In some cases, such as in the composting of grass clippings, the raw material may be too dense to allow for the proper flow of air or may be too moist. A common solution to this problem is to add a bulking agent, such as wood chips, to provide structure to the material and to allow for proper airflow. The amount of bulking agent required is usually determined based on experience. Some facilities add commercial fertilizers to their composting process, but this can usually be avoided by combining different waste streams together in a specific “recipe”. Inoculating the material with microbes is not normally required, since most wastes naturally contain the microbes needed for successful composting to occur.