Silty Clay Loam Soil

What makes a soil silty clay loam

Silty clay loam is the low-sand twin of clay loam. The Soil Survey Manual defines it as 27 to under 40% clay and 20% or less sand, which means silt is always the largest part.^[1] On the triangle: 27 ≤ clay < 40 and sand ≤ 20.

It is the smooth one of the clay loams. Moist, it ribbons one to two inches and feels very smooth rather than gritty, because there is so little sand in it.^[2]

Silty clay loam vs silt loam, clay loam and silty clay

Three neighbors share its borders, and each is one percentage point away in our classifier:^[1]

• Silt loam has less clay. A 10/64/26 sand/silt/clay mix is silt loam; add one point of clay (10/63/27) and it is silty clay loam. See silt loam.
• Clay loam has more sand: 20/45/35 is silty clay loam, 21/44/35 is clay loam. See clay loam.
• Silty clay has more clay: 15/46/39 is silty clay loam, but at 15/45/40 it becomes silty clay. See silty clay.

Why so much silty clay loam is prime farmland

On USDA’s farmland and capability ratings it outscores every other texture. 24.2% of its acres are mapped prime farmland, the highest share of any class, and another 23.6% is prime if drained, protected from flooding or irrigated, also the highest.^[3][4] 71% of rated acres fall in capability Classes I–IV, more than any other class, and 35% in the top two.^[5]

Water and organic matter help. Silty clay loam topsoils tie with silt loam and silt for the most water per foot of any class. The median is 2.4 inches of plant-available water per foot, with 3.0% organic matter. The national picture is on the prime farmland map.^[3]

Silty clay loam drainage

The trouble is water. Only 45.6% of silty clay loam acres are well drained, and 27.9% are poorly or very poorly drained.^[6] Saturated topsoil passes water at a median 3 µm/s.

The hydrologic groups tell the same story: 37.2% is group C and 28.0% group D, and 28.4% carries a dual rating such as C/D, where the soil behaves like the first group only after the land is drained.^[7] Wet ground also explains the large “prime if drained” share, a farmland class that counts as prime only once the land is drained. Wetness (subclass w) is the limit USDA records on 35% of rated acres, second only to erosion (e, 47%). Fixes for a soggy yard are on how to improve soil drainage.

Is silty clay loam good for gardening?

Where it drains, yes. Its pH is often close to what vegetables want: 54% of its acres are slightly acid or neutral (pH 6.1 to 7.3 in USDA’s reaction classes),^[8] next to the 6 to 7 range Oregon State gives for vegetables.^[9]

It holds nutrients well, too. Purdue’s typical range for dark-colored silty clay loams and silty clays in Indiana is 30 to 40 meq/100 g of cation exchange capacity, the highest of its mineral-soil groups;^[10] the full ladder is on the clay page.

For a garden bed, treat it like the other clay-rich soils: compost worked into a new bed and a thin top-dressing on an established one, as compost for clay soil explains. Plants that cope without the work are on plants for clay soil.

Where silty clay loam is found

In Iowa, silty clay loam covers 36% of the state’s classified acres, the largest in-state share anywhere. Hawaii follows at 34%, then Kansas (21%) and Illinois (20%).^[3]

Most widespread silty clay loam soil series

Pewamo (Ohio) has the most acres, but Colo is the most widely mapped of the five, recorded in 192 survey areas.^[3]

Building and digging on silty clay loam

Its median shrink-swell (linear extensibility) is 4.5%, USDA’s moderate class.^[11] Check an address with the expansive soil checker.

In a trench, OSHA says silty clay loam can qualify as Type A in some cases, provided a competent person finds it strong, unfissured and undisturbed on site.^[12] The hub sets the three systems side by side on soil types.

Frequently asked questions

What is silty clay loam?

A USDA texture class with 27–40% clay and no more than 20% sand, so it is mostly silt and clay: smooth, sticky and slow to drain.^[1][2][3]

Is silty clay loam good for plants?

Very, where water can drain: 24.2% of it is prime farmland, and it ties for the most water held per foot of any class.^[3][4]

Does silty clay loam drain well?

Often not. 27.9% of its acres are poorly or very poorly drained, and 23.6% is prime farmland only once drained.^[3][6][4]

What is the difference between silty clay loam and silty clay?

Clay content. Silty clay loam stops below 40% clay; silty clay has 40% or more clay and 40% or more silt.^[1]

What is the difference between silty clay loam and clay loam?

Sand. Both have 27–40% clay, but silty clay loam has 20% or less sand and clay loam more than 20%.^[1]

Where is silty clay loam found?

Iowa, Kansas and Illinois have the most acres, and it covers 36% of Iowa’s classified soil.^[3]