Eating dirt and gravel: how do you stop them?

I have two 12-week old puppies. Everytime we take them out for potty they dig their noses into the ground and eat the dirt. The go right for the peagravel and try to eat it. How do you get them to stop? We've tried the noise cans and firm NO's. We pick them and praise them when we move them to another spot. They just try to run right back. We've tried the boudry spray stuff and that does not stop them. We do have them on leashes, but they constantly tug to try and get to the gravel. But as far as them digging into the dirt and eating it, that happens where ever we have them to relieve themselves. Help! Any suggestions?