I agree with your wife. I am in the midst of a 1/9 figure in a trench section with sandbags, and I made sure to put some dirt between and on top of the bags to set them into the landscape. I just pushed some magic sculpt in the cracks and stippled it with a stiff bristle brush and painted it when set. For dirt on top, I just used some baking soda mixed with acrylic paint to make it chunky and muddy, and gobbed it on. Once it was dry I added various mud colors to make it a combo of wet and dry and blend a bit into theSandbag.
Brilliant vignette BTW!