Septic soil in Clarke County, IA, by the numbers
Very limited
98.2% of Clarke County, IA's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. That reading means a conventional system is a poor bet here - 98.2% of Clarke County lands in the strictest "very limited" bucket USDA uses. Clarke County, IA is 431.111 square miles with a population of 9,524.
What Clarke County's reading doesn't cover
USDA's survey covers the whole of Clarke County, IA at once, not any single address. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Clarke County will actually do.
What a system would cost here
With 98.2% of Clarke 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.
How Clarke County, IA compares
Nationally, Clarke County, IA is the 222-most-constrained of 3,058 rated counties for septic soil. Within Iowa alone it ranks 19 of 99.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems