How to use compost in gardens and landscapes

Overview

• Compost is partially broken-down organic material. You can make compost at home or buy compost. Finished compost is available in bags at plant nurseries and home improvement centers. You can also buy compost in bulk from landscape supply companies.
• Compost is a soil amendment. Choose the right type and amount of compost for your project.
• Use compost to improve soil for planting vegetable gardens, landscape areas and lawns. Adding compost to your soil improves its ability to accept and store water.
• Compost is a food source for soil organisms. Their feeding releases nutrients in the compost over time.
• The fertilizer value of compost is low compared to chemical salt fertilizers. Lawns, vegetables, and fruiting trees and shrubs often need additional fertilizer to thrive.
• Apply the correct amount of compost to your work site. Adding too much compost may reduce water quality. You can apply too much compost at one time or over several years.

Choose the right compost product for your needs

Purchased compost products vary. Compost is made from a wide range of organic materials. Source materials are often mixed together in the composting process. Compost source materials include:

• Yard debris.
• Forestry products such as sawdust and ground bark.
• Crop residue such as straw, processed mint or rice hulls.
• Manures from chicken, cows and other livestock.
• Food scraps from homes and businesses, including wineries and breweries.
• Mushroom production waste.
• Vermicompost, or worm manure.

Compost quality varies. Quality depends on the type of raw materials. It also depends on the compost production method.

How should compost smell?

High-quality compost smells earthy like forest soil. Don’t buy compost that has a foul odor or smells like ammonia or sulfur. These smells indicate that the composting process isn’t complete.

How much compost should you buy?

Get the right amount of compost for your job by determining the volume you need. Volume is the thickness of your compost layer multiplied by the area of your workspace.

• Determine the desired thickness of your compost layer. Convert inches to feet by dividing inches by 12. For example, 3 inches deep equals 0.25 feet.
• Multiply the length of your work area in feet by its width in feet, and then by the desired depth of compost.
• Convert cubic feet to cubic yards by dividing cubic feet by 27. Landscape supplies are sold by the cubic yard.

If these calculations seem daunting, use an online compost calculator to determine how much compost you’ll need. Or, use the calculator on your smartphone.

Bagged compost may be practical in terms of cost and effort. For example, to cover 100 square feet of area with compost 3 inches deep, it would take 25 1-cubic foot bags. Consider your physical abilities and cost before deciding to purchase bagged or bulk (delivered) compost products.

Compost in bulk

Bulk compost comes in increments of cubic yards. One cubic yard equals 27 cubic feet of material. Figure 4 shows compost being delivered.

How to use compost

Vegetable gardens

For new vegetable beds, add 3–4 inches of compost. For existing vegetable beds, add one-quarter to 1 inch of compost per year.

After you spread the compost, follow these steps.

1. Add amendments such as lime and N-P-K fertilizer as indicated by a soil test.
2. Incorporate the materials into the top 8–12 inches with a digging fork, spade or rototiller.
3. Remove debris and chop the soil clods with a hoe, rake or the back of a shovel.

Spread the compost over the area to the desired depth. Wheelbarrows, 5-gallon buckets, shovels and rakes are useful tools for spreading compost (Figure 5).

Use a digging fork, spade,or shovel to mix in the compost and other amendments. Loosening the soil decreases compaction and increases the amount of air in the soil (Figure 6).

Bulk soil mixes from a landscape supply company may be needed for larger jobs. For example, it takes 1.7 cubic yards of soil mix to fill four raised beds (3 feet by 8 feet) with 6 inches of soil mix. Common bulk soil mixes include:

• Three-way mix with equal parts topsoil, sand and compost.
• Four-way mix with equal parts topsoil, sand, compost and lava rock or vermiculite.
• Specialty planting mixes for raised bed and container growing.

Expect your newly filled beds to sink over time as the compost in the mix further decomposes. Refill the raised beds as needed.

For more information about raised beds, see Raised Bed Gardening, FS 270.

Plants thrive in properly amended soil

With proper planning and action, you can ensure the successful establishment of plants in your new landscape areas.

The sequence of photos below shows how soil amended with compost promotes healthy landscape plants:

• The soil in a landscape bed was amended with compost and mixed in with heavy equipment.
• Plants were installed in the fall. They were well watered at that time.
• Then, without any additional irrigation or fertilizer, the drought-hardy plants grow into the space over time.

Mix compost into the soil, adding about half again as much compost as the hole is deep. If you dig 1 foot deep, mix 6 inches of compost into the soil removed. Incorporate the compost into the existing soil (Figure 9B).

The landscape bed is now ready for planting. The surface was raked. Edging was installed between the planting bed and pathway (Figure 9C).

Landscape plants were installed in fall. The plants were watered at the time of planting as shown in Figure 9D.

Mulch was applied on the surface. Woody mulch helps to conserve water in the soil. It also suppresses weed seed germination. (Figure 9E).

After a year, the plants have started to fill in the area without additional irrigation or fertilizer (Figure 9F). If you have appropriately prepared the soil and precipitation is adequate, drought-tolerant plants thrive with minimal inputs.

Lawns

New lawn installation

Use compost to improve the growing conditions for lawns. Adding compost stimulates soil microbe activity, which supports improved soil structure.

The first step in a new lawn installation is to apply compost over the worksite and mix it in with a rake or rototiller. A thin layer of compost may be used as mulch after seeding turf. Add 1–2 inches compost to the area as you prepare the soil (Figure 10). Rototill the compost into the soil. Rototill the compost into the soil, but be aware that excess tillage damages soil structure.

You can use a thin layer of compost as a mulch after seeding. Mulch retains soil moisture, which is essential for seed sprouting and early growth.

Figure 11 shows a wire mesh cylinder or mulch roller tool being used to cover a newly seeded lawn area with mulch. It’s filled with mulch and used to apply a thin layer of mulch to the surface. This tool is available at tool rental centers.

For more information, see Practical Lawn Establishment and Renovation.

Compost for existing lawns

Topdressing is the addition of compost over the top of an existing lawn. Adding compost often improves soil conditions for grass growth. Spread new lawn seed over the area before or after applying compost.

Late spring and early fall are the best times of year for this work. New seeds will sprout quickly when the soil temperature is above 60 degrees Fahrenheit.

Follow the steps below to top dress your lawn with compost:

1. First, mow the lawn short. Use a bagging mower or a rake to collect the cut grass (Figure 12). Even better, use a dethatching machine to thin the grass. The goal is to expose the soil surface for the next steps.
2. Use a core aerating machine on the lawn area (Figure 13). The machine pokes holes in the soil surface and leaves the plugs of soil on the surface. Break up the plugs with a rake.
3. Spread compost one-quarter to one-half-inch deep across the lawn area (Figure 14). The compost will fill in the holes left by core aeration. Amend the soil with lime and other fertilizers if indicated by a soil test.
4. Use a seeder to spread seed over the entire lawn area (Figure 15). Applying seed over an existing lawn (overseeding) will help fill in gaps in the lawn to create a dense stand of turf.
5. Apply water to the seeded area. Keep the seedbed evenly moist until the seeds sprout. Be ready to water three to four times per day to keep the soil and seeds from drying out.

For more information, see Practical lawn care for Western Oregon, EC 1521, and Turfgrass maintenance calendar for Central Oregon.

Before you buy bulk compost or soil mix

Before you purchase a bulk load of compost or soil mix, ask the vendor:

• Whether they test their products for herbicide residues. Basic lab testing might not indicate herbicide contamination. See Herbicide-contaminated compost and soil mix: What to know — and what you should do about it.

• Whether they can provide an analysis of their materials. Test results might include soil pH, percentage of organic matter and N-P-K.

• Whether the finished product is stored under tarps or is otherwise protected from windblown weed seed.

• The moisture content of the finished product. Wet compost (greater than 60% moisture content) is clumpy and doesn’t spread well. Dry compost (less than 40% moisture content) creates dust when spread, is difficult to incorporate and tends to stay on the soil surface.

Avoid problems with compost

Herbicide-contaminated compost and soil mix

Compost and soil mix products sometimes contain herbicide residue. This residue can damage certain vegetable, fruit and flower crops. The residue likely comes from contaminated hay, grass clippings or manure. These materials may make their way into the regional composting system.