To clean dirt from the soles of your white Groundies, we recommend a dirt eraser. For quick direct help, baking powder in combination with water and dishwasher detergent is a good solution.
Mix powder and detergent in a glass in a ratio of 1 to 1, add some water and stir the foam. Use a damp toothbrush to apply the mass to the soiled areas. A damp cloth then removes the cleaning foam from the sole.