How to read a soil suitability rating

The USDA has surveyed almost every acre in the country and published what's under it. Buyers rarely look — and then discover the foundation problem after closing.

The data comes from SSURGO, the USDA's national soil survey. Surveyors mapped the country into map units — areas sharing similar soil — and recorded dozens of measured properties for each. It's free, it's national, and it's one of the most useful public datasets almost nobody in real estate opens.

The limitation ratings — and why "very limited" isn't a no

The USDA rates each soil for specific uses. You'll see the same three-value vocabulary throughout:

This is the single most misread thing in the dataset. "Very limited" is an engineering statement about cost and design difficulty, not a legal restriction and not a verdict on buildability. Enormous amounts of American housing sit on very limited soils. It means budget for it and design for it — and if you're paying raw-land prices for a site that needs engineered foundations, that belongs in your negotiation.

Common ratings you'll see, each rated separately because they stress soil differently:

• Dwellings — houses, typically without basements
• Local roads and streets — driveways, access, subdivision roads
• Small commercial buildings — heavier loads, larger footprints
• Septic tank absorption fields — the one that decides whether an off-sewer lot works at all

The properties behind the ratings

Drainage class

A seven-step scale from excessively drained through well drained down to poorly drained and very poorly drained. It describes how long water sits.

Worth knowing: plain "poorly drained" is completely ordinary across large parts of the country — much of built-out Florida is exactly that. It's very poorly drained, especially combined with a high hydric percentage, that signals genuine wetland conditions.

Hydric percentage

How much of the map unit is hydric — soil formed under saturated, oxygen-poor conditions. Hydric soil is one of the three legal criteria for a jurisdictional wetland (along with wetland vegetation and hydrology). A high percentage is an early warning that a wetland delineation may be needed, and that federal or state permitting may enter the picture.

Depth to bedrock

Shallow bedrock means blasting or hammering for foundations, utilities and septic — one of the fastest ways to blow through a site-work budget. It also constrains septic systems, which need depth to function.

Seasonal high water table

The shallowest depth groundwater reaches during the wet season. This is the number that surprises people, because a site inspected in the dry season looks nothing like the same site in the wet season. A shallow seasonal water table drives wet crawlspaces, floating slabs, failed drainfields and standing yards.

Hydrologic soil group (A–D)

Runoff potential, used directly in stormwater engineering. A soils are sandy and absorb water readily; D soils are clayey and shed it. A D-group site generates far more runoff, which can mean a larger, costlier stormwater system to meet local requirements.

Corrosion of steel and concrete

Rated Low, Moderate or High. This is about the longevity of what you bury — pipes, footings, reinforcement, tanks, foundation steel. High corrosion potential points toward protective coatings, corrosion-resistant materials, or different pipe specifications. It rarely changes whether you build; it routinely changes what you build with, and it's cheap to address at design time and expensive to fix later.

Frost action potential

How prone the soil is to heaving as it freezes and thaws. High frost-action soils lift and crack slabs, footings, driveways and roads placed above the frost line. This is what sets minimum footing depths in cold climates.

Unified and AASHTO classification

Engineering shorthand for the material itself. Unified (USCS) codes like SP-SM or CH tell an engineer grain size and plasticity at a glance. AASHTO codes like A-3 or A-7-6 rate suitability as roadway subgrade, where lower numbers are better. Your geotechnical engineer will recognize these immediately even if they mean nothing to you.

Slope

Steeper ground means grading, retaining, erosion control and drainage design. LandBrief flags a dominant soil at 15% slope or more as terrain likely to add meaningful site cost.

What LandBrief shows you

A LandBrief report pulls the dominant soil at your coordinates and surfaces its name, drainage class, hydric percentage, hydrologic group, depth to bedrock, seasonal water table, slope, the dwelling / roads / commercial ratings, land-capability class, corrosion and frost-action ratings, and the Unified and AASHTO classes — plus a link to the official NRCS series description.

The USDA's own rating wording is passed through unchanged rather than reworded, so what you read is what the survey says. Where the soil name contains a recognizable material — muck or peat, marl, clay, sand, loam — the report adds a plain-English note on what that material tends to mean for building.

For parcels that don't appear to be already improved, LandBrief also derives an approximate septic suitability rating from water table, drainage class and flood frequency. It's derived by us, not issued by USDA, and a county health department permit is the only answer that counts.

What to do with this

• Building? Get a geotechnical report with actual borings. The soil survey tells you what to expect; borings tell you what's there.
• No sewer? Make the contract contingent on a passing percolation test and septic permit. This single item kills more rural land deals than anything else.
• Muck, peat or very poorly drained? Ask specifically about fill depth, surcharge time and engineered foundations before you price the project.
• High hydric percentage? Budget for a wetland delineation before assuming a buildable envelope.
• Visit in the wet season, or at least ask neighbors what the site does in heavy rain.
• Bring the numbers to your engineer. Handing over hydrologic group, water table and Unified class up front saves billable time.

Sources: USDA NRCS SSURGO / Soil Data Access · Web Soil Survey · Official Soil Series Descriptions.