How do I check what the soil on a parcel will support?
Read the soil map units that fall inside the parcel from the USDA soil survey, then read what each unit is rated for — agricultural capability, drainage, depth to water table, and the interpretations for building and septic. It will tell you what to expect and what to test for. It will not tell you what is under any particular corner of the lot.
The mistake almost everyone makes
Treating a map unit as a description of your building site. Soil surveys are mapped at scales from about 1:12,000 to 1:63,360, and most of what you will read has been recompiled onto a 1:24,000 base — where the smallest area that can legibly be delineated is about 5.7 acres, and where the survey manual notes that in practice delineations run larger than that. Every map unit is defined as a mix, and it is normal and expected for it to contain inclusions of quite different soils too small to map. On a one-acre lot, and on a five-acre one, the survey is telling you about the neighbourhood rather than about your building envelope.
How to check it
Pull the map units inside the parcel
Expect several. Note the proportion of the parcel each covers, and treat unit boundaries as generalised rather than surveyed lines.
Read the interpretations, not just the name
The survey publishes ratings for the things people actually ask about: whether a unit is prime farmland, its capability class, depth to the seasonal high water table, shrink-swell behaviour, and limitations for dwellings and septic absorption fields. The rating is the useful part; the unit name is a label for it.
Let the survey tell you what to test for
A high water table rating tells you a perc test may be marginal in wet months. A high shrink-swell rating tells you a foundation design question exists. That is the survey working as intended — as a guide to what site investigation should look at, and when in the year to look.
Notice when nothing has been mapped
Some areas carry no published survey at all — they appear as units coded NOTCOM (not completed), as areas that were never surveyed, or simply as a hole in the data. That is the soils equivalent of an unstudied flood area: it means nobody has mapped here, not that there is nothing here to find. Treat it as a question rather than an answer.
Confirm on site before you rely on it
A septic permit rests on a percolation or soil evaluation test at the proposed field location, conducted under the local health authority's rules. A foundation rests on a geotechnical investigation. Both are site-specific work the survey is explicitly not a substitute for.
What this still does not tell you
- It is not a site-specific soils investigation and does not replace a perc test or a geotechnical boring.
- Map unit boundaries are generalised; a boundary on the map is not a surveyed line on the ground.
- Inclusions of dissimilar soils too small to map are normal within every unit.
- Ratings describe expected limitations, not permit outcomes — the local health authority decides those.
- It is not a substitute for a site investigation, and it is not engineering or legal advice.
Who actually decides
- Your county or state environmental health department — decides septic suitability and issues the permit. Which office, and which test it accepts, varies by jurisdiction.
- A licensed or state-registered soil scientist, and for foundations a geotechnical engineer — the people whose site work the survey is a guide to, not a replacement for.
- USDA NRCS — Web Soil Survey (the data, not the decision)
- USDA NRCS — soil survey manual and interpretations (the method, not the decision)