Silty Clay Soil

What makes a soil silty clay

Silty clay needs two floors at once. The Soil Survey Manual defines it as 40% or more clay and 40% or more silt, so there is never more than 20% sand in it.^[1] On the triangle: clay ≥ 40 and silt ≥ 40.

Moist, it ribbons two inches or more and feels very smooth.^[2]

You may see it called “clay silt” or “silt clay.” Neither is a USDA name: the manual has twelve texture classes, and silty clay is the one for soil that is high in both clay and silt.^[1]

Silty clay vs silty clay loam and clay

The clay floor separates it from silty clay loam, and the silt floor separates it from clay. In our classifier, 15/45/40 sand/silt/clay is silty clay; shift one point from clay to silt (15/46/39) and it is silty clay loam; drop silt to 35% (20/35/45) and it is clay. Pure silt sits far off at the other end, with 80% or more silt.^[1]

Shrink-swell: the trait to plan around

Silty clay topsoils have a median linear extensibility of 7.5%, in USDA’s high shrink-swell class, second only to clay (9.2%).^[4] Linear extensibility measures how much the soil swells as it wets and shrinks as it dries. Before you build or pour, run the address through the expansive soil checker, which reads every layer, not just the surface.

In a trench, OSHA lists silty clay as an example of cohesive Type A soil, its most stable excavation class, once a competent person has confirmed the deposit’s strength and that it is not fissured or disturbed.^[5] The soil types hub covers the OSHA and engineering systems.

How silty clay handles water

Slowly. Saturated topsoil passes water at a median 0.92 µm/s, about 0.13 inches an hour, which USDA’s classes call moderately low.^[6] 70.5% of silty clay acres are in hydrologic group D, the group where water soaks in slowly and runoff is high when the soil is wet, a share topped only by clay.^[7]

It holds a median 1.8 inches of plant-available water per foot, and 28.6% of its acres are poorly or very poorly drained, against 53.6% well drained.^[8] Drainage changes its farm value: 12.5% is mapped prime farmland, and a larger 19.6% counts as prime only if drained, protected from flooding or irrigated.^[3][9]

Is silty clay fertile?

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

On USDA’s capability scale, 58% of rated silty clay acres are cultivable (Classes I–IV), and the limits it records split between erosion (subclass e, 41%) and wetness (w, 38%).^[11] pH leans alkaline: 45.1% of its acres are alkaline, 31.5% neutral and 23.5% acid.

For a garden, the clay rules apply. Utah State advises against tilling clay when it is wet, because it compacts, and names organic matter as the best amendment;^[12] the full method is on how to amend clay soil, and plants that tolerate it are on plants for clay soil.

Where silty clay is found

As a share of each state’s own classified soil, silty clay peaks in Hawaii (17.5%), followed by South Dakota (6.5%) and Arkansas (4.9%).^[3]

Most widespread silty clay soil series

Eckrant (Texas) has the most silty clay acres, ahead of Fargo (North Dakota). Sharkey also ranks among the top clay series, because a series can carry more than one surface texture across its map units.^[3]

Frequently asked questions

What is silty clay?

A USDA texture class with at least 40% clay and at least 40% silt: smooth, sticky, slow to drain and prone to shrink and swell.^[1][2][3][4]

Is clay silt a soil type?

Not in USDA’s system. A soil high in both clay and silt is silty clay; with less than 40% clay it is silty clay loam.^[1]

What is the difference between silty clay and clay?

Silt. Both have 40% or more clay, but silty clay also has 40% or more silt, while clay has less than 40%.^[1]

Does silty clay drain well?

Not quickly. Water moves through it at about 0.13 inches an hour, and 70.5% of its acres are in the slow-infiltrating hydrologic group D.^[3][7]

Where is silty clay found?

Texas, South Dakota and Montana have the most acres; as a share of a state’s soil it is highest in Hawaii.^[3]