In the winter months, when we have had rain or snow, there can be a lot of tracking in that has to be swept up. Mud gets caught in the grooves of the shoes and it's hard to remove once it dries.
I have found it helpful to have a pan, with a little water in it, sitting on the porch. While the dirt is still damp, I can dip the soles of my shoes in the water and get the mud and debris off easily.
After mowing grass, this method also works well for removing wet grass clippings from my shoes.