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While compost and mulch may seem interchangeable, they have distinctly different uses in your sustainable landscaping. Photo: Water Authority Compost vs. Mulch

Using Compost and Mulch to Build Healthy Soil

When undertaking a WaterSmart landscape renovation, strive to continuously feed as much organic matter as possible to the landscape soil to keep it healthy. Start first with compost and mulch to jump-start the process. Eventually, your plants will feed themselves with their own leaf litter.

Organic matter feeds microbes living in the soil that make soil fluffy. It’s similar to bread rising because of yeast.

Learn more about healthy soil in this instructional video

Compost and mulch – what’s the difference?

Compost is a soil amendment. It looks like soil and it’s hard to tell what it once was. That is because it is food scraps, landscape debris and/or manure from livestock, or biosolids (human manure) and other organic matter that already has been partially consumed and mostly decomposed by micro-organisms. Good compost brings oxygen, water, and life in one package.

Compost can be store-bought or made at home. The compost-making process, or composting, involves creating optimal conditions for the microbes to do their transformative work. When compost looks like soil, it can be worked directly into the soil. The more coarse or visible the bits of the compost are, the more likely it is to be used as mulch on top of the soil rather than as an incorporated amendment.

Compost works in several ways. First, the compost itself contains particles improving soil structure. Next, as compost decomposes in soil it encourages the formation of soil macroaggregates. The resulting macroaggregates are composed of existing soil particles and decomposed organic matter, which combine to create a more stable and better functioning soil structure.

Mulch builds soil structure over time and holds in moisture. Photo: Monsterkoi/Pixabay compost

Mulch builds soil structure over time and holds in moisture. Photo: Monsterkoi/Pixabay

Mulch is a soil topping. Mulch may be organic or inorganic material covering soil. It’s made of larger particles and looks like recycled debris. Mulch can be made from organic matter such as grass clippings, leaf litter, and shredded wood trimmings, or inorganic materials such as gravel or decomposed granite.

The microbes in healthy, biologically diverse mulch bind the organic matter together, forming a thick blanket. This cover protects soil and plant roots from temperature change, keeps moisture in by slowing evaporation from the surface of the soil, and keeps weeds from sprouting by reducing sunlight penetration to the soil surface.

Mulch always stays on top of the soil. Unlike compost, it is never worked in. Recycled organic debris is the most effective type of mulch because it builds soil structure over time and provides a durable, protective surface barrier. The smaller the debris and the more mixed leaves with wood chips, the faster it decomposes. When building soil, small and mixed is best.

Composted material, especially coarse composts, also can be used as mulch. Artificial and inorganic mulches (decomposed granite, gravel, rubber chips, and other rubble) are primarily decorative since they do not contribute to soil life or plant health. They may be used in limited applications such as pathways.

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WaterSmart Living-Logo-San Diego County Water Authority

(Editor’s Note: The San Diego County Water Authority and its 24 member agencies offer programs, resources, and incentives to improve water-use efficiency for residential, commercial, and agricultural users. WaterSmart choices are a way of life in the region. Stay WaterSmart San Diego! For more water-use efficiency resources, go to WaterSmart.SD.org.)

Let Nature Be Your Guide: Compost vs. Mulch

You may not know the difference between compost and mulch, but your landscape soil does. Compost and mulch represent two different soil treatments with different advantages when used in a watersmart landscape plan.

Compost is made of organic matter such as food scraps, landscape debris, or livestock manure that have already been partially consumed and mostly decomposed by microorganisms. You cannot tell the original source of compost. Good quality compost brings the helpful OWL formula of oxygen, water, and life together in one package. It is used to amend your landscape soil by adding valuable nutrients it may be missing.

Mulch can be either organic or inorganic material that covers soil. Unlike compost, it’s not worked into the soil itself. Another difference is that the original recycled debris source of mulch is often identifiable. Mulch can be made from organic sources (grass clippings, leaf and yard litter, shredded wood trimmings) or inorganic sources such as gravel or decomposed granite (DG). Mulch is a soil topping.

Healthy, biologically diverse mulch contains microbes with the ability to “knit” the organic material together, forming a thick blanket. Mulch covers and protects the soil and plant roots from temperature swings, retains moisture by slowing down evaporation from the surface of the soil; and keeps weeds from sprouting by reducing sunlight penetration to the soil surface.

Top Tips for Using Compost

Compost the right way, including using compost as mulch to prevent erosion and help soil filter pollution. Photo: Ben Kerckx/Pixabay

Compost can be purchased, or it can be homemade. The compost-making process, or composting, involves creating the optimal conditions for the microbes to do their transformative work.

When compost looks like soil, you can work it directly into the soil. When compost is more coarse or has visible bits of the original materials, it is more likely used on top of the soil instead of as an incorporated soil amendment worked in.

Compost works in several ways. The compost itself contains particles that improve soil structure. Next, as compost decomposes in the soil, it encourages the formation of soil macroaggregates. These macroaggregates are composed of existing soil particles and decomposed organic matter, which combine to create a more stable and better functioning soil structure.

Top Tips for Using Mulch

Mulch builds soil structure over time and holds in moisture. Photo: Monsterkoi/Pixabay compost

Mulch builds soil structure over time and holds in moisture. Photo: Monsterkoi/Pixabay

Mulch always sits on the surface of your soil. It is never worked in. Recycled organic material is the most effective type of mulch because it builds soil structure over time. It creates a durable, protective barrier.

The smaller the debris pieces are, and the more mixed the organic pieces are, such as leaves with wood chips, the faster it decomposes. When building your soil, small mixed mulch is best. When compost is made from course materials like decomposed granite, you can also apply it to the top of your soil as mulch.

Artificial and inorganic mulches (DG, gravel, rubber chips) are mainly decorative. They do not contribute to soil life or plant health. They are best used in limited applications, such as creating natural pathways.

To learn more about your soil, sign up for Soil & Site Assessments virtual workshop.

This article is part of a year-long series inspired by the 71-page Sustainable Landscapes Program guidebook. The Water Authority and its partners also offer other great resources for landscaping upgrades, including free WaterSmart classes at WaterSmartSD.org.

While compost and mulch may seem interchangeable, they have distinctly different uses in your sustainable landscaping. Photo: Water Authority Compost vs. Mulch

Compost vs. Mulch: What’s the Difference?

Compost and mulch may seem like interchangeable terms for the same thing. But they are two different things with different uses in your landscaping. 

Compost is a soil amendment. It is made of organic matter such as food scraps, landscape debris, or livestock manure that have already been partially consumed and mostly decomposed by microorganism. You cannot tell the original source of compost. Good quality compost brings the helpful “OWL” formula of oxygen, water, and life together in one package. 

Mulch is a soil topping. Mulch can be either organic or inorganic material that covers soil. The original recycled debris source of mulch is often identifiable. Mulch can be made from organic sources (grass clippings, leaf and yard litter, shredded wood trimmings) or inorganic sources such as gravel or decomposed granite (sometimes called “DG” for short).  

The microbes in healthy, biologically diverse mulch will “knit” the organic material together, forming a thick blanket. This mulch cover protects the soil and plant roots from temperature swings, retains moisture by slowing down evaporation from the surface of the soil, and keeps weeds from sprouting by reducing sunlight penetration to the soil surface. 

How to Use Compost 

You can buy commercially produced compost, or you can make it at home. The compost-making process, or composting, involves creating the optimal conditions for the microbes to do their transformative work. 

When compost looks like soil, you can work it directly into the soil. When compost is more coarse or has visible bits of the original materials, the most likely it is used on top of the soil instead of as an incorporated soil amendment worked in. 

Compost works in several ways. The compost itself contains particles that improve soil structure. Next, as compost decomposes in soil, it encourages the formation of soil macroaggregates. These macroaggregates are composed of existing soil particles and decomposed organic matter, which combine to create a more stable and better-functioning soil structure. 

How to Use Mulch 

Mulch always stays on the surface of your soil. It is never worked in. Recycled organic material is the most effective type of mulch, because it builds soil structure over time and provides a durable, protective barrier. The smaller the debris pieces are and the more mixed the organic pieces are such as leaves with wood chips, the faster it decomposes. When building your soil, small mixed mulch is best. 

Composted materials, especially coarse composts such as decomposed granite, can also be used as mulch. Artificial and inorganic mulches (DG, gravel, rubber chips) are mainly decorative. They do not contribute to soil life or plant health. They are best used in limited applications, such as creating natural pathways.