Compost and Mulch: What's The Difference

Soil does a great deal of quiet work in a garden. It holds water and nutrients, anchors roots, decides whether a storm soaks in or runs off, and filters what reaches groundwater. Compost and mulch both help it do that work, in different ways, and spring is when most gardeners spread one or the other.

Compost works in the soil

Compost is organic material, such as leaves, grass clippings, kitchen scraps and manure, that microbes have broken down until it is dark, crumbly and smells of earth. Its value lies in what it does once it becomes part of the soil. In sandy soil it adds organic matter that holds water and nutrients that would otherwise drain straight through the root zone. In clay it binds fine particles into larger crumbs, opening spaces for air and letting water enter faster. It also releases a modest, steady supply of nutrients as it continues to decompose, and it feeds the earthworms and microbes that keep soil structure open.

Compost can be dug into a bed before planting or spread on the surface and left for worms and rain to carry down. Make sure it is finished: compost that is still hot, or smells sour or of ammonia, can injure roots, and fresh manure belongs in the compost pile rather than straight on a vegetable bed. How fast it disappears depends on what it was made from, how fine it is and how warm the soil is.

Mulch works on top of the soil

Mulch is any material spread over the soil surface to protect it. Organic mulches include shredded bark, wood chips, pine needles, straw and shredded leaves. Inorganic mulches include gravel and stone. Their main jobs are to slow evaporation from the soil, smother weeds, even out soil temperature, and stop rain from crusting the surface or splashing soil, and the diseases it carries, onto leaves. Organic mulches add some organic matter as they break down, but feeding the soil is not their main purpose.

Depth matters. Extension services in New Hampshire and South Carolina both suggest 2 to 3 inches of shredded hardwood or pine bark. Grass clippings should dry for a few days first, or they mat into a layer that sheds water. Deeper is not better: a thick, fine mulch can starve roots of oxygen.

Keep mulch 2 to 3 inches away from the base of plants, and never heap it into a cone against tree trunks. Bark that stays wet is open to decay and disease, and a mulch pile against the stem gives voles and other rodents shelter in winter, where they can chew the bark unseen.

One caution applies to bulk mulch that has sat in a large, wet pile. Without air it can turn sour, producing compounds that injure plants within days of spreading. Sour mulch smells of vinegar, ammonia or rotten eggs instead of fresh wood or earth. Spread it out and let it air for a few days before using it near plants.

Compost as mulch

Compost can do both jobs. A layer of two to three inches on the surface holds moisture and suppresses some weeds while feeding the soil below. The trade-offs are that it breaks down faster than bark, so it needs topping up more often, and weed seeds that land on it germinate readily. Bought by the yard to cover large beds, it adds up. Made at home from leaves and kitchen scraps it costs little, and it keeps that material out of the trash.

A practical combination for most beds is compost worked into the soil at planting, with a coarser mulch on top to hold in moisture.