What Polk County, TN's soil rates for a septic system
Very limited
In Polk County, TN, 87.7% of the surveyed land carries a "Very limited" rating for a standard septic drain field. Practically, that rules out a simple gravity system for most of the county - 87.7% of it sits in USDA's most-constrained bucket. Polk County, TN is 434.605 square miles with a population of 18,338.
What Polk County's reading doesn't cover
Nothing about Polk County, TN's SSURGO figure applies to a specific address - it's a county-wide average. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Polk County will actually do.
What a system would cost here
With 87.7% of Polk County rating "Very limited", a standard gravity system is not a safe assumption here. The engineered-system range, $10,000-$40,000 (mound or aerobic treatment), is the more realistic number to plan around, against $3,000-$8,000 for a conventional system on the rare lot that passes.
Where this sits nationally
Nationally, Polk County, TN is the 1,492-most-constrained of 3,058 rated counties for septic soil. Within Tennessee alone it ranks 27 of 95.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems