These materials act as an amendment to improve the physical properties of soils and many contain fertilizer (more Compost & Soil Amendment).
"Amending a soil is not the same thing as mulching, although many mulches also are used as amendments. A mulch is left on the soil surface. Its purpose is to reduce evaporation and runoff, inhibit weed growth, and create an attractive appearance. Mulches also moderate soil temperature, helping to warm soils in the spring and cool them in the summer. Mulches may be incorporated into the soil as amendments after they have decomposed to the point that they no longer serve their purpose."
 * Listed by the Organic Materials Review Institute (OMRI) for use in production of organic food and fiber
