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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 available at SustainableLandscapesSD.org. The Water Authority and its partners also offer other great resources for landscaping upgrades, including free WaterSmart classes at WaterSmartSD.org.

How to Compost the Right Way

You can make composting on-site a goal for your sustainable landscape maintenance to reduce waste and help the soil thrive. You’ll know when the compost is ready to use when it has an earthy smell, has cooled off, and doesn’t reheat when stirred. Next, look for a uniformly dark brown or even black color. You shouldn’t be able to identify any of the original particles.

Spread compost directly on the soil surface to use it as mulch. That can prevent erosion and help plants and soil filter pollution, such as hydrocarbons and metals from road surfaces. Most greenwaste-based composts can be applied to a depth of three inches. Use up to two inches of bio-solids.

If you don’t produce your own compost on site, get it from a reputable source that guarantees high quality. Commercially produced quality can vary significantly due to the diverse nature of feedstock, processes, and maturation standards.

Use compost to make healthier soil

For native plants in your sustainable landscaping, use roughly 15 percent compost by volume to repair disturbed or damaged soils.

Clay-based soil amended with compost leads to more productive and healthy plant growth at a lower cost than amending the same soil with the necessary 45 percent sand. Therefore, you can mix poor soils that are compacted, lifeless, or subsoils with about three to six cubic yards of high quality compost per 1,000 square feet to improve the soil structure.

If your compost is based on bio-solids, it can be high in ammonium nitrogen. Use this type of compost sparingly.  When using bio-solids, be sure you know exactly where they came from.

Read more at Gardening Know How: Composting With Biosolids: What Are Biosolids And What Are They Used For https://www.gardeningknowhow.com/composting/ingredients/composting-with-biosolids.htm

This article was inspired by the 71-page Sustainable Landscapes Program guidebook available at SustainableLandscapesSD.org. The Water Authority and its partners also offer other great resources for landscaping upgrades, including free WaterSmart classes at WaterSmartSD.org.