Follow these guidelines to add the right amount of compost to your site for growing vegetables.
Add compost to soil before you install landscape plants. Spread a 3- to 4-inch layer of compost over the area you intend to plant. Mix the compost into the soil as deeply as possible. Then install new plants into the amended soil.
Adding compost improves growing conditions for lawns.
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:
Compost quality varies. Quality depends on the type of raw materials. It also depends on the compost production method.
High quality compost should look like dark topsoil. It has a light, crumbly structure (Figure 1).
Look for compost that doesn’t have large particles. It should pass through a ⅜-inch screen. There shouldn’t be any rocks, trash or other debris. The compost in Figure 2 has large pieces that should be removed for vegetable gardens and lawns. Chunky compost is acceptable for landscape areas.
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.
Before you purchase bulk compost, ask the vendor about the product. Inquire about the following factors:
What to look for in finished compost
Greater than 30%
Carbon to nitrogen ratio (C:N)
Below or equal to 30:1
0.5 to 3.0 percent
Greater than 0.2%
pH (scale of acid to base, 7.0 is neutral)
Meets state and federal agency standards
Finished composts made from mushroom and manure sources may have high soluble salts. Salts from compost can harm vegetable seeds and transplants and lawn seed. Soluble salts from compost can also damage sensitive plants such as blueberries.
Get the right amount of compost for your job.
If these calculations seem daunting, use an online compost calculator to determine how much compost you’ll need.
Bags of purchased compost often contain 1-2 cubic feet of material (Figure 3).
Use bagged compost for smaller jobs.
For vegetable gardening, a single 1-cubic foot bag of compost covers 12 square feet of area to a depth of 1 inch. That’s the largest amount of compost to add each year for existing vegetable areas.
For new vegetable and landscape beds, a single 1-cubic foot bag of compost will cover an area 2.5 feet long x 2.5 feet wide (5 square feet) with 2.4 inches of compost.
It’s practical to buy compost in bags up to a point 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. For this amount of compost, consider purchasing it in bulk and paying for delivery.
Bulk compost comes in increments of cubic yards. One cubic yard equals 27 cubic feet of material.
For larger jobs, you can buy 1–2 cubic yards of compost.
Bulk compost deliveries are based on a flat fee, not on the amount delivered.
People often buy bulk compost in units of 7.5 cubic yards. Figure 4 shows a unit of compost being delivered.
Consider using a full unit of compost for larger jobs. For example, 7.5 cubic yards of compost covers an 800-square-foot area about 3 inches deep.
This example shows the correct volume of compost to amend a larger area for a new vegetable growing area or landscape bed.
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 right amount of compost, follow these steps.
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).
Raised beds use retaining walls to hold in soil (Figure 7). Retaining walls for raised beds can be made from wood, bricks or similar materials.
Raised beds efficiently use space by creating clearly defined paths and growing areas. You can make them into any shape.
Make wheelchair-accessible raised beds 28–34 inches tall.
Before you fill your raised bed, break up the soil surface at the bottom of the area. Add 2–3 inches of compost or soil mix and other amendments such as lime. Mix the added materials into the existing soil.
If you excavated soil while installing your raised beds, mix in compost and use it to fill the raised beds. Add up to 25% compost by volume to fill the depth of the bed.
If your raised bed is not yet filled, you’ll need to purchase a soil mix to fill it.
Use bagged potting soil to fill the raised bed for smaller jobs. For example, it takes 12 1-cubic-foot bags to fill a 3-foot-by-8-foot bed with 6 inches of potting soil.
For larger jobs, purchase a bulk soil mix from a landscape supply company. 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:
Expect your newly filled beds to compact 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.
Don’t fill raised beds with compost by itself. Fill beds with native soil or soil mix. Compost lacks the mineral component of earthen soil and planting mixes. Compost must be mixed with these materials to support plant growth.
Figure 8 shows only compost being used to fill raised beds. This compost application isn’t recommended.
Spread compost over the area where you want to install new plants. Mix the compost in as deeply as possible. Then dig planting holes in the amended soil. Install the new plants. Figure 9 shows a properly amended planting area.
For large areas or compacted soil consider using heavy equipment.
Before digging with a tractor, locate utilities in the area. Visit https://digsafelyoregon.com or call 811 before you dig.
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:
Mix compost into the soil as deeply as possible (Figure 10A). Add about ⅓ compost by volume. So, if you can dig 2 feet deep with a tractor, you could add compost up to 1 foot deep. Incorporate the compost into the existing soil (Figure 10B).
The landscape bed is now ready for planting. The surface was raked. Edging was installed between the planting bed and pathway (Figure 10C).
Landscape plants were installed in fall. The plants were watered at the time of planting as shown in Figure 10D.
Mulch was applied on the surface. Woody mulch helps to conserve water in the soil. It also helps keep weed seeds from germinating (Figure 10E).
After a year, the plants have started to fill in the area without any additional irrigation or fertilizer (Figure 10F). When the soil is prepared the right way, drought-tolerant plants thrive with minimal ongoing inputs in areas with adequate rainfall.
Use compost to improve the growing conditions for lawns. Adding compost helps to improve soil structure.
The first step if you plan to install a new lawn 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 11). Rototill the compost into the soil. Don’t overtill the soil.
You can use a thin layer of compost as a mulch after seeding. Mulch helps to keep seeds moist during sprouting and early growth.
Figure 12 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, EC 1550.
Topdressing is adding compost on top of an existing lawn. Adding compost often improves soil conditions promoting 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:
For more information, see Practical Lawn Care for Western Oregon, EC 1521, and Turfgrass Maintenance Calendar for Central Oregon.
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.
Before you purchase a bulk load of compost or soil mix, contact the vendor.
Gardeners and landscapers should be aware of symptoms of plant injury from herbicides.
Herbicide damage from contaminated compost or soil mix is caused by growth regulator herbicides. These herbicides more often affect broadleaf plants. Look for distorted or cupped leaves. The damage occurs in new growth, including the end of the shoots (Figure 17).
When too much compost is added to the soil, plants growing in the amended area fail to thrive. Plants often look stressed, stunted or burned growing in soil with too much compost.
Plants’ response to too much compost looks similar to fertilizer burn. Figure 18 shows a chrysanthemum growing in a container with too much fertilizer. The leaves look burned and purple.
If too much compost is added to the soil, nutrients including phosphorus and potassium can leach past the root zone and move beyond your garden. The nutrients may leach into surface and ground water from rain or irrigation. Nutrients in surface water supports algae growth and low-oxygen water.
The surface water shown in the pond in Figure 19 is covered with a thick layer of algae. Algae growth often results from too much nutrients. Excess nutrients in surface waters often come from fertilizer. Adding too much compost to soil can have a similar impact on surface water and groundwater.
Bell, N., D. Sullivan, L. Brewer, J. Hart. 2017. Improving Garden Soils with Organic Matter, EC 156, Corvallis, Oregon: Oregon State University Extension Service.
Edmunds, B. 2020. Raised Bed Gardening. FS270, Corvallis, Oregon: Oregon State University Extension Service.
Landschoot, P. 1997. Using Composts to Improve Turf Performance, University Park, Pennsylvania: Penn State Extension
Starbuck, C. J. 2020. Making and Using Compost. g6956, Columbia, Missouri: University of Missouri Extension.
Traunfeld, J. 2019. Soil to Fill Raised Beds. College Park, Maryland: University of Maryland Extension.
Cooperative Extension Fresno County. 2020 Accessible Gardening.UC Extension Fresno County. Fresno, California: University of California Agriculture and Natural Resources.