Yes, it could be. It could also be wet dirt. Wet dirt will grow a bacterium that produces a smell that can get into your home. If the space under your home is damp, try using powdered lime to alleviate the smell. Take proper precautions when using it, as it could burn your skin and eyes. Sprinkle it on the grade in sub areas. Also try improving the ventilation by adding foundation vents that will free the air throughout the crawl space.