Mulches are often made of woody material such as wood chips and ground-up wood particles but can include straw, pine needles, shredded paper, and other finely ground organic materials that have not yet decomposed*. To serve various purposes, mulch needs to be placed as a layer on the surface of the soil. It must be absorbent, yet light in weight, somewhat porous, allowing both water and air to penetrate. For many uses, the layer of mulch must be opaque to sunlight and insulative, reducing extremes of temperature in the soil below it while preventing sunlight from reaching the soil surface.