety or distress in your dog.
This sounds like a chronic diarrhea. It is mainly triggered by food Allergies, chronic digestion of foods and indigestible objects such as rocks and sticks. It would be wise to consult a vet for help. Inflammatory Bowel Disease will be high on your veterinarian's diagnostic list as soon as you tell him/her the symptoms your dog is experiencing.