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How to read a soil map

The USDA soil survey is the most useful layer most land buyers never open. It was drawn by soil scientists walking the county with augers, and it predicts drainage, septic suitability, bearing strength and farm value for every acre. LandCheck draws it, colours it as a stoplight, and reports the map unit under every click. Here is how to read what it says.

The map unit name

Each coloured area is a map unit, named for its dominant soil: Glenelg silt loam, 3 to 8 percent slopes. The first word is the soil series, a named recipe of layers found across a region. The texture word says what the surface is made of. The slope range tells you how steep the unit usually is. A unit can also be a complex of two soils too tangled to map apart, or a miscellaneous area such as rock outcrop, pits or urban land, which the ratings treat as unsuitable for almost everything.

The ratings that matter for land

Septic tank absorption fields	The one rating to read first off the sewer line. Not limited, somewhat limited and very limited, with the reason: slow percolation, wetness, depth to rock, flooding, slope. "Very limited" usually means an engineered system or none.
Drainage class	From excessively drained to very poorly drained. Poorly drained soils stay wet in spring, resist basements and septic fields, and often sit inside or next to wetlands.
Hydric soil	A soil that formed under saturation. Hydric map units are where regulators expect wetlands even where none are drawn yet; delineations start here.
Depth to water table and bedrock	Shallow water means wet basements and failed drain fields; shallow rock means blasting for foundations and trenches, and a septic field with nowhere to go.
Flooding and ponding frequency	Independent of FEMA. A soil rated for frequent flooding or ponding is telling you where water sits, whatever the flood map says.
Dwellings and small commercial buildings	Building-site ratings for basements, slabs and local roads. Shrink-swell clays, low strength and wetness are the usual limits.
Farmland classification	Prime farmland and farmland of statewide importance. Useful for agricultural value and tax programs, and some counties protect it from subdivision.

What the stoplight means

Green: the map unit is rated not limited or only slightly limited for septic fields and buildings; a conventional system is likely
Yellow: somewhat limited; expect a larger field, a mound or a raised bed, or a site-specific design
Red: very limited; wetness, rock, slope or slow percolation make a conventional system unlikely without engineering or a different spot
The colour follows the dominant soil in the unit; a unit can hide a better or worse inclusion covering a tenth of its area

What the survey cannot promise

Survey maps were drawn at about 1:24,000, so the smallest area shown is a few acres and lines are approximate on the ground. The ratings describe the typical soil in the unit, not the exact spot where the drain field would go. A county health department issues a septic permit from a perc test or a soil scientist's evaluation of pits dug on the site, and that result overrides the map in both directions: red units pass sometimes, green units fail. Use the survey to decide which parcels deserve the test, and to choose where on the parcel to dig.

How to check

Turn on the Soil layer and zoom to the parcel. The colours are the septic stoplight; the legend sits at the bottom of the map.
Click inside the likely building site, not the middle of the parcel. The Soil detail panel names the map unit, its drainage class, the septic rating and the reason for any limitation.
Click a second and third spot if the parcel crosses a colour boundary. Many lots have one good unit along the ridge and a wet unit along the drain; the house and the field go on the good one.
Cross-check with Wetlands and Contours. A hydric or poorly drained unit next to a mapped wetland and a flat contour pattern is a wet corner, whatever the listing says.
Call environmental health with the map unit name. They know which units pass in that county and what system the red ones end up with, and they will tell you what the perc test costs and who is licensed to run it.

Open the map with the soil layer on →

Soil data is read live from the USDA Soil Data Access service at the survey's published scale. It screens a parcel; the county's perc test or soil evaluation is the determination that counts for a permit.