Clay Soil

What makes a soil clay

In USDA terms a soil is clay when it has 40% or more clay, 45% or less sand, and less than 40% silt.^[1] That is the single defining number: it is the clay-sized particles (under 0.002 mm), not the color or the stickiness, that put a soil in this class.

The Royal Horticultural Society’s gardening shorthand calls clays “heavy soils” and notes they are potentially fertile because they hold nutrients; the six gardening types it uses are set out on the soil types hub.^[5]

Is clay soil good or bad?

Both, and for the same reason. Clay holds a median 1.7 inches of water per foot and water creeps through it at just 0.22 µm/s, the slowest of any class, so it stays wet and short of air after rain. 87.7% of clay acres fall in hydrologic soil group D, the slowest-infiltrating group (another 2.7% are dual-rated), while only 52.9% are well drained and 22.2% poorly or very poorly drained. Its median linear extensibility is 9.2%, in USDA’s very-high shrink-swell class, so it swells when wet and cracks when dry.^[4][6][7][8]

The upside is fertility. Clay particles carry the negative charge that gives a soil its cation exchange capacity (CEC), the measure of how many nutrient cations it can hold, so clay holds nutrients that sand lets go. For all the drainage trouble, 48% of rated clay acres are cultivable (Classes I–IV) and 22% is prime farmland.^{[9][4][10][11]}

Why clay holds so many nutrients

Cation exchange capacity is a soil’s total negative charge, measured in milliequivalents per 100 grams, and it comes from the clay and organic matter present. The more of either, the more nutrients the soil can hold against leaching. Purdue’s typical ranges run from light sands near the bottom to clays and organic soils at the top.^[9]

Typical ranges, Purdue Extension AY-238; Indiana soil groups.

Clay soil colors

Color tells you about drainage and iron, not clay content. Organic matter darkens soil, so the more of it the blacker the surface. Goethite, a common iron oxide, colors soil brown or yellow; hematite colors it red. Where oxygen is shut out by waterlogging the iron is reduced and gray colors remain, as the dominant color or as mottles (flecks of one color in a background of another), and gray indicates poorer natural drainage.^[12]

So a clay can be red, gray, brown or black and still be clay. Red clay gets its color from iron oxides through the profile, and it is amended the same way as any other clay; the detail is on how to amend clay soil.^[12]

Putting topsoil over clay

Layering a different soil on clay rarely works the way people hope. Topsoil laid over compacted subsoil forms an interface that is nearly impervious, and amendments mixed in unevenly can create vertical and horizontal textural barriers that reduce how water and oxygen move, perching a water table above them. Counter-intuitively, adding a coarse layer such as sand or pebbles under finer soil decreases drainage rather than improving it.^[13]

The fix is to blend organic matter through the clay rather than cap it, and to raise beds where the site stays wet. Both are covered in how to amend clay soil and compost for clay soil.

Clay soil pH

Clay topsoils skew alkaline in USDA’s survey: 55.1% are alkaline, 26.8% neutral and 18.0% acid. pH is a separate property from texture, and it is the one worth testing before you plant or lime; see how to raise or lower soil pH.^[4][14]

Where clay soil is found in the US

Texas, South Dakota and Louisiana hold the most clay, in that order. Its most extensive series is Tarrant (Texas), a shallow soil over limestone bedrock^[15], followed by Sharkey (Louisiana).

Most widespread clay soil series

Clay and excavation safety

In OSHA’s excavation rules, clay is given as an example of a Type A soil (the most stable), but only once a competent person has confirmed its strength and that it is not fissured, previously disturbed or vibration-prone.^[16] That is a safety call made on site, not something map data can settle; the construction systems are set out on the soil types hub.

Frequently asked questions

Is clay soil good or bad?

Both. Clay holds water and nutrients well (high CEC, 22% is prime farmland) but drains slowly, runs short of air, and cracks and sets hard with the seasons.^{[9][4][11][2]}

What color is clay soil?

Any color. Red and yellow come from iron oxides, black from organic matter, and gray from waterlogging. Color signals drainage and iron, not clay content.^[12]

Should I put topsoil over my clay?

Not as a cap. A layer of different soil (or a coarse layer like sand) over clay perches water at the interface. Blend organic matter through the clay instead, or build a raised bed.^[13]

Where is clay soil found?

Most of all in Texas, then South Dakota and Louisiana.^[4]