Soil Map vs. Perc Test: What You Actually Have to Hire

Two different things get called "the soil test," and confusing them costs people weeks. Here is what each one is, who is allowed to do it, and who pays.

The soil map

Prepared by a TDEC-Approved Soil Consultant — a private professional the state has vetted. Approval is not casual: it requires a soil science or agronomy degree (or fifteen semester hours of biological, physical, or earth science plus fifteen hours of soil science), two years of documented full-time soil evaluation experience, a written exam on the Tennessee Soils Handbook, a field mapping test requiring 80% to pass, and evaluation of five completed maps.

There are two map types:

• General soil map — a second-order survey using a "Percable / Non-percable" legend at 1:100 scale, 2,500 square foot minimum delineation. This decides whether a site even qualifies for percolation testing.
• High-intensity soil map — a first-order survey identifying individual soil series and phases at 1:100 scale, 625 square foot minimum delineation, with absorption rates and management notes.

The state maintains a public roster of active approved consultants. Several are based in Franklin and Thompson's Station.

The percolation test

A different job with a different rule. Under Rule 0400-48-01-.05, tests "shall be conducted by an engineer or surveyor licensed in the State of Tennessee." Approved soil consultants and registered professional environmentalists may also perform them, so long as they are not employed by a state or local environmental agency.

The procedure is prescribed, and the timing is why this cannot be rushed: holes six to twelve inches wide with vertical sides, two inches of coarse sand or gravel at the bottom, filled to at least twelve inches, presoaked for a minimum of four hours and preferably overnight, with measurements beginning twenty-four to thirty hours after initial filling. Only rates generated from the complete four-hour measurement period are valid for plat approval or permit issuance.

What it costs

Unpublished. Neither the state nor either county sets or publishes a price for this work — it is private professional services priced by the market. The University of Tennessee Soil, Plant and Pest Center confirms it does not perform perc testing or soil mapping and refers people to the approved consultant roster.

We would rather tell you we do not know than repeat a number we cannot source. Call two or three consultants off the state roster and compare.

Why any of this matters

Soil is not a formality here — it is the treatment system. A septic tank only settles and partially digests solids. The actual treatment happens when liquid effluent percolates out through the drain field and soil microbes finish the job. If the soil cannot absorb at a workable rate, the field does not function, and no amount of tank matters. That is what the soil work is measuring.