Septic soil in Decatur County, IA, by the numbers
Very limited
Soil across 94.4% of Decatur County, IA rates "Very limited" in USDA's septic-absorption-field interpretation. That reading means a conventional system is a poor bet here - 94.4% of Decatur County lands in the strictest "very limited" bucket USDA uses. Decatur County, IA is 531.883 square miles with a population of 7,656.
What Decatur County's reading doesn't cover
Nothing about Decatur County, IA'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 Decatur County will actually do.
What a system would cost here
With 94.4% of Decatur 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 Decatur County, IA compares
Nationally, Decatur County, IA is the 738-most-constrained of 3,058 rated counties for septic soil. Within Iowa alone it ranks 53 of 99.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems