In its essence, all soil improvement involves the concentration of nutrients. Manure represents, in concentrated form, all the plants eaten by the animal that produced it. Compost and leaf mold concentrate the nutrients from a variety of plants growing over a large area into a dense, soil-like material. Cover crops and green manures, which grow by scavenging the depths of the soil for nutrients, concentrate all that goodness in the top few inches of the soil when we turn them under. Mulching, either in paths or cropping beds (where appropriate) provides nutrients and organic matter as the mulch breaks down.