Your Comprehensive Guide To Greene County Soil Map Unit D99A (2026)

This guide focuses on the USDA NRCS Soil Map Unit D99A (Darroch silt loam) within Greene County, Indiana, and its implications for agricultural productivity, drainage engineering, and land development. For inquiries regarding local county drainage district infrastructure (such as Joint Drainage District 99A), please consult the Greene County Surveyor's Office.

Understanding the specific properties of land parcels is critical for agricultural yield optimization, civil engineering compliance, and environmental risk assessment. Within Greene County, the USDA Natural Resources Conservation Service (NRCS) designates specific map units to classify land tracts. One of the most critical, yet complex, units found in the region's outwash plains and till plains is Map Unit D99A.

Representing Darroch silt loam with 0 to 2 percent slopes, Map Unit D99A is a highly productive agricultural asset when properly managed, but it presents significant engineering challenges due to its seasonal high-water table and moderate permeability. This technical guide outlines the physical, hydrologic, agricultural, and engineering characteristics of Greene County D99A to assist developers, farmers, and civil engineers in making informed land-use decisions for the 2026 planning year.

Geotechnical Profile and Soil Taxonomy of D99A

The Darroch series, classified under the map symbol D99A, consists of very deep, somewhat poorly drained soils formed in loamy outwash or lacustrine deposits over till or outwash. These soils are primarily situated on outwash plains, till plains, and lake plains.

The taxonomic classification of Darroch silt loam is Fine-loamy, mixed, active, mesic Oxyaquic Argiudolls. This classification reveals key soil characteristics:

• Fine-loamy: Indicates a texture containing 18 to 35 percent clay and 15 percent or more fine sand or coarser particles, providing a balanced mix of water retention and drainage capacity.
• Mixed: Denotes a mineralogy containing various clay minerals without one dominating the profile.
• Active: Reflects a moderate cation-exchange capacity (CEC), indicating healthy nutrient retention capabilities.
• Mesic: Refers to the mean annual soil temperature regime (8 to 15 degrees Celsius / 46 to 59 degrees Fahrenheit), typical of the Midwestern United States.
• Oxyaquic Argiudolls: Indicates a mollisol (rich organic surface layer) with an argillic (clay-accumulation) subsoil horizon that experiences saturated conditions within 100 centimeters of the surface for significant periods during the year.

The typical soil profile of Greene County D99A is stratified into three primary horizons:

• Ap Horizon (0 to 9 inches): Very dark grayish brown silt loam; moderate medium granular structure; friable consistency; neutral pH; rich in organic matter.
• Bt Horizon (9 to 32 inches): Dark yellowish brown to brown clay loam and silt loam with common fine prominent yellowish brown iron accumulations and grayish brown depletions (redoximorphic features indicating seasonal wetness); moderate medium subangular blocky structure; firm consistency; slightly acid to neutral pH.
• C Horizon (32 to 60+ inches): Yellowish brown stratified loamy sand and gravelly loam; massive structure; loose consistency; neutral to slightly alkaline pH.

Hydrologic Properties and Drainage Requirements

Managing soil hydrologic dynamics is paramount when operating on D99A tracts. Under natural, undrained conditions, Darroch silt loam is classified as a somewhat poorly drained soil with a seasonal high-water table that rises to within 12 to 24 inches of the surface from January to April.

Important Hydrologic and Conservation Regulations for 2026

Wetland Inclusions and Swampbuster Compliance While Darroch silt loam itself is not classified as a hydric soil, it frequently features small depressions of highly hydric soils, such as the Rensselaer or Patton series. Landowners must secure a certified wetland determination from the USDA Natural Resources Conservation Service (NRCS) before modifying any subsurface drainage tiles to avoid non-compliance with Food Security Act wetland conservation provisions.

Saturated Hydraulic Conductivity (Ksat) The saturated hydraulic conductivity of the upper profile is moderate, ranging from 0.6 to 2.0 inches per hour. This moderately slow rate in the clay-rich Bt horizon necessitates structured subsurface drainage systems to pull water away from the crop root zone or structural foundations during heavy spring precipitation events.

To make D99A soils viable for intensive agricultural production or structural foundations, systematic tile drainage is required. For the 2026 crop year, drainage engineers recommend the following design parameters for subsurface tile installation in D99A tracts:

• Tile Depth: 3.0 to 3.5 feet below the surface.
• Tile Spacing: 40 to 55 feet apart, depending on the clay content of the local Bt horizon and the crop rotation program.
• Outlet Requirements: High-capacity gravity-fed outlets discharging into established county legal drains or constructed bioswales. Because of the level topography (0 to 2 percent slopes), surface drainage ditches should be constructed with flat-bottom designs and stabilized banks to prevent siltation.

Agricultural Productivity Metrics and Yield Benchmarks

When artificial subsurface drainage is successfully implemented, Greene County D99A represents some of the most productive farmland in the region. The thick, organic-rich Ap horizon provides an excellent seedbed with high water-holding capacity and superior nutrient exchange capabilities.

The table below outlines the physical parameters, productivity indexes, and projected 2026 crop yield benchmarks for D99A compared to adjacent Greene County soil map units under high-level management:

The high organic matter content of D99A (typically 2.5% to 4.0% in the topsoil) ensures excellent retention of applied fertilizers. However, due to seasonal wetness, nitrogen management requires careful timing. Agronomists in 2026 recommend split-nitrogen applications (pre-plant combined with side-dress) to minimize denitrification losses in these soils during wet spring cycles.

Engineering Suitability and Civil Infrastructure Limitations

Civil engineers and developers must approach Greene County D99A with caution. The physical properties that make this soil exceptional for agriculture present severe limitations for structural development.

Dwellings with Basements

Building structures with basements on D99A is highly discouraged. The seasonal high-water table (often within 12 inches of the surface during winter and spring) generates significant hydrostatic pressure against basement walls, leading to structural cracking, shifting, and water intrusion. If basements are built, they require heavy-duty continuous exterior foundation drains, heavy waterproofing membranes, and commercial-grade dual sump-pump systems with battery backups.

Dwellings Without Basements (Slab-on-Grade)

Building on slab-on-grade foundations is feasible but requires mitigation. The soil presents moderate shrink-swell potential due to clay accumulations in the subsoil. Developers should remove the organic-rich Ap horizon completely and backfill with compacted structural aggregate. Perimeter curtain drains should be installed around the foundation to lower the localized water table.

Septic Tank Absorption Fields

D99A presents severe limitations for conventional onsite wastewater treatment systems (septic systems). The combination of a high seasonal water table and moderately slow permeability in the subsoil prevents traditional gravity-fed leach fields from operating safely. Effluent is at high risk of backing up or discharging directly into local water tables without adequate soil filtration.

To overcome these constraints, developers must install alternative septic systems:

• Mound Systems: Elevated sand mounds raise the absorption field above the natural ground level, providing the required unsaturated soil depth for proper filtration.
• Perimeter Sand Filters: These require a subsurface curtain drain to artificially lower the water table surrounding the absorption field, discharging the clean water down-gradient.
• Aerobic Treatment Units (ATUs): Advanced treatment units that utilize oxygen injection to accelerate organic waste breakdown before effluent discharge.

How to Access and Interpret D99A Spatial Data via Greene County GIS

Landowners and professionals can easily locate and analyze D99A parcels using digital mapping tools. To view this data, utilize the following steps:

1. Access the Greene County GIS Portal: Navigate to the official Greene County, Indiana Beacon GIS page.
2. Locate Your Parcel: Search using your 18-digit parcel identification number, owner name, or address.
3. Activate Soil Map Layers: Open the "Layers" panel on the left side of the screen, expand the "Environmental" or "Natural Resources" tab, and check the box for "USDA Soils" or "Soil Map Units."
4. Identify D99A Boundaries: The screen will render green or brown boundary lines overlaying the satellite map. Look for the label "D99A" inside the parcel boundaries.
5. Access Web Soil Survey (WSS) for In-Depth Reports: For professional engineering reports, export the Shapefile (SHP) or Area of Interest (AOI) from the GIS portal and upload it to the USDA NRCS Web Soil Survey. This platform allows you to generate customized soil properties, engineering ratings, and soil health matrices for your specific project coordinates.

Frequently Asked Questions About Greene County Soil Map Unit D99A

Is Map Unit D99A classified as a wetland or hydric soil?

No, the Darroch silt loam series mapped under D99A is officially classified as a "somewhat poorly drained," non-hydric soil. However, due to its flat topography (0 to 2 percent slopes), it frequently contains small, depressed inclusions of fully hydric soils such as Rensselaer or Patton. These inclusions are subject to wetland regulations under state and federal law.

What are the main limitations of D99A soils for residential construction?

The primary limitations are the high seasonal water table, moderate risk of frost heave, and slow permeability. These properties restrict the use of basement foundations and conventional gravity-fed septic systems, requiring specialized foundation engineering, curtain drains, or mound-type wastewater systems.

Can I install standard agricultural tiling on D99A without a permit?

Under the USDA conservation compliance rules (Swampbuster), you can perform routine maintenance on pre-existing drainage systems. However, if you are installing new subsurface tile lines, extending existing lines, or draining a previously undrained wet area on a D99A plot, you must submit Form AD-1026 to your local USDA Service Center to obtain a certified wetland determination prior to starting excavation.

What are the best cover crops for improving D99A soil structure?

Because D99A is prone to compaction and spring wetness, deep-taproot cover crops such as tillage radishes or daikon radishes are highly effective at fracturing the clay-rich Bt horizon. Combining radishes with a winter-hardy grass like cereal rye helps stabilize the topsoil, scavenge leftover nitrogen, and accelerate soil drying in early spring.

Strategic Land Management Recommendations for 2026

To maximize the value of land tracts containing Map Unit D99A in 2026 and beyond, managers must adopt a balanced conservation and engineering strategy. For agricultural producers, shifting toward strip-tillage or no-tillage operations preserves the valuable organic matter in the Ap horizon, reducing the risk of crusting and preserving soil structure.

Controlled drainage systems, featuring inline water-table control structures, should be integrated into drainage networks. These structures allow operators to raise the outlet elevation after spring planting, holding water and dissolved nutrients in the field during dry summer cycles, and lowering the water table in autumn for harvest.

For municipal planners and developers, incorporating low-impact development (LID) standards on D99A soils is essential. Due to the high-water table, standard detention ponds can easily become saturated and lose capacity. Permeable pavements, dry retention ponds with engineered sub-bases, and bioretention cells designed to function with shallow groundwater tables are the preferred stormwater management tools for any projects breaking ground in 2026.

Contact the Greene County Soil and Water Conservation District (SWCD) or your local Purdue University Extension office to secure localized soil tests, drainage design approvals, and cost-share opportunities for conservation practices.