Soil vs Dirt: The Measured Difference in 7 U.S. Soils

The soil vs dirt difference can be measured, and the numbers sit in a public USDA database. We pulled the horizon records for 7 benchmark U.S. soils from USDA NRCS Soil Data Access (the data behind Web Soil Survey): organic matter runs 0.75 to 5.5 percent in the plow layer and falls to 0.1 to 0.55 percent in the deepest layer, the material an excavator hauls off as fill dirt. None of the top 5 search results we read in November 2026 gave a single measured figure for either one.

That matters when you buy or move material. Fill is sold for grading, and builders are told to strip topsoil out from under slabs (IRC 2021, R506.2), while New York’s highway spec will not accept lawn topsoil under 5 percent organic content. Prices per yard and ton are in our topsoil and fill dirt cost guide, and bagged products are covered in what 9 topsoil and garden soil bag labels contain.

What do soil scientists call soil, and what is dirt?

The Soil Science Society of America Glossary of Soil Science Terms defines soil as “the layer(s) of generally loose mineral and/or organic material” near the surface that “usually hold liquids, gases, and biota and support plants.” The same glossary has no entry for dirt: its D list runs from “direct shear test” straight to “discharge” (SSSA glossary, letter D, read Nov. 12, 2026).

Dirt gets two working definitions from soil scientists. One is about place: J. Patrick Megonigal of the Smithsonian Environmental Research Center told Science News Explores in 2017 that “dirt is displaced soil.” The other is about function: SSSA’s own Soils 4 Kids site calls dirt soil “out of place” and also soil that has “lost the characteristics that give it the ability to support life.”

Source	Term	What it says	What it means on a job site
SSSA Glossary	Soil (a)	Loose mineral or organic layers near the surface that usually hold liquids, gases and biota and support plants	The working top layer, in place
SSSA Glossary	Dirt	No entry between “direct shear test” and “discharge”	Not a defined technical term
Soils 4 Kids (SSSA)	Dirt	Soil out of place, or soil that has lost the ability to support life	Tracked-in soil, or stripped and dead material
Megonigal, Smithsonian (2017)	Dirt	“Displaced soil”	Anything dug up and moved
USDA NRCS Soil Quality sheet (2001)	Soil	Mineral particles, organic matter and numerous species of living organisms	Biology is part of the definition
IRC 2021, R506.2 and R506.2.1	Top soil, fill	Remove all vegetation and top soil inside foundation walls; fill must be free of vegetation and compacted	Builders treat topsoil as waste under slabs and want inert fill

Soil vs dirt by the numbers

A typical mineral soil is about 50 percent solids and 50 percent pore space, split as 45 percent minerals, 5 percent organic matter, 25 percent air and 25 percent water (Illinois Extension, Soil Basics; Oregon State EM 9544). NRCS adds that organic matter is under about 5 percent of the volume in most soils (NRCS Soil Quality Indicators: Organic Matter), so small percentages carry most of the difference.

Property	Soil (topsoil in place)	Dirt (displaced or subsoil fill)	Source
Volume makeup	About 45% minerals, 5% organic matter, 25% air, 25% water	Same minerals; organic share near zero at depth	Illinois Extension
Organic matter, 7 U.S. soils	0.75% to 5.5% in the plow layer	0.1% to 0.55% in the deepest layer	USDA SSURGO
Organic content in a lawn topsoil spec	5% to 12% (NYSDOT 713-01, lawn)	No minimum for fill	NYSDOT, Jan. 2026
Bulk density and pore space	1.33 g/cm3 is about 50% pore space, the ideal	Miami 2Cd at 1.85 g/cm3 is 30% pore space	SDSU Extension; SSURGO
Bacteria per teaspoon (1 gram)	100 million to 1 billion in healthy soil	No count published; carbon limits biology	NRCS Soil Biology Primer
Smell and color	Humus is dark brown with a pleasant, earthy smell	Subsoil colors run lighter: Cecil subsoil is red (10R 4/8) clay	NRCS; Cecil OSD
Job-site rule	Removed from under slabs	Must be compacted; 8 inch limit for earth fill unless approved	IRC R506.2.1

Pore space is the number to watch, because air, water and organisms all live there. SDSU Extension gives the formula: pore space equals 1 minus bulk density divided by 2.65 g/cm3, the density of mineral particles (SDSU, Bulk Density). Its NRCS table marks 1.75 g/cm3 as the point where a silt loam starts to restrict roots; the dense till under a Miami silt loam sits at 1.85.

Soil and dirt in the same hole: 7 U.S. soils

Every soil survey map unit carries horizon-by-horizon estimates of organic matter, bulk density and texture. We queried Soil Data Access on Nov. 12, 2026 for the main component of one map unit each from 7 benchmark soil series, from the Corn Belt to the Georgia Coastal Plain and California’s Central Valley. The top layer is the soil you garden in; the bottom layer is what a basement dig or a grading job turns into fill dirt.

Soil series (county, map unit)	Top layer	Organic matter	Bulk density g/cm3 (pore space)	Deepest layer	Organic matter	Bulk density g/cm3 (pore space)	Top vs deepest
Miami (Hamilton Co., IN, MmA)	Ap, 0 to 7 in	2%	1.45 (45%)	2Cd, 34 to 79 in	0.25%	1.85 (30%)	8x
Drummer (Champaign Co., IL, 152A)	Ap, 0 to 14 in	5.5%	1.3 (51%)	2Cg, 47 to 60 in	0.1%	1.55 (42%)	55x
Cecil (Wake Co., NC, CeB)	Ap, 0 to 8 in	0.75%	1.58 (40%)	C, 50 to 80 in	0.25%	1.56 (41%)	3x
Houston Black (Williamson Co., TX, HoA)	Ap, 0 to 6 in	3%	1.23 (54%)	BCkss, 70 to 80 in	0.55%	1.29 (51%)	5.5x
Tifton (Tift Co., GA, TfA)	Apc, 0 to 11 in	0.75%	1.63 (38%)	C, 65 to 80 in	0.15%	1.4 (47%)	5x
Plano (Dane Co., WI, 7199A)	Ap, 0 to 11 in	4%	1.34 (49%)	2C, 46 to 79 in	0.25%	1.5 (43%)	16x
Hanford (Madera Area, CA, HaA)	H1, 0 to 12 in	0.75%	1.55 (42%)	H3, 36 to 60 in	0.25%	1.55 (42%)	3x

1. Miami2%
2. Drummer5.50%
3. Cecil0.75%
4. Houston Black3%
5. Tifton0.75%
6. Plano4%
7. Hanford0.75%

Source: USDA NRCS SSURGO via Soil Data Access, representative values, queried Nov. 12, 2026 Top layer ends 6 to 14 in down; deepest layer starts 34 to 70 in down.

Show the numbers

	Top layer (soil)	Deepest layer (fill dirt)
Miami	2%	0.25%
Drummer	5.50%	0.10%
Cecil	0.75%	0.25%
Houston Black	3%	0.55%
Tifton	0.75%	0.15%
Plano	4%	0.25%
Hanford	0.75%	0.25%

Two patterns stand out. In the Midwest prairie soils the gap is wide: Drummer silty clay loam in Champaign County, Ill., holds 5.5 percent organic matter in its top 14 inches and 0.1 percent below 47 inches, a 55-fold drop. In the Southeast and the Central Valley, the plow layer of Cecil, Tifton and Hanford is only 0.75 percent, which is less than Drummer’s subsoil at 1.2 percent.

So no single organic matter cutoff separates soil from dirt nationwide. What separates them on your lot is position and life: the top 6 to 14 inches carry the roots, the granular crumb and the biology, and the layer 3 to 6 feet down does not. Sandy soils like Tifton and Hanford hold so little organic matter that the question becomes whether grass can grow in sand at all, and compost does most of the lifting there.

How much life is in a teaspoon of soil?

The NRCS Soil Biology Primer slide set gives typical counts per gram, about a teaspoon, for healthy soils under 3 land uses. Bacteria hold steady at 100 million to 1 billion; fungi, protozoa and arthropods climb sharply from farm fields to prairie and forest. Ohio State’s Role of Soil Bacteria fact sheet quotes the same 100 million to 1 billion range and puts the bacterial mass at about two cows per acre.

Organism	Farmland	Prairie	Forest
Bacteria, per gram (teaspoon)	100 million to 1 billion	100 million to 1 billion	100 million to 1 billion
Fungi, per gram	Several yards of hyphae	10s to 100s of yards	1 to 40 miles (conifers)
Protozoa, per gram	1,000s	1,000s	100,000s
Nematodes, per gram	10 to 20	10s to 100s	100s
Arthropods, per sq ft	Under 100	500 to 2,000	10,000 to 25,000
Earthworms, per sq ft	5 to 30	10 to 50	10 to 50 (0 in conifers)

Dirt has no matching table because nobody publishes one, but NRCS names the two limits. Carbon from residue, compost and manure is typically the limiting resource for soil life, and compaction removes the large pores the organisms live in (NRCS Soil Quality: Soil Biodiversity, 1998). Subsoil at 0.1 to 0.55 percent organic matter is short on carbon, and the densest layers, like the till under Miami silt loam at 30 percent pore space, are short on room.

What several soil vs dirt guides get wrong

Four claims recur across the pages ranking for this question. We checked each against the primary source in November 2026; the table names the page we compared, not every site that repeats it.

Claim	Where we read it	What the primary source shows
The Soil Science Society of America defines dirt as “displaced soil”	Nature’s Path blog	The SSSA glossary has no dirt entry. The phrase is Megonigal’s (Science News Explores, 2017); SSSA’s kids site says “out of place”
Dirt “has none of the minerals” found in soil	Nature’s Path blog	Displaced soil keeps its sand, silt and clay. Cecil’s red subsoil is 53% clay and Tifton’s Btc2 subsoil is 63.5% sand (SSURGO)
Dirt “does not compact when wet”	Nature’s Path blog	IRC R506.2.1 requires earth fill to be compacted; NRCS says moist or wet soil compacts more easily than dry (NRCS, Compaction)
A teaspoon of healthy soil holds more organisms than there are people on Earth	MyLand blog	NRCS lists 100 million to 1 billion bacteria plus thousands of protozoa per teaspoon of farmland, against 8.2 billion people in 2024 (UN WPP 2024)

Fill dirt, topsoil or garden soil: which goes where?

Use the material whose organic content matches the job. New York’s DOT, one of the few agencies that writes numbers into a topsoil spec, asks for 3 to 8 percent organic content in roadside topsoil, 5 to 12 percent under lawns and 10 to 15 percent in planting mix (NYSDOT Standard Specifications, Section 713-01, Jan. 1, 2026). Fill carries no organic minimum because organic matter decomposes and settles.

Material	Typical organic matter	Use it for	Avoid it for	Source
Fill dirt (subsoil)	0.1% to 0.55% at depth in our 7 soils	Raising grade, backfill, base under slabs (compacted)	The top 4 to 6 inches under a lawn or bed	SSURGO; IRC R506.2.1
Native topsoil in place	0.75% to 5.5% in our 7 soils	Keep and amend it; restore it after grading	Under slabs, where IRC R506.2 requires removal	SSURGO; IRC
Purchased lawn topsoil	5% to 12% in the NYSDOT lawn spec	A settled 4 to 6 inch layer for new turf	Pots and containers	NYSDOT 713-01; Penn State
Bagged topsoil or garden soil	Often little or no mineral soil	Mixing into existing beds	As the only fill in raised beds	UMaine Bulletin 2283
Compost	Mostly organic matter	3 to 4 inches mixed 6 to 8 inches deep in new beds	Filling beds on its own	Oregon State EM 9308

For a new lawn, Penn State calls for at least 4 to 6 inches of good topsoil, measured settled, over the graded subsoil, with 2 inches of it mixed into the top 2 to 3 inches of subsoil so water and roots cross the line (Penn State Extension, Lawn Establishment). Delivered prices for each grade are in our topsoil cost per yard tables, and the bag-by-bag contents are in our comparison of garden soil vs potting soil.

How to tell whether you have soil or dirt

These 6 checks, from free to lab, settle it for a bed or a lawn area. Do them in moist weather, not after a week of drought, because earthworms and smell both fade in dry ground.

1. Dig a 1 sq ft hole one spade deep (about 8 inches). Topsoil in place is darker than what lies under it: Drummer’s top 7 inches are black (10YR 2/1) with “many fine roots,” while Cecil turns from dark yellowish brown to red clay at 8 inches (Drummer OSD; Cecil OSD).
2. Count earthworms in the spoil. NRCS lists 5 to 30 per sq ft for healthy farmland and 10 to 50 for prairie (Soil Biology Primer); none in moist ground points to dirt.
3. Smell a handful. Well-decomposed organic matter is “dark brown, porous, spongy” with “a pleasant, earthy smell” (NRCS Soil Quality: Organic Matter). Pale, odorless material is low in it.
4. Squeeze and crumble. NRCS says organic matter makes soil “more friable, less sticky, and easier to work” (NRCS); Drummer’s topsoil is described as “weak fine granular.” Dense clods or loose single grains point to subsoil.
5. Run a jar test for texture: fill a straight-sided jar 1/3 with sieved soil, top with water, shake, then mark sand at 1 minute, silt at 2 hours and clay at 48 hours (Clemson HGIC 1656; Montana State marks silt at 6 hours, MSU). Texture tells you sand, silt and clay, which soil and dirt share; it does not measure life.
6. Send a sample to a lab. A standard report gives organic matter percent, which is the one number that settles it; our guide to lab soil tests and kits lists what labs charge.

1. 1Did the material come from a pile, a trench or below the top 14 inches?

   YesTreat it as fill dirt: use it for grade, then cap lawn areas with 4 to 6 inches of settled topsoil (Penn State).

   NoGo to the next question

2. 2Is it dark brown to black, crumbly and earthy-smelling?

   Yes

   NoOrganic matter is likely low: get a lab organic matter test before planting.

3. 3Did a spade-deep 1 sq ft hole in moist ground turn up 5 or more earthworms?

   Yes

   NoBiology is thin: mix 3 to 4 inches of compost 6 to 8 inches deep (Oregon State EM 9308).

4. 4Does a wire flag push in 6 inches by hand?

   Yes

   NoCompacted: loosen to the depth of the hard layer before planting.

5. It is working soil: test pH and nutrients, then plant.

Source: NRCS Soil Biology Primer; NRCS Soil Quality sheets; Penn State; Oregon State EM 9308

If a wire flag or screwdriver stops within the top few inches, the bigger problem is compaction, not the material. Our guide to planting grass seed on hard dirt matches the fix to the depth of the hard layer.

Can you turn dirt into soil?

Yes, for the top 6 to 12 inches, and the recipe is organic matter plus time. Oregon State says to spread 3 to 4 inches of compost and mix it 6 to 8 inches deep for landscape beds, or 8 to 12 inches for new vegetable beds, then add 1/4 to 1 inch a year (OSU EM 9308). Penn State limits each tilled layer to 2 inches of organic material, because thicker layers do not mix evenly (Penn State).

Biology follows the carbon. NRCS reports that organic matter decomposes very slowly below 38°F and fastest up to about 102°F in moist soil, so compost worked in during spring feeds organisms through the growing season (NRCS Soil Quality: Organic Matter). For the depth and moisture rules, see how to till soil without wrecking its structure; to choose between amendments, compost and mulch do different jobs. Heavy red clay has its own steps in our guide to improving clay soil with compost and gypsum and in whether clay soil is acidic.