There is a great deal of confusion, especially for novice gardeners, about the differences between compost and mulch. After all, if mulch is made up of compostable materials, it seems like they should amount to the same thing.
The truth is, however, that there are several key differences distinguishing mulch and compost. Knowing the proper uses of each item will save you from inadvertently damaging your soil and your plants.
When choosing between compost and mulch remember that compost is used to improve the quality of soil while mulch is primarily used to protect the soil and plants by retaining moisture and inhibiting weeds. Compost can be used as mulch but mulch cannot be used as compost.
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Let’s dig into this a little deeper so that we are clear on the purpose and proper use of each.
What’s the difference between compost and mulch?
As you likely already know, compost is the decomposed remains of organic matter such as leaves, twigs, fruit and vegetable rinds, coffee grounds, and paper. Depending on the organic matter, moisture levels, and temperature of the compost bin, it can take anywhere from three to twelve months for the matter to decompose and become usable compost (source).
Mulch can also be made up of leaves, twigs, paper, and other organic materials. However, mulch may also be inorganic; some gardeners favor mulches like landscape fabrics, plastic sheeting, or shredded tires. For the remainder of this article, we will focus on organic mulches, since those are much more likely to be confused with compost.
So what are the differences between the two? The differences come down to two factors:
- The primary purposes of compost and mulch
- The stage of decomposition
Primary Purposes of Compost and Mulch
Compost is first and foremost a soil amendment; mixing it into your soil is one of the most effective ways to improve the quality and structure of your soil. Compost can improve your soil’s moisture retention, drainage ability, nutrient retention, and aeration, all while maintaining a healthy habitat for worms and other beneficial organisms.
Some gardeners use compost in the place of fertilizer. While compost does release some essential nutrients into the soil, it does so very slowly and in low quantities. The best use of compost is to till it into the soil as a soil amendment rather than a fertilizer.
The primary purpose of mulch is to protect your plants and your soil. When you apply it correctly, mulch keeps weeds from overtaking your garden beds while also regulating the temperature and moisture levels of your soil.
Over time, mulches do break down and can be mixed into the soil. However, that is simply a bonus of using organic mulch materials. Their main purpose is to stay on the surface of the soil as a protectant against environmental factors.
Stage of Decomposition
As mentioned above, for compost to be usable as a soil amendment, it needs to decompose in a pile or bin for several months. When it is ready for “harvesting,” compost will look a bit like damp coffee grounds. At this stage of decomposition, compost is very easy to mix into the soil.
Mulch, on the other hand, is often fresh and hasn’t yet begun to decompose. As it starts to break down, gardeners can freshen it up by “fluffing” it and adding new mulch to the surface. This keeps the mulch working as effectively as possible while also keeping garden beds looking attractive.
Can mulch be used as compost?
Compost can be applied to the soil’s surface and used as mulch. However, the reverse is not true–mulch cannot be used as compost. The main reason for this is that mulch and compost are not at the same stage of decomposition.
The reason compost is such an effective soil additive is because of its level of decomposition. All compostable materials contain varying levels of carbon and nitrogen, both of which are necessary for the microorganisms in the compost pile that do the hard work of breaking down organic matter.
Woody matter, like chips, branches, and sawdust, has a very high ratio of carbon to nitrogen. Within a compost bin or pile, that is not a problem, since hay, yard waste, and food waste help balance the overall carbon to nitrogen ratio.
However, if a gardener were to use woody mulch, like wood chips or bark, as compost and mix it directly into the soil, that would be a mistake. The wood chips would break down, but they would do it very slowly because their only source of nitrogen would be the soil itself. In other words, the wood chips would deplete the soil of nitrogen, which is an essential nutrient for all plants, and the plants themselves would starve (source).
That said, mulch does break down over time, and when it does, it can safely be tilled into the soil. Just bear in mind that mulch’s primary purpose is to stay on the soil’s surface.
Do you need to use both compost and mulch?
Unless you are very lucky and have ideal soil and growing conditions, your garden could probably benefit from both compost and mulch.
Compost is extremely versatile; it will improve the condition and structure of a wide range of soils, from sandy soil to heavy clay. If you mix one or two inches of compost into the top eight inches of your soil every year, you will see major improvements over time. Your soil will become more fertile, easier to work, and better at regulating moisture and nutrient levels.
While compost does its work underground, mulch provides necessary above-ground benefits. It blocks sunlight to weeds, making it difficult for their seeds to germinate; that alone is a good reason to add mulch to your garden beds! Furthermore, because mulch also protects the soil from erosion, temperature fluctuations, and water evaporation, all gardeners should consider adding mulch layers to their beds (source).
Can you get by without compost and mulch? You can, but you can get more from your soil in the long run when you give it the extra care that compost and mulch provide.
Compost works underground to enhance the overall condition of the soil, while mulch works above-ground to protect it. Just like your plants, your soil needs care and maintenance, too. Compost and mulch are efficient, all-natural materials that will help you do just that.