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.  


Compare your soil to these diagrams to determine your landscaping's soil composition. You can then adjust amendments to reach the optimum mix. Illustration: SDCWA Jar Soil Test

What Kind of Soil Do You Have? Take the Test

If you have ever put a shovel into the ground in San Diego County, you have likely encountered the region’s impermeable soils. Impermeable soils are defined by their poor infiltration areas. This means water doesn’t flow through the soil to replenish the groundwater, because the soil is too dense.  

It also means water doesn’t soak evenly into the ground or flow through living soil to plants in a healthy way. No matter where you do your landscaping, you should concentrate on improving your soil structure as much as possible. That will help irrigation be more efficient and more cost-effective, and your landscape plants will receive the nutrients and water they need to flourish. 

Why Does Particle Size Matter in Soils? 

 Before you can build better soil, you need to figure out what type of soil you are working with. The three basic types of soil are: 

  • Clay: Soil made up of the smallest particles 
  • Silt: Soil made up of a mixture of particle sizes 
  • Sand: Soil made up of the largest particles 

 In general, sandy soils drain faster than clay soils, because there is more space between the larger particles. Soil structure also influences soil quality. Lifeless, compacted, sandy soil will not absorb water, while healthy clay soil will be more sponge-like, holding and releasing water.  
The “just right” soil – an even blend of sand, silt and clay – is called loam. 

Determining Your Soil Type Using The Jar Soil Test 

 Some tests can be done onsite to figure out what kind of soil you have. Others require lab analysis. Certain conditions require specialized tests, such as soil used for food production or soil receiving a lot of storm water. 

You can test your home landscaping soil yourself using a “Jar Test.” This is a fun project to do with kids. 

  1. Use a one-quart glass container. 
  1. Add one cup of soil from the garden. You can select one area or take samples from several areas and blend them together. 
  1. Add three cups of distilled water. 
  1. Close the jar and shake it until all the soil solids are suspended in water. 
  1. Put the jar on a shelf and wait 24 hours.  
  1. If the container is still cloudy, wait another 24 hours. 
  1. After 48 hours, the soil layers should be settled on the bottom. 
  1. Measure the layers in proportion to each other, with the total adding up to 100 percent. 
  1. Sand will be on the bottom, silt in the middle, and clay on top. 

 Refer to the graphic to determine your soil type, based on the proportions of sand, silt, and clay. 

 Which jar does your home sample look most like? 

Now you can work to improve your soil condition, providing the best possible foundation for your landscaping plants and the most efficient irrigation.  


This article was inspired by the 71-page Sustainable Landscapes Program guidebook available at Hardcopies are available free of charge at the Water Authority’s headquarters, 4677 Overland Ave., Kearny Mesa. The Water Authority and its partners also offer other great resources for landscaping upgrades, including free WaterSmart classes at