Compost vs Mulch: Lab Numbers, Depths and Same-Yard Prices

The compost vs mulch question comes down to particle size and carbon. Finished compost runs about 10 to 15 parts carbon per part nitrogen (Cornell Composting); wood chips run 100 to 500. Below, the two are set side by side using two US Composting Council lab sheets, same-yard prices from 4 supply yards checked Nov. 8, 2026, and the federal heat rule that most compost pages quote only half of.

Property	Compost	Organic mulch (wood chips, bark, shredded wood)	Source
Job	Feeds and restructures soil: water and nutrient holding, aeration	Covers soil: cuts evaporation and runoff, blocks weeds, buffers temperature	Colorado State 7.235
Where it goes	Mixed into the top 6 to 12 in., or a thin topdressing	Left on the surface	Colorado State 7.235
Depth	1 to 2 in. (lawn), 3 to 4 in. (new bed), 0.25 in. a year (dug beds)	2 to 4 in.; fine composted material 2 to 3 in., coarse chips 3 to 4 in.	CSU Table 1; ISA
Particle size	Curb It and WLSSD sheets: largest aggregate 0.64 in.	Best weed mulches: about 2/3 in. to 3 in.	STA sheet; City of Bellingham
Carbon to nitrogen (C:N)	10 to 15:1 finished; WLSSD Garden Green measured 11:1	Bark 100 to 130:1; wood chips or sawdust 100 to 500:1	Cornell; WLSSD 2025
Total nitrogen	2.3% (Curb It) and 2.8% (WLSSD), dry weight	Ties up soil nitrogen if mixed in fresh	Curb It; WLSSD; CSU
pH and salts	pH 8.54 and 8.18; soluble salts 3.2 and 6.0 dS/m	Not reported on mulch; manure and biosolids composts often run high in salts	Curb It; WLSSD; CSU
How long it lasts on top	Finely ground compost may not last a growing season	Hardwood chips may last a full year	Morton Arboretum
Weed control as a top layer	Poor; fine compost often supports seedling growth	Good at 3 to 4 in.; weed germination rises once it thins below 2 in.	City of Bellingham; Iowa State
Testing program	Voluntary USCC Seal of Testing Assurance: 10 parameters	No equivalent lab program	USCC STA description
Bulk price, same 4 yards, Nov. 2026	$39.50 to $64 per cu yd (lowest listed at each yard)	$25 to $45 per cu yd (lowest undyed wood mulch)	Yard price lists, table below

The carbon gap explains most of the rules. Colorado State Extension fact sheet 7.235 says fresh wood products tie up soil nitrogen when they are mixed in, because the microbes breaking down the wood use it, and that the hazard is greatest with sawdust. Compost has already finished that work, which is why it goes into the soil and wood stays on top.

1. Finished compost (Cornell)10
2. WLSSD Garden Green (lab, 2025)11
3. Grass clippings15
4. Autumn leaves30
5. Straw40
6. Bark100
7. Wood chips or sawdust100

Source: Cornell Composting C/N table; WLSSD 2025 STA results (Minnesota rule form) Cornell gives typical ranges; the WLSSD bar is one measured product.

Show the numbers

	Low end	High end
Finished compost (Cornell)	10	15
WLSSD Garden Green (lab, 2025)	11	11
Grass clippings	15	25
Autumn leaves	30	80
Straw	40	100
Bark	100	130
Wood chips or sawdust	100	500

Where each one goes, and how deep

The rates below come from Colorado State for compost and the International Society of Arboriculture for mulch. CSU writes its compost rates for plant-based compost or compost known to be low in salts, and notes that 3 cubic yards covers 1,000 sq ft about 1 inch deep.

Job	Compost	Mulch	Source
New vegetable or flower bed	3 to 4 in., mixed 8 to 12 in. deep	2 to 4 in. on top after planting	CSU Table 1; ISA
Existing vegetable or annual bed dug each year	0.25 in. a year, mixed 8 to 12 in. deep	Straw or shredded leaves in season	CSU Table 1; Iowa State
New lawn, before seed or sod	1 to 2 in., mixed 6 in. deep	None	CSU Table 1
Established lawn	1/4 in. of fine compost with core aeration	None	Iowa State
Trees and shrubs, well-drained soil	Optional thin layer under the mulch	Coarse chips 3 to 4 in., at least a 3 ft radius ring, off the trunk	ISA
Wet or poorly drained soil	Fix drainage first	No more than 2 in. of organic mulch	ISA
Perennial bed, weedy	Thin layer to build soil	Coarse mulch topped back up to 4 in. when it thins below 2 in.	City of Bellingham

For both in one bed, the City of Bellingham mulching fact sheet gives the order: fine compost in a thin layer to build soil, then coarse mulch over it for weeds. In practice:

1. Pull weeds first. Mulch can smother tiny seedlings, but perennial and grassy weeds grow up through it (City of Bellingham).
2. Spread compost at the CSU rate for the job, usually 3 to 4 inches for a new bed, and work it in 8 to 12 inches deep. On an existing bed with plants in place, spread 1/4 inch and leave it on the surface.
3. Plant, then cover with 2 to 4 inches of coarse wood chips or bark, kept at least 1 inch off stems (City of Bellingham) and pulled several inches back from tree trunks (ISA).
4. Each spring, check depth with a trowel. Below 2 inches of coarse mulch, top it back up to 4 inches; rake any matted fine layer loose first (ISA).

Our mulch calculator and coverage chart turns those depths into cubic yards, and our clay soil guide covers compost rates where clay is the problem.

Can you use compost as mulch?

You can, but it does the weed job badly. Iowa State Extension says compost improves structure and fertility but does not suppress weeds as well as other mulches. The City of Bellingham fact sheet goes further: fine-textured compost often supports seedling growth, and it lists screened compost and composted manures under materials to avoid for weed suppression.

The reason is nutrients and water at the surface. A UF/IFAS 2024 publication, ENH1380 says nutrient-dense materials like compost can increase weed germination and benefit weed growth, and the Morton Arboretum adds that already-decomposed compost may not last a full season on top, where hardwood chips may last a year. The ISA still lists compost mixes as an organic mulch and caps fine composted material at 2 to 3 inches.

There is one exception, and it is not for home beds. In a University of Florida trial, 4-week-old municipal waste and biosolids compost laid 7.5 cm (about 3 inches) or deeper in vegetable row alleys stopped weeds for 240 days in fall 1995 but only 65 days in spring 1996, when yellow nutsedge pressure was higher. The authors credit part of the effect to acetic acid at 1,221 to 4,128 mg per kg, which is phytotoxic, and they found glass and plastic in the compost. Immature compost that kills weeds can hurt plant roots too.

Can mulch go into the soil, and will it turn into compost?

Fresh wood should stay on top. CSU 7.235 says fresh wood products are typically not used as amendments because they can cause nitrogen deficiency for several months to years, and that composting them first avoids it. On the surface the effect is small: Linda Chalker-Scott of Washington State University describes a zone of low nitrogen at the mulch and soil interface that inhibits weed seeds without affecting established roots below.

Yes, mulch does turn into compost over time. Iowa State notes that high C:N mulches such as hardwood bark, ground pallets, straw and sawdust lower available nitrogen most for herbaceous plants and new transplants, and that the effect fades as they decompose. CSU says organic mulches can be worked in as amendments once they have broken down too far to work as mulch. The wood chips vs mulch trials cover which wood products last longest.

What a compost lab sheet tells you before you buy

Bagged and bulk compost carry no required quality label. Colorado State says there are no regulations on the salt content or other qualities of bagged soil amendments and points buyers to the US Composting Council’s Seal of Testing Assurance (STA). Under the STA program description, certified products are tested for 10 properties, including pH, soluble salts, nutrients, particle size, stability, a seedling bioassay, pathogens and trace metals, and the producer publishes a Compost Technical Data Sheet.

Line on the sheet	Curb It Compost (Republic Services, Kuna, Idaho)	WLSSD Garden Green (Duluth, Minn.)	What to do with it
Date sampled	Apr. 10, 2025	Feb. 11, 2025	Ask for a sheet under 12 months old
Total nitrogen, dry weight	2.3%	2.8%	Useful, but USCC says compost is typically not characterized as a fertilizer
C:N ratio	Not on the STA sheet	11:1 (Minnesota Class I limit: under 20:1)	Under 20:1: low risk of tying up soil nitrogen
pH	8.54	8.18	Above 7: a poor match for acid-loving plants; check your soil test
Soluble salts (EC)	3.2 dS/m	6.0 dS/m	Higher salts: use the low end of the rate and water in
Organic matter, dry weight	42.3%	63.7%	Higher means more soil building per yard
Moisture	26.5%	49.1%	You pay for water in a wet load
Largest particle	0.64 in.	0.64 in.	Fine enough to mix in; too fine to mulch for weeds
Seedling emergence and vigor, % of control	100.0 and 100.0	93.3 and 75.3	Vigor under 100 is a reason to mix in rather than plant straight into it
Fecal coliform or Salmonella; metals	Pass; pass	Pass; pass	Fail on either: do not use on food beds

The two sheets show why a seal is not a grade. Both passed pathogens and metals, but WLSSD’s salt reading is nearly twice Curb It’s, and CSU warns that salt burn from salty amendments is a common cause of plant death. Ask the yard for its current sheet and compare those two lines before you order 3 or more yards.

The 131°F rule has two versions

Most compost pages say compost is safe once it holds 131°F for 3 days. That is the EPA Process to Further Reduce Pathogens (PFRP), 40 CFR Part 503, Appendix B, but only for in-vessel and aerated static piles. For windrows, open rows turned by machine, the same rule requires 55°C (131°F) or higher for 15 days or longer, with at least 5 turnings in that period.

The rule binds sewage sludge compost by law; the USCC definition of compost borrows the same PFRP standard for all compost. CSU adds that composted manure heated above 145°F kills most pathogens and weed seeds, while aged manure piled 6 months without turning still carries viable weed seeds. When you buy, ask which method the site uses and whether it logs 15 days and 5 turns for windrows.

Herbicide carryover: run a pea test first

Some broadleaf herbicides survive composting. Oregon State University Extension EM 9307 names clopyralid and aminopyralid, and says chemical lab tests may miss them. NC State Extension lists products that carry them, including Milestone (aminopyralid), Transline and Lontrel (clopyralid) and Grazon P+D (picloram), and says deactivation can take 30 days to several years. NC State traces the risk to treated hay, straw, grass clippings and manure, and to compost made from them. Our grass clippings guide has the label lines that bar treated clippings from compost.

OSU’s bioassay takes 3 to 4 weeks and costs a few pots and a packet of peas:

1. Fill 3 pots with a purchased peat-based potting mix that lists no compost (the control). Fill 3 more with 2 parts test compost to 1 part potting mix.
2. Plant 3 garden pea seeds 1 inch deep in each pot and record the planting date.
3. Grow all 6 pots in the same light, temperature and water until 3 sets of leaves appear, 14 to 21 days after germination.
4. Compare: cupped, twisted or stunted leaves in the test pots and not the controls mean herbicide is present. Keep it away from beans, peas, tomatoes, peppers, potatoes, roses and grapes, which OSU lists as prone to damage.

If contaminated compost is already in a bed, OSU says the herbicide can take 2 to 5 years to break down; plant grasses or corn, which do not show damage, and repeat the bioassay once or twice a year until peas grow normally.

Which costs more at the same yard?

Compost usually does. At 3 of 4 supply yards we checked on Nov. 8, 2026, the lowest-priced compost cost $13 to $20 a cubic yard more than the lowest-priced undyed wood mulch; at Cheaper Than Dirt in Belton, Texas, cottonseed compost was $5 below cedar mulch.

Supply yard	Compost, per cu yd	Undyed wood mulch, per cu yd	Price list
Sequoia Landscape Materials, Santa Rosa, Calif.	Sonny’s Organic Compost $39.50; Cold Creek Compost $54.00	Arbor Mulch $25.00; Wood Chips $44.95; Redwood double grind bark $47.95	Sequoia
Cheaper Than Dirt Landscape Supply, Belton, Texas	Cottonseed compost $40; organic compost / garden mix $50; cow manure compost $70	Cedar mulch $45; hardwood mulch $50	Cheaper Than Dirt
Mr. Mulch Landscape Supply, Columbus, Ohio	Zoo Brew bulk compost $52	Natural brown double shredded hardwood $39; triple shredded $52	Mr. Mulch
Cummin Landscape Supply, Atlanta area	Compost $64	Mini pine nuggets $44; oak mulch $48.50	Cummin

1. Sequoia, Santa Rosa, Calif.39.50
2. Cheaper Than Dirt, Belton, Texas40
3. Mr. Mulch, Columbus, Ohio52
4. Cummin, Atlanta area64

Source: Posted yard price lists, checked Nov. 8, 2026 Pickup or base prices; delivery and tax extra.

Show the numbers

	Compost	Undyed wood mulch
Sequoia, Santa Rosa, Calif.	39.50	25
Cheaper Than Dirt, Belton, Texas	40	45
Mr. Mulch, Columbus, Ohio	52	39
Cummin, Atlanta area	64	44

Coverage changes the bill more than the yard price. A 200 sq ft new bed takes about 1.85 cu yd of compost at 3 inches (200 x 3 / 324) and the same 1.85 cu yd of mulch at 3 inches; at Mr. Mulch that is roughly $96 of compost and $72 of double-shredded mulch before delivery. Our mulch cost survey and topsoil cost data cover delivery fees and blends.

Compost, mulch or both: pick by the spot

1. 1Is it a lawn?

   YesCompost only: 1/4 inch after core aeration (Iowa State), or 1 to 2 inches mixed 6 inches deep before seeding (CSU). No wood mulch.

   NoGo to the next question

2. 2Is the soil wet or slow to drain?

   YesFix drainage first; then no more than 2 inches of organic mulch (ISA).

   NoGo to the next question

3. 3Are you building a new bed in bare, worked soil?

   YesBoth: 3 to 4 inches of compost mixed 8 to 12 inches deep (CSU), then 2 to 4 inches of coarse mulch on top after planting.

   NoGo to the next question

4. 4Is it a vegetable or annual bed you dig every year?

   YesCompost: 0.25 inch a year worked in (CSU); straw or shredded leaves on top in season.

   NoGo to the next question

5. 5Is it a tree ring or an established shrub or perennial bed?

   YesMulch: coarse chips 3 to 4 inches, off the trunk (ISA); optional 1/4 inch of compost under it.

   NoTest the soil first: if pH or salts are the limit, neither compost nor mulch fixes it.

Source: Colorado State 7.235; ISA Proper Mulching Techniques; Iowa State Extension

If the spot is a lawn, compost is the only one of the two that belongs there. If it is a tree ring, mulch does nearly all the work, and our guide to mulching around trees sets the ring size and trunk gap. If you are weighing compost against bagged soil, topsoil vs garden soil reads the bag labels, and a soil test tells you whether pH or salts, not organic matter, are the real limit.