What is the difference between mulch and compost?

Compost can be applied on the soil surface or mixed directly into the soil. Photo: 123rf

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Dear Plant Doctor,

What are the differences between compost and mulch? How do I use them in my garden? – John

Mulch serves as a protective layer for the soil, offering multiple benefits to gardens. When applied to the soil surface, it helps prevent erosion, conserve moisture, suppress weed growth and regulate soil temperature.

Gardeners can choose between two main types of mulch: organic and inorganic.

Organic mulches, including materials such as straw, wood chips, leaves, bark and well- composted animal manure, offer additional advantages.

As these natural options decompose, they gradually release nutrients into the soil, thereby enhancing their fertility.

However, this decomposition also means that organic mulches are relatively short-lived, typically lasting from several weeks to a few months, depending on the material and climate conditions.

In contrast, inorganic mulches such as gravel, stones, landscape fabric and plastic provide a more permanent solution.

Highly resistant to decomposition, these materials maintain their protective function for extended periods.

While they do not supply nutrients to the soil, inorganic mulches excel at long-term weed suppression, erosion control and moisture conservation.

Compost can be applied on the soil surface or mixed directly into the soil. Comprising decomposed organic materials such as kitchen scraps, garden waste, animal waste and other biodegradable matter, compost’s primary purpose is to improve soil structure, provide nutrients and enhance overall soil health.

When applied to the soil surface, the compost functions as an organic mulch. It offers the same benefits as other organic mulches, including protection against erosion, conservation of soil moisture, suppression of weeds, and regulation of soil temperature.

Like organic mulches, compost also decomposes over time, gradually releasing stored nutrients. Both organic mulches and compost increase soil microbial populations and activity, thereby improving overall soil health.

When applying mulch or compost, spread it evenly across the soil surface, but keep it away from plant stems and tree trunks to prevent fungal growth.

Avoid over-mulching, which can harm the plants. Typically, 3–5kg of organic mulch or compost is applied per square metre.

While mulch is always applied on the surface, compost can either be spread on top, like mulch, or mixed into the soil.

Assoc Prof Dr Christopher Teh heads the Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of specialty is in soil and water conservation. The views expressed are entirely his own.