Sandy Clay

What makes a soil sandy clay

Sandy clay is a clay with a lot of sand in it. USDA’s Soil Survey Manual defines it as 35% or more clay and more than 45% sand, which leaves under 20% for silt.^[1] On the triangle: clay ≥ 35 and sand > 45.

A moist sample ribbons two inches or more and still feels very gritty.^[3]

How rare is sandy clay?

Very. USDA’s survey records about 56,725 acres of it, fewer than any other texture class, spread over just 63 soil components. Silt loam covers roughly 11,100 times as much ground, and clay about 924 times as much.^[2]

With so few components, its averages are fragile: one survey area can move them. Read the medians below with their counts in mind.

What you get when you mix sand and clay

If you searched for “clay and sand,” you may be trying to turn one into the other. The classifier shows how touchy the boundary is: a 46/14/40 sand/silt/clay mix is sandy clay, one point less sand (45/15/40) is clay, and at 50/20/30 it is sandy clay loam.^[1]

Diluting a clay with pure sand never lands on loam. We ran every USDA clay composition on a 1% grid through 29,260 sand mixes, from 5% to 95% sand added, and none came out as loam: the sand dilutes the silt along with the clay, and loam needs at least 28% silt.^[1]

That matters in a garden. Utah State calls sand a poor amendment for clay, because in the wrong proportions the mix comes out too compact, and names organic matter as the best amendment for clay soils.^[4]

UC Master Gardeners call sand impractical for clay: it takes large quantities, and too little leaves the soil concrete-like.^[5] Rates and methods for organic matter are on how to amend clay soil.

Sand, loam and clay next to sandy clay

Colorado State University’s short version: sandy soils drain quickly and don’t hold water and nutrients well, while clay soils hold a lot of both but drain slowly.^[3] In the survey, sandy clay holds the same median water per foot as clay.

The three particles behind these numbers are on sand, silt and clay.

What the survey shows about sandy clay

Most of it drains well: 63.7% of its acres are well drained, and 19.6% are poorly or very poorly drained (7.0% very poorly). 13.9% carries a dual hydrologic group, C/D.^[6][7] pH leans alkaline: 58.8% of its acres are alkaline and 38.3% acid, and the largest single class is moderately alkaline, at 47.9%.^[8]

Little of it is farmland. Only 24% of rated acres fall in capability Classes I–IV (0% in Classes I–II), 37% is Class VII, and the limit USDA records most is erosion (subclass e, 42% of rated acres).^[9] 12.0% is prime farmland.^[2][10]

Where sandy clay is found

Arizona holds 27% of the class, South Carolina 23% and Texas 18%. Its two largest series, Gish (Arizona) and Denhawken (Texas), each come from a single survey area. At this scale one soil survey can put a state on the list, so treat the chart as a map of where sandy clay has been recorded, not of where it is common.^[2]

Most widespread sandy clay soil series

Sandy clay, clayey sand and construction

Engineers have a near-namesake: clayey sand, the Unified Soil Classification System’s SC group. That system sorts earth by sieve sizes and plasticity, not by USDA’s percentages, so an SC fill and a USDA sandy clay are not the same thing.^[11]

For trenches, OSHA names sandy clay among its examples of cohesive Type A soil, though a competent person still has to confirm the strength and condition of each deposit on site.^[12] Both systems are explained on the soil types hub.

Frequently asked questions

What is sandy clay?

A USDA texture class with 35% or more clay and more than 45% sand. It ribbons like a clay but feels gritty.^[1][3]

Can you mix sand into clay to make loam?

No. In our classifier, no mix of a USDA clay soil with pure sand comes out as loam. Sand in the wrong proportions makes clay more compact; organic matter is the amendment to use.^[1][4]

Is sandy clay the same as clayey sand?

No. Sandy clay is a USDA texture class; clayey sand (SC) is an engineering group in the Unified system, defined by sieve sizes and plasticity.

How much sandy clay is there in the US?

About 56,725 acres, from 63 soil components in USDA’s survey, the least of any texture class.^[2]

Where is sandy clay found?

In scattered patches: Arizona, South Carolina and Texas have the most recorded acres.^[2]