Both compost and mulch are incredibly useful for enriching soil and helping plants thrive. However, while compost and mulch share some similarities and can even be made from some of the same materials, they serve different primary purposes in the garden. Understanding the differences between compost and mulch allows gardeners to use each to its full potential.

What is Compost?

Compost is organic material that has been allowed to break down into a nutrient-rich, soil-like substance. Good compost contains a mixture of:

• Brown materials like dead leaves, woody plant trimmings, and cardboard
• Green materials like fresh grass clippings, fruit and vegetable scraps, and coffee grounds
• A healthy layer of beneficial microorganisms like fungi and bacteria

As these organic materials decompose, the microorganisms generate heat and break down the matter into rich humus. This finished compost is teeming with nutrients that plants need to grow and thrive.

Compost enriches soil by:

• Improving soil structure, texture, and aeration
• Retaining moisture in the soil effectively
• Promoting healthy microbial activity in the soil
• Providing a slow, steady release of nutrients to plant roots

Adding compost to soil or potting mixes fundamentally changes the nature of the growing medium and almost always enhances plant growth and vigor. Compost is especially beneficial for establishing new gardens, improving poor or depleted soil, growing hungry vegetables, and amending potting soil for containers and houseplants.

What is Mulch?

Mulch refers to any loose material placed on top of the soil as a protective ground cover. Common organic mulch materials include:

• Wood chips
• Shredded bark
• Leaves
• Straw
• Compost (when used as a topdressing)
• Grass clippings

Other popular mulches include stones like pebbles or gravel. Plastic sheeting is also sometimes used, although plastic mulch has some downsides environmentally.

Whatever the material, mulch forms a protective layer on the soil surface. Mulch Offers many benefits:

• Prevents weeds by blocking light from reaching seeds and smothering small weeds
• Retains soil moisture by preventing evaporation
• Insulates plants’ roots and buffers soil temperature
• Helps prevent erosion, compaction, and mud
• Can reduce soil-borne fungal diseases
• Gives beds a tidy, finished look

A 2-4 inch layer of mulch is ideal for most garden applications. Mulch is commonly used around trees, shrubs, perennials, and even annuals and vegetables. It is generally applied once or twice per year and can be added to an existing bed or laid down before planting.

Key Differences Between Compost and Mulch

While compost and mulch are both organic materials used by gardeners, they serve very different primary purposes:

Location

• Compost is worked down into the soil or potting mix before planting.
• Mulch is laid on top of the soil around plants after planting.

Purpose

• Compost is primarily used to feed plants by enriching the soil.
• Mulch is primarily used to protect the soil surface and plant roots.

Appearance

• Compost becomes incorporated with soil and indistinguishable.
• Mulch forms a visible, distinct surface layer.

Decomposition

• Compost is fully decomposed organic matter.
• Mulch is only partially decomposed and remains visibly fibrous.

Nutrient Content

• Compost provides a balanced range of major and micronutrients.
• Mulch provides very limited nutrients as it decays slowly.

Timing of Application

• Compost is added before planting and mixed into soil.
• Mulch is added after planting around existing plants.

Frequency of Application

• Compost only needs reapplied every few years.
• Mulch usually needs reapplication yearly (or more often).

Water Retention

• Compost absorbs and holds onto moisture well underground.
• Surface mulch prevents evaporation but does not hold much water.

Weed Prevention

• Compost has little weed-blocking ability.
• Mulch is excellent at preventing weed growth when fresh.

Cost

• Compost can often be made on-site for free.
• Mulch usually must be purchased bagged or in bulk.

Microbial Activity

• Compost contains abundant microbes that enrich soil.
• Mulch has far less microbial life and activity.

So in summary, compost provides food for plants, while mulch protects them. Compost and mulch work together to create the ideal environment for healthy plant growth.

Using Compost and Mulch Together

Compost and mulch truly excel when combined in a holistic garden care plan. Here are some great ways to use compost and mulch together:

Amend Garden Beds

When preparing new garden beds:

1. Loosen soil and remove weeds/debris
2. Mix 2-4 inches of finished compost into the top 6-12 inches of soil
3. Smooth bed surface
4. Top dress bed with 2-3 inches of mulch like wood chips once planted

This gives plants nutrient-rich soil and a protective mulch barrier for optimal growth.

Potted Plants

When potting containers:

1. Mix compost into potting soil as up to 25% of total volume
2. Fill containers and transplant plants
3. Top dress containers with mulch, leaving about 1 inch around stems

The compost nurtures plant roots while the mulch retains moisture and prevents weeds. Reapply compost or mulch as needed.

Tree and Shrub Beds

To care for established trees and shrubs:

1. Spread compost in a wide ring around dripline and work gently into top few inches of soil
2. Cover entire bed with 2-4 inches of mulch from trunk to dripline

The compost feeds the plants’ expansive root zones while the mulch protects surface roots and keeps weeds down. Reapply mulch annually.

Vegetable Gardens

For vegetables and annuals:

1. Work 1-2 inches of compost into veggie garden soil in spring
2. Plant seeds/transplants in compost-enriched soil
3. Mulch between rows and around plants with grass clippings or straw

The compost supplies nutrients for fast veggie growth while the mulch retains moisture and reduces weeds. Replenish compost after crop harvest.

Compost vs. Mulch: An Easy Summary

• Compost is decomposed organic matter used to enrich soil with nutrients and beneficial microbes. It is worked down into the soil or potting mix before planting.
• Mulch is a partially decayed organic (or inorganic) material placed on the soil surface after planting to protect plants.
• Compost provides food for plants through nourished soil while mulch protects plants in a number of ways.
• Though made from similar inputs, compost and mulch have distinct appearances, decomposition levels, nutrient contents, and uses.
• Combining compost for enriched soil and mulch as a protective topdressing provides the ideal environment for vigorous plant growth.

FAQs About Compost vs. Mulch

Can compost be used as mulch?

Finished compost certainly can be used as mulch. It forms a protective layer on the soil surface and will slowly release more nutrients as it continues breaking down. However, compost may not suppress weeds or retain moisture as long as coarser, woodier mulches. An easier solution can be simply topdressing beds with compost and then covering the compost layer with mulch.

Is mulch better than compost?

Mulch and compost serve different primary purposes, so one is not necessarily better. Compost is superior for improving nutrient-poor soil, while mulch is better for retaining moisture and blocking weeds in an existing bed. Using both compost to enrich soil and mulch to protect it creates ideal growing conditions for plants.

How do you use compost and mulch together?

Here are some great ways to combine compost and mulch:

• Amend new garden beds by mixing compost into soil, then topdressing with mulch once planted.
• For potted plants, blend compost into potting mix, then topdress containers with mulch.
• For trees and shrubs, spread compost around the dripline, then mulch the entire bed area.
• In veggie gardens, mix compost into soil, plant, then mulch between rows and around plants.

Should compost go on top or mixed in?

Compost should be mixed down into the soil or potting medium rather than placed on the surface. This allows plant roots to access the nutrients and humus. After mixing in compost, topdress beds with mulch for further protection and weed prevention.

Should mulch touch plants?

It’s best to leave 1-2 inches of space between plant stems and the mulch surface. Piling mulch right against plants can trap moisture and lead to rot. For trees, keep mulch about 6 inches back from the trunk. Mulch rings around plants help retain soil moisture.

Does compost need mulch on top?

It is not strictly necessary to cover compost with mulch. However, mulching over a layer of compost or compost-enriched soil provides many additional benefits. The mulch preserves moisture, prevents weeds and erosion, and insulates plants after the compost feeds and nourishes the soil.

How long does compost last?

Added nutrients from compost amendments may last 2-3 years in garden soil, often longer in clay soils. However, the soil structure benefits imparted by compost can persist much longer. Replenishing compost annually or biannually is optimal for vegetable gardens, while trees and perennials may only need compost every 2-4 years.

Do you fertilize when using compost?

If ample compost has been recently incorporated, additional fertilizer may not be necessary for that season’s crops. The nutrient release from decaying compost can provide sufficient fertility in many cases. However, gardens low in certain nutrients like nitrogen can still benefit from supplemental organic fertilizer or amendments even when compost is used.

Does mulch decompose and turn to soil?

Mulch decomposes very gradually. Thin layers may decompose fully over 2-3 years. Thicker mulch layers may take 5+ years to fully break down. However, mulch usually does not decompose into soil-like material. Mulch leftovers tend to become unappealing matted layers once they stop suppressing weeds. It is best to replenish mulch before it decomposes completely.

Conclusion

While compost and mulch share some common traits – organic origins, garden uses, decomposition properties – they serve distinct primary purposes for the gardener. Nutrient-rich compost physically changes and biologically enhances the growing medium, providing food for plants via the soil life it cultivates. Protective mulch creates a beneficial environment for plants aboveground by retaining moisture, insulating soil, and preventing weeds and erosion. Mulch also lets soil life flourish by keeping soil undisturbed. Used together, compost and mulch create the ideal conditions for plant health and vigor. Understanding the differing roles of compost and mulch allows gardeners to leverage both to their full advantage. With compost and mulch working together in harmony, gardens and gardeners thrive.