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Unlike the air we breathe or the water we drink, soil is often overlooked as an essential element in a balanced, sustainable environment. But healthy soil is critical for good air and water quality and the health of our lawns and gardens. In the Pacific Northwest, the top layer of soil is thin. And in many yards, construction and years of neglect have removed any trace of healthy soil, leaving only poor soil behind. Learn how you can improve your soil and how composting can restore soil to make plants grow and yards healthy.

Compost is a natural organic material that is produced when leaves, plant residue, grass clippings and other yard waste break down over time. Organic materials decompose in nature to feed soil and make it healthy. You can imitate nature in your own yard by composting your yard waste and kitchen waste. Compost is used as a soil amendment rather than a fertilizer because its nutrients become available slowly. Worm castings are effective as a nutrient rich-top dressing and soil amendment, providing nutrients for your plants in a form that plants can use as needed. Apply two to three inches of worm castings to your soil as a top dressing in small areas of your garden to feed the plants and nurture their growth.

You can buy compost and worm castings or make your own. Buy bagged compost and worm castings at local home improvement, nursery and garden stores. For large amounts of compost, there are several suppliers that sell bulk supplies.

Benefits of composting

  • Encourages the growth of earthworms and other macro-organisms, whose tunneling makes room for water and air
  • Provides nitrogen, phosphorus, potassium, sulfur and micro-organisms that are essential for plant growth
  • Acts as a glue, holding water and soil particles together, and makes soil resistant to erosion
  • Binds itself to polluting metals, pesticides and other contaminants to prevent them from washing into waterways or being absorbed by plants
  • Suppresses soil-borne diseases and plant pathogens (a number of plant and lawn diseases are suppressed by micro-organisms found in compost)

Using compost

How and where should you use compost?

  • Topdress lawns: Spread a thin layer of compost (about a half inch) on your lawn during the spring or fall. This technique works best if you first aerate the lawn.
  • Plant gardens: Mix compost to a 10- to 12-inch depth before each year's planting.
  • Mulch with it: Place a several-inch-thick layer of mulch around plants to limit weed growth, reduce evaporation, keep soil temperature even and reduce soil erosion.
  • Slope stabilization: Secure slopes and reduce effects of erosion by adding a 3-inch layer evenly to the slope surface.

View short videos demonstrating each of these uses.

Mix in organic matter with existing soil before planting perennials or lawns, each time garden beds are replanted and when dividing perennials or repotting container plants. Sandy soils need more compost than do clay soils.

How much compost should you use?

Amounts vary for compost use, and it is possible to use too much. Below are some general guidelines based on 100 square feet of planting area. Check with your local nursery for specific directions.

  • New lawns: Mix compost at least 6 inches but preferably 8 inches deep as follows:
    • clay soils – 8 cubic feet = 1-inch layer of compost
    • Sandy soils – 13 cubic feet = 1-½-inch layer of compost
  • Established lawns: Spread a ½-inch layer and rake in; grass should be standing up, not bent over or buried, when finished. Mixing grass seed with compost encourages new growth.
  • New and established gardens: Mix compost to a 10- to 12-inch depth as follows:
    • clay soils – 16 cubic feet = 2-inch layer of compost
    • Sandy soils – 24 cubic feet = 4-inch layer of compost
shovel in compost
Compost holds water and soil particles together and makes soil resistant to erosion.
hands in compost
Compost can limit weed growth, reduce evaporation and keep soil temperature even.

For very poor, unhealthy lawns, you may want to consider starting over. For a list of local laboratories that can test the amount of sand, silt and clay in your soil, call the Washington State University/King County Extension at 206-205-3100.

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