Seattle Public Utilities

Growing Healthy Soil

Soil is alive! Billions of soil organisms create soil structure that allows air, water, and plant roots into the soil, while recycling nutrients, storing water, and protecting plants from disease. Those organisms live on organic matter such as dead leaves, mulch, and compost.

Feed your soil with compost

Dig or rototill compost into the soil before planting.

  • Lawns: mix 1-2 inches of compost 6 inches into the soil.
  • Gardens: mix 2-3 inches of compost 8-12 inches deep.
  • Use less on clay soils, more on sandy soils.
  • Amend the whole bed, not just planting holes.
  • On existing gardens add 1 inch every year or two.

Mulch your plantings

Mulch is any organic material spread on the surface to conserve water, control weeds, and slowly feed the soil. Different mulches work better for different plants:

  • Flower beds and vegetable gardens: spread 1-3 inches of fall leaves, compost, grass clippings, or straw. Keep mulch at least an inch away from plant stems.
  • Trees, shrubs, and perennials: spread 2-4 inches of woody mulches, like wood chips (often available from tree services, or in bags or bulk from garden stores) or if chips aren’t available, coarse bark (fine bark can plug the soil). Fall leaves also work well to prevent winter weeds and soil erosion.
  • Lawns: mulch mow (leave the clippings). On lawns in poor condition, aerate and then rake in ½ inch of compost in spring or fall. Learn more in Natural Lawn Care.

Use less fertilizer – go organic

Chemical fertilizers can pollute our waterways and damage soil and plant health. The best start for all plants is to amend the soil with compost before planting.

  • Trees, shrubs, and most perennials get all the nutrients they need from healthy soil, and regular mulching with organic matter like compost, leaves, or wood chips.
  • If lawns are yellow or thin, apply a “natural organic” or “slow release” fertilizer” once a year in September, and top-dress with compost – see Natural Lawn Care.
  • Vegetables may need an organic fertilizer and mineral supplements as well as compost – see Growing Food in the City or for flower beds see Growing Healthy Soil (pdf).
  • Vegetable gardens and lawns may need lime every few years, which supplies calcium and makes other nutrients more available by changing the pH – see Growing Healthy Soil (pdf) for lime types, timing, and use recommendations, or call the Garden Hotline.

Reality check – get a soil test

Using too much or the wrong fertilizer damages plants, your soil, and our streams. Call the Garden Hotline to diagnose plant problems, pick the right fertilizer, and to find out how to get an inexpensive soil test that will tell you what’s really in your soil, and what your lawn or garden needs. Free soil testing is available through the King Conservation District.

Where to buy compost and mulch?

Compost is available in bags or bulk directly from composting companies and through local nurseries and garden stores. See the Compost Resource List (pdf) or call the Garden Hotline for a supplier near you.

Fall leaves make a great free mulch. Arborist wood chips are often free from tree services. Cedar Grove and other suppliers sell wood chip mulch in bulk, or it’s available in bags at garden stores. The Garden Hotline can help with other mulch ideas and local sources.

Learn more

Growing Healthy Soil (pdf) – How to select and use compost, mulch, and natural fertilizers.

Get to Know Your Soil (pdf) – Diagnose and solve soil problems.

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For more information on building healthy soil and other natural lawn and garden tips, contact the experts at the Garden Hotline at (206) 633-0224 (language interpretation available) or at Garden Hotline.

Related Links

Soils for Salmon – Construction Best Practices - How to protect and restore soil during construction projects, to help restore wildlife and Puget Sound. Includes links to Seattle’s soil restoration code requirements.

Ecologically Sound Lawn Care Manual (pdf) - Tells professionals (and interested residents) how to amend soil for lawns (page 28 of that (pdf)), and how to improve existing lawns by topdressing with compost (page 35).

RainWise: Managing Stormwater at Home - Learn how to slow and filter runoff with compost-amended soil, trees, rain gardens, cisterns, and other RainWise techniques.

Links to Other Sites

WSU Master Gardeners and WSU Soil Management - More information on soil, nutrients, and solving soil-related plant problems.