The Dirt on Soil, Compost, and Mulch in Seattle
Professional Member, APLD
What is soil? What is compost? What is mulch? What's the difference? Does it matter? The answer to the last question is yes, it matters, and if you know the difference you can have a healthier landscape.
SoilWhen your jeans are dirty and need washing, you might say your jeans have dirt on them-dirt is not a medium for growing plants. Plants are grown in soil, and soil has four principal components: air, water, mineral (weathered rock fragments), and organic matter such as decomposed plants. Its composition varies from region to region, and different plants grow better in different types of soil. Healthy soil is full of life. A quarter teaspoon of topsoil can contain a billion microorganisms, and these organisms, such as bacteria, fungi, and algae, help break down the remains of plants and other organisms that release energy and nutrients to growing plants.
Undisturbed topsoil (the dark brown layer at the top) provides energy and nutrients to growing plants. Image courtesy of Janine Anderson
A soil's acidity is determined by rainfall. The greater the rainfall, the more acidic the soil. The pH of Western Washington soils is usually in the range of 5 to 7 (pH values below 7 are acidic), while the soils in Eastern Washington are more alkaline, with higher pH values. “Acid-loving” plants such as rhododendrons and blueberries are often recommended for Western Washington gardens because of the lower pH values of the soil.
CompostCompost is created by the breakdown of organic matter such as dairy manure and grass clippings. Over time, the addition of organic compost to a clay soil can transform it from a mucky mess into a viable plant medium, and compost can improve the water-holding capacity and fertility of a sandy soil. Most tree roots are in the top 6 to 9 inches of soil, so that is the area most critical to plant health.
Cedar Grove Compost is a good commercial compost made from food scraps and yard waste generated by Seattle residents. Bailey Compost in Snohomish is a combination of yard waste and dairy manure and is available in bulk for pick-up or delivery. Avoid products composted with chicken manure; according to Dr. Linda Chalker-Scott of Washington State University Extension Urban Horticulture, chicken manure inhibits beneficial microbes in the soil. More information can be found in the Seattle Composting Resources Guide published by Seattle Public Utilities.
Applying mulch around a new planting helps retain soil moisture; prevents soil erosion; aids in weed suppression; and protects plant roots from heat, cold, and drought. Image courtesy of Janine Anderson
If you use organic compost as your mulch, you will still have to weed. Arborist wood chips are highly recommended as mulches, and they are often available at no charge from area tree companies. Arborist chips are a mix of bark, wood, and often leaves that together create a healthier soil community than is possible from more uniform products, such as finely shredded bark. Unlike compost, arborist chips are also useful for weed suppression. As with compost and other mulches, wood chips should be kept away from the trunks of woody plants to prevent moisture build-up around their bases.
Arborist wood chips used as mulch below a conifer in the University of Washington Botanic Gardens provide many benefits, including protecting the tree roots from compaction caused by foot traffic. Image courtesy of Janine Anderson
Perhaps the topsoil has been scraped off and the ground compacted by machinery and human traffic. Over time, we can improve the fertility and texture of our garden soil by incorporating compost and applying mulch.
Soils science is complex, and this article hardly scratches the surface. If you would like to know more about your particular soil and how you might improve it, consider having your soil analyzed by the King Conservation District.
More resources about soils, soil testing, compost, and mulch have been compiled by the Elisabeth C. Miller Library at the University of Washington Botanic Gardens.