What Is the Difference Between Compost & Mulch?

Compost and mulch are components of healthy gardens, but they contribute differently. When the soil in a garden is blanketed with a layer of mulch and amended with nutrient-rich compost, plants flourish. Although their form and function differ, compost and mulch can work together to offer a two-fold benefit to your plants.

Mulch is any material that blankets the soil as topdressing. It can be organic, such as wood chips, or inorganic, such as plastic. The many benefits of mulch include retaining water, mitigating soil temperature, reducing erosion and suppressing weeds and disease. Organic mulches decompose over time, releasing nutrients into the soil. Inorganic mulches provide no nutritive value, but they typically last longer. Mulched beds, islands and walkways also add aesthetic value by making a garden look neat and tidy.

Compost is a soil amendment, which is the result of decayed organic matter. Although compost may also be used as a mulch to cover the soil, it is more effective when incorporated into the soil. When worked into the garden to a depth of 6 to 8 inches, compost improves soil structure, helps aerate the soil and provides nutrients to plants. Making compost may be intentional, such as removing kitchen scraps to a bin and allowing them to decompose. Compost has decomposed sufficiently for use in the garden when the product has an earthy smell, is dark and crumbles when handled. Compost may also result as the natural byproduct of fallen leaves and grass clippings that break down into crumbly humus. When used as mulch to suppress the growth of weeds, apply at least a 4 inch layer of compost around plants to sufficiently block light that weed seeds require to germinate.

When used together, compost and mulch offer a two-for-one garden benefit -- the valuable combination of insulating the soil while enriching it. Before incorporating kitchen scrap compost into the garden, make sure it has decomposed to the point that its components are no longer recognizable. If it has not aged sufficiently, unfinished compost attracts wildlife that dig in the garden to feed on it, and it may burn plant roots. Don’t work wood chip mulch into the soil, because as it decomposes it pulls nitrogen away from plants. Keep mulch on top of the soil and only well-aged compost beneath the soil for optimal results.

Rubber mulch is made from crumbled tires. It is a popular mulch choice because of its perception as a responsible way to recycle discarded tires. Although rubber mulch retains water and suppresses weeds, research suggests the dangers of using it outweigh the benefits. Rubber is not a static material; it degrades over time. As it breaks down, toxic leachates contaminate the soil and aquatic environments. Rubber mulch is flammable and hard to extinguish when burning. Its metallic elements also pose health risks, making it an unsuitable choice for the garden.