Naturally, the best time to improve the soil for a lawn is before you plant the grass. But that doesn't mean you can't treat a poor existing lawn for healthy results. You can top-dress your lawn with various lawn and turf dressings by spreading a thin layer over the grass. You may need to do this if nutrients have been depleted, when there are bare spots on the lawn, when water pooling occurs or where there are rotting tree roots. Top-dressing with a good organic substance slowly filters down through the existing lawn soil and improves the grass at root level.