How Much Compost to Add to a Garden Bed: Rates, Chart and Math

How much compost to add to a garden

For an existing garden bed, add a layer of compost 1/4 inch to 1 inch deep each year. For a new bed on unimproved soil, spread 3 to 4 inches once and mix it into the top 8 to 12 inches. Both figures come from Oregon State University Extension. In volume terms, 1 inch of compost over 1,000 square feet takes about 3 cubic yards, a conversion that Colorado State and Utah State both use.

The type of compost changes the yearly number. Colorado State University Extension splits its rate for gardens that have been amended for more than three years: 1 to 2 inches a year of plant-based compost, and 1/2 inch a year of compost made from manure or biosolids, which tends to carry more salt. If you do not know what your compost was made from, stay within Oregon State’s 1/4 to 1 inch.

Situation	Depth	Source
Existing vegetable or flower bed, yearly	1/4 to 1 inch	Oregon State
Established garden, plant-based compost, yearly	1 to 2 inches	Colorado State
Established garden, manure-based compost, yearly	1/2 inch	Colorado State
New garden bed, one time	3 to 4 inches, mixed 8 to 12 inches deep	Oregon State
New lawn, one time	1 to 2 inches, tilled in	Oregon State
Existing lawn, topdressing	1/4 to 1/2 inch	Oregon State

Compost chart: volume by bed size and depth

Compost is sold by the cubic foot in bags and by the cubic yard in bulk, so a depth has to be converted before you buy. The chart below does the conversion for four common areas.

Area	1/4 inch	1/2 inch	1 inch	3 inches
4 by 8 foot bed (32 sq ft)	0.7 cu ft	1.3 cu ft	2.7 cu ft	8 cu ft
100 sq ft	2.1 cu ft	4.2 cu ft	8.3 cu ft	25 cu ft
1,000 sq ft	0.8 cu yd	1.5 cu yd	3.1 cu yd	9.3 cu yd
1 acre (43,560 sq ft)	34 cu yd	67 cu yd	134 cu yd	403 cu yd

For any other size, use this calculation:

1. Multiply length by width in feet to get square feet.
2. Multiply by the depth in inches and divide by 12. The result is cubic feet.
3. Divide cubic feet by 27 to get cubic yards.

A 10 by 20 foot plot is 200 square feet. At 1/2 inch, that is 200 times 0.5 divided by 12, or 8.3 cubic feet, about a third of a cubic yard. One 1 cubic foot bag covers 12 square feet at 1 inch deep, which is a handy check at the store. For deciding between bags and a bulk delivery, see bagged vs. bulk compost.

How much compost per acre

Scaling the garden rate to an acre gives large numbers. At 3 cubic yards per 1,000 square feet, a 1 inch layer over 43,560 square feet takes about 131 cubic yards by that rounded rule and about 134 by exact geometry. A 1/4 inch application, the low end of Oregon State’s yearly range, takes about 34 cubic yards per acre, and 1/2 inch takes about 67.

These are garden-style depth rates applied to a larger area. For a market garden or a large food plot, get a soil test before ordering by the truckload.

New beds: 3 to 4 inches, once

A new bed gets the heavy application because the goal is to change the soil. Oregon State’s method is to spread a 3 to 4 inch layer on the surface and incorporate it into the top 8 to 12 inches. For landscape plantings the same depth is mixed 6 to 8 inches deep, which matches Colorado State’s general instruction to cultivate amendments into the top six to eight inches. Utah State offers a simple check on tilling depth: for every inch of compost added, till at least twice that deep.

Colorado State frames the long-term target as raising soil organic content to 3 to 5 percent over a period of years. That target explains why the big dose happens once. After the first year, small yearly additions maintain what the first application built.

Raised beds and containers

A raised bed filled from scratch should not be mostly compost. Oregon State’s guidance is to add up to 25 percent compost by volume when filling the bed. A 4 by 8 foot bed that is 12 inches deep holds 32 cubic feet, so the compost share tops out at 8 cubic feet, with soil or a soil mix making up the rest. Our guide to soil for raised bed vegetables covers the other 75 percent.

In later years, treat the raised bed as an existing bed and top it up with 1/4 to 1 inch each year. Oregon State notes that newly filled beds sink over time as the compost in the mix decomposes, and says to refill them as needed.

Containers follow a similar proportion. Utah State’s potting mix recipes use compost at 20 to 33 percent of the mix, depending on the plant, blended with peat moss and perlite or vermiculite. Compost for potting mixes should also be low in salt; Utah State lists less than 2.5 mmhos per centimeter for that use. More on blends is in the potting mix guide.

How much manure for a garden

Composted manure is applied at a lower rate than plant-based compost. Colorado State’s figure for an established garden is 1/2 inch a year, and the same publication cautions against adding more than one inch per season of manure, manure-based compost or biosolids without a soil test to check for salt build-up. From the chart above, 1/2 inch on 100 square feet is about 4.2 cubic feet.

Fresh or aged manure is a food safety question before it is a rate question. Aged manure has not necessarily been through the heat that kills pathogens. University of New Hampshire Extension states the waiting periods plainly:

• At least 120 days between applying raw or aged manure and harvesting crops that grow in or near the soil, such as root crops, leafy greens and strawberries.
• At least 90 days for other crops.

University of Wisconsin Extension gives the same 120 and 90 day limits, using sweet corn as an example of a crop in the 90 day group. Colorado State’s practical version is to apply fresh manure only in fall after harvest, at least four months before any edible crop is picked. For manure to count as composted, New Hampshire says the pile must reach 131 to 140 F for several weeks and be turned so that all of it is exposed to that heat.

Penn State Extension lists cow, horse, poultry, sheep, goat and llama manure as appropriate for vegetable gardens and advises against pig manure and against dog, cat and human waste. Poultry manure is high in nitrogen and can burn seedlings, so compost it first.

When more compost stops helping

Compost carries nutrients and salts along with organic matter, and both accumulate. Oregon State warns that soluble salts from compost can damage sensitive plants such as blueberries, that manure-based composts in particular may be high in soluble salts that harm seeds and transplants, and that excess compost leads to nutrients leaching toward groundwater.

Phosphorus is the nutrient to watch. New Hampshire’s guidance ties compost and manure use to the soil test: with phosphorus in the low to optimum range, up to 50 ppm on the UNH test, use manure or compost freely. When phosphorus tests very high, over 100 ppm on that test, consider nutrient sources other than manure. Laboratories report phosphorus on different scales, so read the rating on your own report; how to read a soil test report explains the categories.

Unfinished compost causes a different problem. New Hampshire notes that when the carbon to nitrogen ratio exceeds 30 to 1, most of the nitrogen is immobilized and unavailable to plants for an extended period. Woody, strawy or still-warm compost belongs back in the pile until it finishes.

When and how to apply it

On a new bed, spread the full 3 to 4 inches and dig or till it in before planting. On an existing bed, spread the yearly 1/4 to 1 inch on the surface and mix it into the soil with a digging fork, spade or tiller, along with any lime or fertilizer a soil test calls for. Fresh manure belongs in fall only, for the waiting periods given above.