Artificial turf on clay soil: how to tell if your lot is clay, and what the install does differently

01How do I look up my lot on Web Soil Survey?

Web Soil Survey is the federal soil map, run by the USDA’s Natural Resources Conservation Service (NRCS). Following NRCS’s getting-started steps:

1. Click Start WSS. In the Quick Navigation panel, open the Address form, enter your address and click View.
2. Draw an Area of Interest around your lot with the rectangle or polygon tool; no soil data shows until you do.
3. Open the Soil Map tab. It names the soils inside your outline, such as “Nunn clay loam, 0 to 1 percent slopes.”
4. Open Soil Data Explorer, then Soil Properties and Qualities. In the Soil Physical Properties folder, run Linear Extensibility, NRCS’s measure of shrink-swell.
5. In the Soil Qualities and Features folder, run Hydrologic Soil Group, which rates how fast water soaks in.

What the results mean, in NRCS’s own classes (NRCS technical guide; Soil Data Access):

Set the depth range deep enough. Linear Extensibility asks for a depth range in centimeters and averages across it, so a shallow range can hide the clay. In NRCS’s profile for Nunn clay loam, the top 15 cm (about 6 inches) rates 4.2, moderate; the layer under it, down to about 26 inches, is 38% clay and rates 6.8, high. Weld loam’s clay-rich layer starts at about 8 inches and rates 8.1. Strip the sod and some topsoil for a turf base, and the base can end up sitting on that layer.

NRCS says soil surveys serve general planning, and onsite investigation is needed for some engineering uses. And a lot graded for a new house may not match the map; CSU Extension notes that soils become compacted during home construction (CSU Extension). Builder grading is in turf on a new-build lot.

02What does the builder’s soils report tell me?

For a house built new in Colorado, the soils report is the document closest to your actual lot. Colorado requires the builder to give the buyer a summary of the soils analysis and the site recommendations at least 14 days before closing. Where the site has significant potential for expansive soil, the builder also owes each buyer a publication on those soils and how to care for the home (C.R.S. 6-6.5-101). The Colorado Geological Survey says its homeowner guide to swelling soils satisfies part of that disclosure and covers landscaping (CGS). This is general information, not legal advice.

Pull the summary out of your closing papers and look for what it says about:

• the soil the engineer found, and any swell test results;
• grading and drainage around the house: which way the ground must fall, and for how far;
• irrigation or planting next to the foundation;
• fill brought in during construction.

Those recommendations still apply once the lawn becomes turf; give a copy to anyone quoting the work.

03How can I tell clay by hand?

Take a trowel of soil from a few inches down for CSU Extension’s feel tests (CSU Extension):

• Feel. Wet it to the consistency of moist putty and rub it between your fingers. Clay feels sticky; sand feels gritty.
• Ball. Squeeze a moist ball. Clayey soil resists breaking. Sand breaks with slight pressure.
• Ribbon. Push the moist soil out between thumb and fingers, squeezing upward. A ribbon longer than 2 inches before it breaks means clayey soil.

CSU adds that as little as 20% clay makes a soil behave as clay. The Colorado Geological Survey’s warning signs of swelling clay can be spotted from the patio: a puffy, popcorn look to dry surface soil, soil that turns very sticky when wet, open cracks in dry ground, bare patches on heavy clay, and soil that is plastic when wet but rock-hard when dry (CGS).

04Which Northern Colorado towns sit on clay, and which on sand?

A lookup, not a verdict on your lot: each row is what NRCS soil surveys map in a sample around the town center (USDA-NRCS soil survey data), and subdivisions differ. The last column uses the classes above.

In Greeley, the city’s water department warns of hard clay a few inches down, under the sandy soils the map shows (City of Greeley).

05What does clay change about a turf install?

It moves with water. The Colorado Geological Survey says expansive clays can swell up to 20% by volume when wet, and that clay subsoils, usually fairly dry, swell once construction lets water in (CGS). Turf stays flat only while the ground under its base does, so the aim is steady moisture in the clay. Winter adds frost heave: what a Northern Colorado winter does to turf.

Water gets through the turf, then slows. Every Nunn and Weld soil mapped in the area is NRCS hydrologic group C, slow infiltration (Soil Data Access). So the clay has to be shaped to shed water sideways, to somewhere it can go, before any rock goes on it. Puddles and their fixes: what happens to artificial turf when it rains.

It compacts only at the right moisture. Ground must be at or near its optimum moisture content to reach full density, and ground left short of it keeps compressing and deforming after the job (Pavement Interactive). Clay is the fussy case: compaction-equipment maker Mikasa says it is plastic when wet, very hard when dry, and needs even moisture plus impact or pressure (Mikasa). Snowmelt and summer storms move that window; see when to install artificial turf.

It makes the ground by the house matter most. The Colorado Geological Survey lists poor surface drainage, and irrigation water soaking in next to the foundation, among the landscaping problems on expansive soils. The grade has to fall away from the house before the base goes down.

What to ask an installer about clay, and what a good answer sounds like:

• “How will you find out what’s under my lawn?” They dig and look, and they ask for your soils report if you have one.
• “Where does the water go?” They trace it through the turf and base, across the shaped clay, to a spot away from the foundation that can take it.
• “What if the clay is soaked or rock-hard that week?” They name the conditions they won’t compact in, and what they do instead.
• “What is the base, and how deep, on my soil?” Compare with what base goes under artificial turf.

06What changes if my yard is sandy?

Water stops being the main worry. Holding shape takes its place.

• It drains fast. Otero and Vona soils are hydrologic group A. Vona loamy sand’s surface layer passes water at 92 micrometers per second, NRCS’s “high” class and about 100 times the rate of Nunn’s clay subsoil.
• It compacts more forgivingly. Sand isn’t cohesive; it settles under shaking or vibration, and Mikasa says it reaches maximum density dry or saturated, so moisture matters far less than on clay.
• Its edges don’t hold themselves. Mikasa gives dry sand little or no cohesive strength, and in CSU’s ball test sand breaks with slight pressure. A cut edge in sandy soil won’t stand on its own, so the perimeter needs something firm that keeps the base in and gives the turf edge a hold: artificial turf edging options.
• Sandy on top isn’t always sandy below. Olney fine sandy loam, the largest soil in the samples around central Windsor and Greeley, has 15% clay near the surface but 26.5% clay from about 10 to 20 inches down, past the 20% at which CSU says a soil behaves as clay. Dig before you assume.

Turf that settles or dips later, on either soil: why artificial turf sinks.