Silt Soil: What Silt Is and Where It's Found

What is silt?

Silt the particle is the middle size, finer than sand and coarser than clay. Silt the soil is the strictest of USDA’s twelve classes: 80% or more silt and less than 12% clay.^[1] Dry, it feels smooth, like flour, and it is not sticky the way clay is.

USDA splits silt itself into coarse silt (0.05 to 0.02 mm) and fine silt (0.02 to 0.002 mm); below that, clay runs from coarse clay (0.002 to 0.0002 mm) down to fine clay.^[1] The three separates together are explained on sand, silt and clay.

What is silt made of? For loess, the wind-laid silt of Iowa’s Loess Hills, USGS lists tightly packed grains of quartz, feldspar, mica and other minerals.^[4]

Where does silt come from?

One source is loess, in USGS’s words “a gritty, lightweight, porous material” deposited by the wind. Glaciers ground rock into “glacial flour,” meltwater carried enormous amounts of sediment down the Missouri River valley, and the wind piled it up. In Iowa’s Loess Hills the layers reach as much as 200 feet thick, matched only by Shaanxi, China. USGS calls loess the source of most of the nation’s rich agricultural soils.^[4]

Silt is also laid down by water, on floodplains and in river valleys. For those alluvial deposits, see alluvial soil.

Is silt soil good for plants?

Potentially very good, with a few catches. The Royal Horticultural Society calls silt soils fertile and fairly well drained but easily compacted.^[5] The survey agrees on the upside: silt ties for the most plant-available water of any class, a median 2.4 inches per foot, with a high 5.0% organic matter. The catches start with compaction, which the RHS names; a silty surface also seals and crusts, and silt erodes more easily than any other texture. 31.8% of silt acres are also poorly or very poorly drained. Working in organic matter is the remedy for crusting and lowers how easily the soil erodes.^{[3][6][7][8][9]}

Silt and erosion

Silt is the most erodible texture. By soil-erodibility (K), the order runs silt > silt loam = fine sand > loam > clay loam > clay > coarse sand: silts and fine sands are not cohesive and wash or blow away easily, whereas clays cohere and coarse sand is too heavy to move. Raising organic matter lowers the K value.^[9]

Loess shows this at scale: Iowa’s Loess Hills have one of the highest erosion rates in the country, about 40 tons per acre per year.^[4]

Where can I find silty soil?

True silt is rare: it is only 0.3% of classified US acres, rank 11 of 12. Alaska holds about 93% of it, so across the contiguous 48 states silt is under 0.1% of the classified acres. Most “silty” US ground is really silt loam (33.0% of acres against silt’s 0.3%); if your soil feels floury but is not pure silt, start on the silt loam page.^[3]

Most widespread silt soil series

Can I buy silt soil?

Rarely, and you would not want to. UConn puts silts on its avoid list for purchased topsoil because they are hard to work and drain poorly; the texture to buy is a sandy loam. What to look for is on the sandy loam page.^[10]

Frequently asked questions

What is silt soil?

A soil that is at least 80% silt and under 12% clay: smooth and flour-like, fertile and water-holding, but easily compacted and eroded.^[1][5][9]

Is silt the same as clay?

No. Silt particles (0.002–0.05 mm) are larger than clay (under 0.002 mm) and feel smooth rather than sticky.^[1][2]

Where is silt found?

Overwhelmingly in Alaska; in the lower 48 true silt is under 0.1% of soil, because most floury US ground is silt loam, not pure silt.^[3]

Can I buy silt soil?

Not readily. Silt is on UConn’s avoid list for purchased topsoil; buy a sandy loam instead.^[10]