Soil Amendments V. Mulch: What’s The Difference (And How To Use Them)

Much like baking a cake, improving garden soil is about getting the right ingredients in the right proportion. The “recipe” for a healthy garden includes soil amendments and mulch.

But what’s the difference between the two?

Soil amendments are used to improve the fertility and condition of the soil, while mulch is a material spread over the surface of the soil to retain moisture, prevent erosion, and prevent weeds. Both are beneficial for the garden but are not interchangeable so choose your material carefully.

Knowing the differences between these two materials will help you choose the right one for your soil’s needs.

What are soil amendments?

Soil amendments are various forms of organic matter which are added to soil to improve its structure and drainage. Popular amendments include compost, worm castings, peat moss, lime, and sulfur. Soil amendments help improve the quality of your garden’s soil so that it can better support plant growth.

Common soil amendments:

• Compost is a natural fertilizer made from decomposed organic materials. It is rich in nutrients like nitrogen and potassium, which promote plant growth, and it also serves to improve the water retention and drainage of soils. You can make compost at home, but it does have a learning curve.

• Lime, also known as calcium carbonate, is naturally found in the soil, and it acts to neutralize any excess acidity while providing essential minerals that plants need to thrive.

• Sulfur is an essential nutrient that supports the growth and development of roots, leaves, and other plant tissues. Additionally, sulfur helps improve soil texture, holding it together and preventing erosion. 

• Specific soil amendments like bloodmeal, bonemeal, kelp meal are also used to provide essential nutrients to garden soil. Bloodmeal is a natural source of nitrogen, bonemeal is a rich source of phosphorus, and kelp meal provides both nitrogen and potassium.

• There are many more single-ingredient amendments available, each with its own benefits. If you can, talk to someone at your local garden center or landscape supply company to see which amendments you might need to help your garden thrive.

When should you use soil amendments?

The best time to use soil amendments is before you plant. This gives the amendments enough time to break down and be absorbed by the soil. You can also add amendments to existing planted areas, but be sure not to disturb the roots of your plants as you do so.

How to apply soil amendments

The amount of amendment you’ll need will depend on the size of your garden, the needs of your soil, and which amendment you’re using. A soil test can tell you which nutrients your soil is lacking, and the packaging of the soil amendment will guide you to apply it at the correct rate.

As a general rule, you should mix amendments into the top three inches of soil if your garden is already in progress. If you’re at the beginning of the season with an empty garden bed, you can mix the amendment in more deeply with a cultivator, so the bed is prepped for planting.

Once you’ve mixed in the amendment, water your garden well to help the nutrients reach the roots of your plants.

What is mulch?

Mulch is a protective layer of material that is spread on the surface of the soil to suppress weeds, retain moisture, maintain even soil temperature, and prevent runoff and erosion. It comes from various materials, such as wood chips, grass clippings, straw, or leaves.

Common mulches:

• Wood chips are one of my favorite mulches. They provide a thick layer of soil protection, and are easy to get for free from tree trimming companies. They also last the whole season, so I don’t have to reapply often.

• Grass clippings are equally easy to come by as woodchips. Every time I mow my backyard, I use the clippings to freshen and reapply mulch where needed. Be sure that any grass clippings you use aren’t coming from a lawn sprayed with herbicides or pesticides to avoid bringing those chemicals into your garden.

• Leaves come in abundance every fall, making a fantastic mulch as long as they don’t blow away. If you have strong winds, try chopping the leaves up in your lawnmower before applying them to your garden beds, as the smaller pieces are less likely to blow away than a full-sized leaf.

• Straw (not hay!) is an excellent mulch for protecting the soil, and it adds a ton of organic matter to the ground as it breaks down. It’s more expensive than free leaves or grass, but it’s convenient to buy a bale of straw and apply it as needed throughout the season.

Why you should use mulch

In my opinion, mulch is one of the best solutions for the garden. Soil is meant to be covered, not left bare. Bare soil is exposed to rainfall, wind, and weed seeds. With a thick layer of mulch, even in an unplanted garden, you help protect the ground from the elements while also providing time for the mulch to decompose into the soil.

• Mulch helps suppress weeds by blocking sunlight and preventing weed seeds from germinating.

• Mulch helps to retain moisture in the soil by slowing evaporation, especially during the hot summer months.

• Mulch helps to keep the soil temperature consistent through summer highs and winter lows. A consistent soil temperature is beneficial for both your plants and the microbes that live in the soil.

• Mulch can help to prevent runoff and erosion by holding the soil in place during heavy rains.

When should you use mulch?

Mulch can and should be applied any time of year. Here are some of the best times to apply it:

• In spring, after planting to prevent weeds from germinating
• Reapplied during summer if you pull out or replant any garden beds
• At the end of the season to keep the bed covered through the winter

How to apply mulch

Apply the mulch in a layer that is two to four inches thick. Be sure to leave some space around the base of plants so you don’t smother the stems. Mulch will break down over time, so it is important to reapply it every few months to maintain a thick layer, especially if you replant midseason and some of the mulch gets into the soil.

Are mulch and soil amendments interchangeable?

Except for compost, mulch and soil amendments are not interchangeable. A thick layer of a soil amendment such as kelp meal would be virtually useless as it blows away in the wind. Chunky wood chips mixed into the soil will take months to decompose and add nutrients to the soil.

Compost is one exception, as it can be applied to the soil surface or mixed in to protect or amend the soil.

Learn more

Colorado State University Extension has a convenient guide with the general application rates for soil amendments and some expert guidance on choosing the best amendment for your soil. You can read it here: Choosing A Soil Amendment.

From a home gardener’s perspective (mine!), wood chips and high-quality compost are the two best materials to use with your soil. Of course, the best options for you will depend on your soil type, the amount of rainfall in your area, and the types of plants you’re growing, so it’s worth experimenting over the gardening seasons.

Find out why I love woodchips so much: Best (Free!) Organic Mulches For The Home Garden

FAQ

Can I use soil amendment or mulch as potting soil?

Compost is the only soil amendment or mulch you can plant directly into, although for many garden plants, this will make too rich of an environment. The result will be vigorous plant growth but little to no fruits or flowers. The compost might also be too fluffy for the plant roots to anchor themselves firmly, making plants vulnerable in windy weather. 

Can I plant directly into a soil amendment? 

Unless compost is your soil amendment of choice, planting directly into an amendment such as lime, sulfur, or bloodmeal will spell absolute disaster for your plants. Soil amendments must be diluted and mixed into the soil to support plant health and growth.