Silt Loam

What makes a soil silt loam

Silt loam is the silt-rich loam: USDA defines it as 50% or more silt with 12 to under 27% clay, or 50 to under 80% silt with less than 12% clay.^[1] On the triangle: (silt ≥ 50 and 12 ≤ clay < 27) or (50 ≤ silt < 80 and clay < 12).

In the hand it feels smooth, almost floury, and ribbons a little under an inch.^[2]

A note on a common search: “sandy silt loam” is not one of USDA’s twelve classes.^[1] A silt loam that picks up sand stays silt loam until it crosses the triangle into loam; a genuinely sandy soil is a sandy loam.

What silt loam is good for

Growing crops, above all. Silt loam ties for the most plant-available water of any class, a median 2.4 inches per foot, and carries 3.0% organic matter. That water, plus a workable texture, shows in the survey: 44% of rated silt-loam acres are cultivable (Classes I–IV), 19% reach the best cropland (Classes I–II), and 15% is prime farmland. Because it covers so much ground, that adds up to 96.4 million acres of prime farmland, the most of any class.^[3][4][5]

Three cautions. Silt loam skews acid (77.0% of its acres are acid), so a lime test pays off before planting; see how to raise or lower soil pH. And 23.3% is poorly or very poorly drained.^[3][6][7]

The third is the surface. Silt loams and sandy loams are particularly prone to crusting: raindrops break the surface crumbs down and the fine particles fill the pores, which delays seedling emergence and raises runoff.^[8] Sealing hits soils low in clay and organic matter but high in silt, most silt loams among them, and it can sharply cut how fast water soaks in.^[9] Working in organic matter is the remedy.^[8]

Its acres spread across the hydrologic soil groups, which rate how fast water soaks in: 28% are in group B, 23% in C and 23% in the slow group D, with the rest in the fast group A or in dual groups such as B/D.^[3][10]

Where silt loam is found in the US

Silt loam is the single most common US surface texture, 33.0% of classified acres. One state skews that: Alaska holds 224.3 million of the class’s acres (about 36%), so across the contiguous 48 states silt loam is 24.7% of the classified acres, still the most of any class there.^[3]

Measured against each state’s own soil, silt loam covers 84% of Alaska’s classified acres and 72% of Kentucky’s, the two largest in-state shares, then Missouri at 68%.^[3]

Most widespread silt loam soil series

The most widespread is Gilpin (West Virginia), a well-drained upland soil formed in residuum from siltstone, shale and sandstone.^[11] Harney (Kansas) and Uly (Nebraska) come next; the table above links each series to its USDA description.

Related reading

Silt loam is at least half silt, the particle that makes up pure silt, and that silt fraction makes it erosion-prone: silt loam ranks just below silt on the erodibility ladder, which the silt page sets out. For how much nutrient it can hold, its cation exchange capacity sits in the 10–25 meq/100 g loam-and-silt-loam band on the CEC ladder.^[12][13]

Frequently asked questions

What is silt loam?

A smooth, silt-rich loam: 50% or more silt with 12 to 27% clay. It ties for the most water held of any texture class and has the most prime-farmland acres of any class (96.4 million).^[1][3]

Is “sandy silt loam” a soil type?

No. USDA has only twelve texture classes, and sandy silt loam is not one of them. Add enough sand and a silt loam becomes a loam; a truly sandy soil is a sandy loam.^[1]

What is silt loam good for?

Crops, above all: it ties for the most water held of any class, and 15% of its acres are prime farmland. Watch for acidity and surface crusting.^[3][5][8]