Loam Soil: What Loam Is and Why Gardeners Want It

What makes a soil loam

Loam is one of USDA’s twelve texture classes. By the Soil Survey Manual it is 7 to under 27% clay, 28 to under 50% silt, and 52% or less sand: a balanced band rather than any single recipe.^[4] On the triangle: 7 ≤ clay < 27 and 28 ≤ silt < 50 and sand ≤ 52.

Enough sand to drain and warm up, enough silt and clay to hold water and nutrients: that balance keeps loam away from the extremes of its neighbors. It has no single particle size, because it is a mix of sand (0.05–2 mm), silt (0.002–0.05 mm) and clay (under 0.002 mm); for how those three separates behave, see sand, silt and clay.^[4]

Is loam 40-40-20?

You will see loam described as “40% sand, 40% silt, 20% clay.” Run those numbers through our texture classifier and they do come out as loam, but it is a rule of thumb, not the definition. Loam spans a range, so mixes like 45/40/15, 35/45/20 and 50/35/15 (sand/silt/clay) all classify as loam too. Any blend inside the band above is loam; 40-40-20 is just the middle of it.^[4]

How loam behaves

Loam topsoils in USDA’s survey hold a median 1.9 inches of plant-available water per foot and carry about 2.0% organic matter, with water moving through at 9 µm/s. Water held sits in the upper-middle of the twelve classes: more than any sandy soil, a little less than the silt-rich classes. It drains readily yet keeps a useful reserve, which is why roots rarely drown or thirst in loam.^[3]

Most loam is well drained (78.2% of its acres), and only 4.6% are poorly or very poorly drained, so wet loam is usually a landscape-position problem, not the texture. Its low shrink-swell means it does not crack and heave the way the clay classes do.^[3][5][6]

Loam holds nutrients moderately well. Cation exchange capacity comes from clay and organic matter, and Purdue puts light-colored loams and silt loams at 10–20 meq/100 g (15–25 when darker), against 30–40 for dark silty clay loams and silty clays. The full ladder is on the clay page.^[7]

What is loam good for?

Growing almost anything. The University of Georgia puts it plainly: vegetable plants grow best in a fertile, loamy soil that drains well.^[8]

The Royal Horticultural Society agrees, calling loam “fertile, well-drained and easily worked,” the mix that avoids the extremes of clay or sandy soils, and lists it among the six gardening types.^[1]

The survey backs that up. 36% of rated loam acres fall in land-capability Classes I–IV (the classes suited to cultivation), 14% in Classes I–II (the best cropland, with few limits), and 9% is mapped prime farmland. Strawberries do best on loam or sandy soils; the sandy loam page covers those. For lawns, loam is a strong base, but grass choice follows your climate, not the texture (see choosing a lawn grass).^{[3][9][10][11]}

The disadvantages of loam

Where loam acres fall short of top cropland, USDA records why as a capability subclass. Across rated loam acres, 56% carry an e (erosion risk, usually slope), 26% an s (a root-zone limit such as shallow depth or stoniness), 9% a w (wetness) and 7% a c (climate). So the main thing to watch on loam is erosion on a slope; keep it covered and add organic matter to hold its structure. You cannot amend a soil into the loam class, because texture is set by its sand, silt and clay, but you can give clay loamier behavior by working in organic matter, covered in how to amend clay soil.^[3][9]

Where loam is found in the US

Loam is spread widely but concentrates in the northern Great Plains and the West. Montana, Texas and California hold the most acres. The single most extensive loam series is Williams, mapped most in North Dakota.^[3]

Most widespread loam soil series

Gravelly, heavy and light loam

Gravelly loam is the word for a loam with rock fragments. USDA adds a fragment modifier once a soil is 15% or more rock by volume: gravelly at 15 to under 35%, very gravelly at 35 to under 60%, and extremely gravelly at 60 to under 90%.^[4] The loam texture is unchanged; the modifier just flags the stones, which drain faster and get in the way of digging.

“Heavy” and “light” loam are informal, not USDA terms: a heavy loam leans toward the clay side and works stickier, a light loam toward the sandy side and drains quicker.

Is loam the same as topsoil?

No. Loam is a texture, a particular mix of sand, silt and clay. Topsoil is the surface layer of a soil, the A horizon, and it can be loam, sand, clay or anything in between.^[12] What to look for when buying topsoil is on the sandy loam page, the texture UConn rates ideal for it.

Frequently asked questions

What is loam good for?

Gardens, lawns and most field crops. It balances drainage with water and nutrient holding, which is why 36% of rated loam acres are cultivable cropland and vegetables grow best in a fertile, well-drained loam.^[3][9][8]

What is the disadvantage of loam soil?

Few. The commonest limit USDA records on loam acres is erosion on slopes (56% carry an erosion subclass), and like any soil it needs organic matter replenished to stay fertile and keep its structure.^[3][9]

Is loam the same as topsoil?

No. Loam is a texture (a sand/silt/clay mix); topsoil is the surface layer of a soil and can be any texture.^[12]

Is loam 40-40-20?

40% sand, 40% silt, 20% clay does classify as loam, but it is only the middle of the loam band. Any mix with 7–27% clay, 28–50% silt and 52% or less sand is loam.^[4]

Where is loam found?

Across the US, most of all in Montana, Texas and California. Its most widespread series, Williams, is a North Dakota soil.^[3]