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.
Spreading topsoil for grass is best done in spring or early in the fall because the grass needs time to grow healthy and strong. You may need to add compost soil mixes to some trouble spots more than once but you shouldn't need to treat the whole lawn too often since you're not aiming to raise the grade level of your garden. Ideally, you would spread lawn and turf dressing before predicted rainfall but otherwise water the treated grass following application. Look for a variety of lawn soil additives at your local garden centre.
