The best dog foods* will often cause softer stool because they're richer and more digestible. "Rich" means they're made of high-grade protein that's fit for human consumption: chicken meat, lamb meat, etc. Such nutrients are easier for the dog to utilize, which means less waste (i.e,. poop) and an increase of healthy mucous lining the colon.
If the soft stool creates too much problem for you OR your dog (gas, butt-dragging, straining, etc.), here's an easy solution that works wonders: add a sprinkling (1/4 tsp. to 1 Tbls., depending on dog's size) of Metamucil powder to your dog's dry food. Add a splash of water and serve. Do this at every meal. Within a day, you'll notice a change. The stool will be soft, but it will be "encased," as if in a thin membrane, and it will roll easily and neatly onto your clean-up scoop.
Costco carries a copycat version of Metamucil (psyllium dietary fiber) that's several dollars cheaper than the brand name.
*A few of the good brands of dry kibble:
Chicken Soup for Dog Lover's Soul
Taste of the Wild