Sheet mulching is a procedure that imitates the natural patterns found in the forest. Just as the leaves and needles fall from the trees and create a cover that kills weeds and keeps the soil moist, in your garden you can start your planting beds in the same manner; newspaper and cardboard can be laid down directly on the grass and topped with manure, soil and straw. These layers kill the weeds and grass and trap the moisture in the soil. When the grass has completely broken down, the result is a new garden bed that is nutrient rich.