Soil series profile

Millboro soil series

Millboro is a well drained soil with a silty clay surface, mapped on 547,081 acres in 16 counties across 2 states. Hydrologic group D.

#486 by mapped area fine Typic Haplusterts (Vertisols)

Drainage class: Well drained
Hydrologic group: D
Typical topsoil pH: 7.0
Mapped area: 547k

Typical Millboro profile

Texture and pH by depth

Depth to scale. More strongly shaded layers hold more clay. Horizon names are the soil survey's (A = topsoil, B = subsoil, C = parent material).

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Typical Millboro horizons (representative values)
Depth	Texture	pH	Clay	Sand	Organic matter
0–7 in (Ap)	Silty clay	7.0	50%	4%	3%
7–19 in (Bt)	Clay	7.3	58%	4%	1%
19–36 in (Bkss)	Silty clay	7.9	57%	3%	0.5%
36–79 in (C)	Silty clay	8.0	59%	1%	0.25%

Representative values from the most extensive Millboro map unit (Millboro silty clay, 3 to 6 percent slopes, survey area SD085). Other map units of this series vary within the series range.

Stock photo for illustration, not a sample of this soil series. Credit on the Sources & method page.

What this means

What Millboro soil means for gardens, lawns and septic systems

For gardens

Typical topsoil in Millboro soils is neutral: pH 7.0, the range most vegetables, flowers and lawn grasses prefer.

Clayey topsoil covers 90% of mapped Millboro acres. Clay holds water and nutrients but drains slowly and compacts if worked wet: add compost, avoid tilling soggy soil and consider raised beds. The most common surface texture is silty clay (90% of mapped Millboro acres).

100% of mapped Millboro acres is moderately well to excessively drained, which suits most garden plants.

For lawns and yards

On 100% of mapped acres, rain soaks in slowly (hydrologic groups C and D, including dual groups that are D until drained). Expect runoff and puddles after heavy rain: core-aerate compacted lawns and keep ground sloping away from foundations.

For a septic system

For conventional septic absorption fields, NRCS rates 100% of the acres in mapped Millboro soils very limited, 0% somewhat limited and 0% not limited.

Where the soil is very limited (slow permeability, a high water table, flooding, shallow bedrock or steep slopes), alternative systems such as mound or sand-filter designs are common and cost more.

Only a licensed site evaluation or perc test at the actual lot decides whether a system is allowed and which design.

Generated from the mapped numbers above using fixed rules, not a site visit. See how these notes are written.

Where Millboro soil is found

Mapped acres

Millboro is mapped in South Dakota and Nebraska.

Counties with the most mapped Millboro soil
County	Acres	Share of county land
Lyman County, SD	274,977	26.4%
Tripp County, SD	178,774	17.3%
Gregory County, SD	34,565	5.3%
Mellette County, SD	23,923	2.9%
Jones County, SD	11,854	1.9%
Todd County, SD	10,592	1.2%
Stanley County, SD	7,108	0.8%
Brule County, SD	1,474	0.2%
Buffalo County, SD	1,474	0.2%
Jackson County, SD	1,019	0.1%
Boyd County, NE	880	0.3%
Ziebach County, SD	165	0.0%
Hughes County, SD	132	0.0%
Dewey County, SD	94	0.0%
Haakon County, SD	33	0.0%
Keya Paha County, NE	17	0.0%

Septic absorption fields

NRCS rating, share of Millboro acres

Not limited 0%

Somewhat limited 0%

Very limited 100%

Septic tank absorption field rating across mapped Millboro acres. Scale 0 to 100% of mapped land.

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Septic tank absorption field rating across mapped Millboro acres.
Category	Share of acres
Not limited	0.0%
Somewhat limited	0.0%
Very limited	100.0%

Drainage, water table and flooding

Share of Millboro acres

Depth to seasonal high water table: None reported within about 6.5 feet (median where reported).
Flooding: Not mapped as flooding on most acres.
Slope: Typically 2–8% (median 4%).
Topsoil organic matter: Median 3%.

Drainage class

Excessively drained 0%

Somewhat excessively drained 0%

Well drained 100%

Moderately well drained 0%

Somewhat poorly drained 0%

Poorly drained 0%

Very poorly drained 0%

Drainage class across mapped Millboro acres. Scale 0 to 100% of mapped land.

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Drainage class across mapped Millboro acres.
Category	Share of acres
Excessively drained	0.0%
Somewhat excessively drained	0.0%
Well drained	100.0%
Moderately well drained	0.0%
Somewhat poorly drained	0.0%
Poorly drained	0.0%
Very poorly drained	0.0%

Hydrologic soil group

A: fast infiltration (mostly sandy or gravelly) 0%

B: moderate infiltration 0%

C: slow infiltration <1%

D: very slow infiltration (clayey, shallow or wet) >99%

A/D: D as is, A if drained 0%

B/D: D as is, B if drained 0%

C/D: D as is, C if drained 0%

Hydrologic soil group across mapped Millboro acres. Scale 0 to 100% of mapped land.

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Hydrologic soil group across mapped Millboro acres.
Category	Share of acres
A: fast infiltration (mostly sandy or gravelly)	0.0%
B: moderate infiltration	0.0%
C: slow infiltration	0.1%
D: very slow infiltration (clayey, shallow or wet)	99.9%
A/D: D as is, A if drained	0.0%
B/D: D as is, B if drained	0.0%
C/D: D as is, C if drained	0.0%

Sources & data dates

Source: USDA NRCS Soil Survey Geographic Database (SSURGO), queried through Soil Data Access. Public domain.
Data date: SSURGO as published on November 3, 2026 (most recent survey area update October 26, 2026). NRCS refreshes SSURGO each October.
Scope: All map unit components named Millboro (component kind "Series") in SSURGO: 547,081 acres.
Method: Shares are by mapped acres of Millboro components. The profile comes from the single largest Millboro component. Full methodology.

Independent reference site. No government affiliation or endorsement.

Full narrative description: USDA Official Soil Series Description for Millboro.