Find out what distinguishes compost from mulch, when you should be using either compost or mulch and when a combination of the two is needed.
What Is Compost?
Compost is organic material in various stages of decomposition. It can be anything from kitchen scraps, grass clippings, leaves, manure, weeds, and plant debris as long as it does not harbor plant diseases, insects, or chemicals that could be spread through the compost. How long compost breaks down depends not only on the material itself—decomposition can take from two weeks to two years—but on many other factors. The proper balance of green and brown in a compost pile is important. As well as the right amount of moisture, aeration, turning the pile, its size, the ambient temperature, and the presence of worms and microorganisms that break down the compost. Once the compost is fully decomposed, it becomes organic matter or humus. You won’t recognize any of the original materials any longer. Mature compost is dark brown and crumbly with soil-like particles, and it has an earthy smell.
Uses for Compost
Compost is the ultimate form of recycling. Mature compost returns nutrients back to the soil in a form that is readily absorbed by plants. However, because the nutrient makeup of compost depends on the original materials, only adding compost to a garden as a fertilizer is not enough. Especially when soils are depleted by crops every year, such as in a vegetable garden. The targeted application of nutrients in the form of fertilizer is also needed. The other important function of compost is as a soil amendment to improve soil health. Organic matter like mature compost loosens up the dense texture of heavy clay soils, so it’s easier for plants to grow. It also improves soils at the other end of the spectrum, sandy soils, because organic matter holds water and nutrients better, so the sandy soil does not wash out as quickly.
What Is Mulch?
Mulch can be any material that you use to cover the soil surface. While all compost by definition consists of organic material, not all mulch is organic. The most popular form of mulch, sold in bulk or bags at garden centers, are wood chips or shavings, hardwood, and softwood bark. Other mulches are pine needles, pine cones, hay, straw, cocoa, rice, buckwheat hulls, other crop residues, tree leaves, and grass clippings. Synthetic, man-made mulches are rubber, plastic sheeting, or geotextiles such as landscape fabric, cardboard, and newspaper. Only the last two decompose, whereas plastic breaks down into smaller pieces over a long period of time, and eventually end up contaminating the environment as microplastics. The third group of mulches are materials that are also not degradable, but they are natural: crushed seashells, gravel, pebbles, stone chips, and slate.
Uses for Mulch
Mulch is used to cover the soil of garden beds and landscapes to accomplish three things: retain moisture, control weeds, and improve the visual appearance. Mulch helps the soil to retain moisture so it dries out less fast. The effectiveness of mulch against weeds depends on the type of mulch—the thicker and denser the mulch, the more difficult it is for weeds to grow. Lastly, a mulched flower bed looks tidier and neater than one with bare soil, but that is based on preference.
Can You Use Compost and Mulch Interchangeably?
Because the uses for compost and mulch are different—soil fertilization and amendment vs. soil covering—it is usually not a good idea to use them interchangeably. When mulches of organic materials break down, they eventually enrich the soil with organic matter and nutrients but there is a caveat. Bark mulch and other woody materials take a long time to decompose. Grass clippings and other fresh plant material, on the other hand, decompose faster but the microorganisms in the soil need nitrogen to do their decomposition job. During that time, they snatch it away from any nearby plants. This can stress the plants and lead to chlorosis. That’s also the reason why adding compost to the soil before it is fully decomposed is not recommended. Using other not fully decomposed organic materials as mulch can be equally problematic. For example, piling up a thick layer of whole leaves without chopping them up beforehand prevents air and water to reach the soil. The moisture buildup can lead to fungal diseases and cause root rot in nearby plants.
When to Use Compost vs. Mulch
The choice between compost or mulch depends on your goal. If you primarily want to enrich the soil to add nutrients and improve its texture, you should be working mature compost into the top few inches of your garden soil. If your main concern is to keep the weeds down and cut down on irrigation, apply mulch, preferably one that decomposes over time so you get the added benefit of improving your soil. Ideally, you do both—recycle as much of your yard waste and kitchen scraps as compost and mulch your garden beds.
