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Using Compost

Benefits of Compost Compost can limit weed growth, reduce evaporation and keep soil temperature even.

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.

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

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 206-296-3900.

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