While mulch isn't a type of soil in itself, it's often added to the top layer of soil to help improve growing conditions. It's made from wood chips, tree bark, leaves, yard waste and many other types of organic materials. Mulch helps keep the surface of the soil porous, allowing water and air to reach the roots of the plants. It also helps to keep sunlight and rain from reaching the soil directly, which can keep heat levels in check and minimize erosion and evaporation [source: Better Homes and Gardens].
One of the most beneficial properties of mulch is that it tends to decompose over time, adding even more organic nutrients to the soil. Mulch can be spread over planting beds and gardens, or can be used around individual plants and bushes. To discourage bugs and other pests from taking up home in your mulch beds, keep layers thin and leave a gap in the mulch bed around the base of each plant [source: Chesman and Lloyd].