Web Soil Survey: how to read the soils on a parcel

What Web Soil Survey is

Web Soil Survey is the public website of the USDA Natural Resources Conservation Service that publishes the national soil survey as maps, tables, and ratings. It is free. It draws on the Soil Survey Geographic Database, the detailed soil data that NRCS keeps for each survey area. A survey area often matches a county.

How to use Web Soil Survey for a parcel

You use Web Soil Survey by drawing the parcel, reading the soil map, and opening the ratings. The steps run in this order.

1. Define the area of interest. Find the parcel by address or coordinates, then draw its boundary with the area of interest tool. Trace the parcel line from county GIS as closely as you can.
2. Read the soil map. The Soil Map tab divides the area into map units and lists the acres each one covers.
3. Open the Soil Data Explorer. Its sections for suitabilities and limitations, and for soil properties and qualities, rate each map unit.
4. Save the report. Add the maps and ratings you need to the free report, then save it as a PDF.

How to read a soil map unit

A soil map unit is a mapped area where one or more named soils dominate. Its name gives the soil series, a slope range, and sometimes erosion or flooding. The map unit description lists the main soils and the minor soils mixed in. Those minor soils matter. A map unit named for a well-drained soil can hold wet pockets the map does not draw.

The ratings that matter for land

The ratings that matter most for land are drainage, hydric soils, septic suitability, depth to rock, and shrink-swell. Each answers a different question.

• Drainage class. It runs from excessively drained to very poorly drained. Poorly drained soils point to a high water table.
• Hydric rating. Hydric soils formed under saturation. They are one of the three signs of a wetland, and a wetland delineation confirms whether one exists.
• Septic absorption fields. This rating grades soils as not limited, somewhat limited, or very limited, and it names the reasons.
• Depth to a restrictive layer. It shows where bedrock or a dense layer begins, within the depth the survey describes.
• Shrink-swell. The survey reports it as linear extensibility. High values mean clays swell when wet and shrink when dry, which cracks slabs and foundations.

When to use it

You use Web Soil Survey before an offer and before you order fieldwork. It shows which parts of a tract deserve a closer look and which professional to call first. It is one line on a land due diligence checklist, not the whole list.

What Web Soil Survey cannot tell you

Web Soil Survey cannot tell you what the soil is at a specific spot. The survey is drawn at a scale meant for broad planning, not for a single building pad. Map unit lines are approximate, and grading or fill may have changed the ground since mapping. The ratings describe typical soil properties, not a site investigation.

A licensed soil scientist answers septic questions. A geotechnical engineer answers questions about rock, bearing, and foundations. A wetland scientist answers wetland questions. See what desktop research cannot replace.

How FOUND reads it

FOUND reads the soil survey for each tract we review as a first warning. We record wet soils, hydric ratings, shallow rock, and shrink-swell clays, then name the fieldwork that settles each one. See what we check on flood, wetlands, and soils.

Questions

Is Web Soil Survey free?

Yes. NRCS publishes Web Soil Survey at no charge, and the reports it produces are free to download.

Can Web Soil Survey tell me if a lot will pass for septic?

No. It rates soils for septic use at survey level. A licensed soil scientist or county environmental health evaluates the lot itself.

How current is the soil survey?

Each report shows the version date of the survey area data. Check that date, and remember the land may have changed since the soils were mapped.