Clarke County Soil Data
Clarke County, Iowa
USDA SSURGO / PHZMAvg pH
6.2
Texture
Silty clay loam
Drainage
Poorly drained
Organic Matter
3.0%
Hydrologic Group
D
Data Story
About Soil in Clarke County, Iowa
Acidic Alfisols of the South
This county is dominated by Alfisols with an average pH of 6.24, which is notably more acidic than the national median of 6.5. These soils often develop under hardwood forests and provide a sturdy foundation for diverse plant life.
Dense Silty Clay Loams
A silty clay loam texture prevails, driven by 49.0% silt and 28.1% clay. This dense mix provides excellent nutrient storage capacity, but it can be heavy and difficult to till when the ground is wet.
High Moisture Capacity
The organic matter content of 2.99% falls below the Iowa average of 3.81%. Despite this, the available water capacity of 0.197 in/in is slightly better than the state benchmark, ensuring a steady moisture supply for crops.
Challenges with Slow Drainage
The soil is classified as poorly drained and belongs to hydrologic group D. These soils hold onto water for long periods and have high runoff potential, requiring careful planning for garden drainage and building foundations.
A Longer Season in Zone 5b
Situated in hardiness zone 5b, Clarke County offers a slightly longer growing season than northern Iowa. The heavy soil is particularly well-suited for moisture-loving plants like squash and pumpkins.
Soil Composition
Soil pH
Soil Series
Detailed soil series data not yet available for this county.
Hydrologic Group
Group D — High Runoff
Clay soils with very slow infiltration rates. Highest runoff potential. These soils have a permanent high water table or are shallow over impervious material.
Drainage Class
Poorly drained
Water is removed so slowly that the soil is wet at or near the surface much of the year. Drainage improvement usually needed.
Soil Profile
Dominant texture: Silty clay loam
Typical soil profile. Actual depths vary by location.
USDA Soil Orders
Clay Soil Detected
Clay soils can struggle with drainage. These products improve soil structure and water flow.
Browse Soil AmendmentsFrequently Asked Questions
What type of soil does Clarke County have?
Clarke County's soil is classified as Silty clay loam. This indicates the relative proportions of sand, silt, and clay — 23% sand, 49% silt, and 28% clay. Loamy soils hold moisture well and support a wide range of plants.
Is Clarke County's soil acidic or alkaline?
Clarke County's soil has a pH of 6.2, which is moderately acidic. This range suits many crops and is common across much of the eastern US.
What is the drainage class for Clarke County?
Clarke County has a dominant drainage class of "Poorly drained." Poorly drained soils retain water for extended periods, which can limit crop choices and may require tile drainage or raised beds.
What crops or plants grow well in Clarke County's soil?
Based on Clarke County's Silty clay loam texture, pH of 6.2, and Poorly drained, the soil is well suited for vegetables, grains, and most field crops, row crops and vegetable gardens, rice, cranberries, and other water-tolerant species.
What USDA soil order is found in Clarke County?
Clarke County's dominant soil order is Alfisols. Alfisols are moderate to high base-status soils with a clay-enriched subsoil. They support productive forests and cropland across temperate regions.
Soil & Planting Advisory: Clarke County
Soil Verdict
Clarke County is in USDA hardiness zone 5b, a cold climate zone. where winter lows can reach -15°F. Freeze-thaw cycles are moderate; soil biology slows in winter but recovers reliably in spring, supporting a broad range of plants.
Planting Guidance
A wide plant palette is possible: deciduous trees, most shrubs, cool-season vegetables, and many perennials thrive here. Winter protection (mulch, burlap wraps) helps push the zone boundary for desired species. Always match plant selections to the specific microclimate of your property — low spots, slopes, and wind exposure can shift effective hardiness by half a zone.
Soil Implications
Moderate freeze-thaw cycles naturally aerate the soil but can also cause compaction in clay-heavy areas; annual core aeration is beneficial. Organic matter amendments decompose at a moderate pace — aim for 3-5% organic content for optimal fertility. When soil temperatures drop below freezing, microbial nitrogen cycling stops; spring fertilization is more effective than late-fall application.
Regional Context
Clarke County closely matches the Iowa average hardiness conditions.
Planning your lawn? View lawn care data for Clarke County on LawnByCounty.com →
Other Counties in Iowa
Soil data sourced from USDA SSURGO via Soil Data Access. Data is informational only and should not be used as a substitute for on-site soil testing. Contact your local NRCS office for site-specific soil information.