A soil map divides a landscape into colored or labeled zones, each representing a different soil type with distinct properties like drainage, texture, and depth to bedrock. Most soil maps you’ll encounter in the U.S. come from the USDA’s Web Soil Survey, which covers nearly every county in the country. Learning to read one takes about 15 minutes once you understand what the labels, colors, and tables are telling you.
Finding Your Soil Map Online
The USDA’s Web Soil Survey (websoilsurvey.sc.egov.usda.gov) is the primary tool for viewing soil maps in the United States. It’s free and runs in your browser. The interface follows four tabs that walk you through the process in order.
Start on the Area of Interest tab. You can zoom into your location on the map or type an address into the Quick Navigation menu. Once you’ve found your spot, use the drawing tool to outline the area you want to examine. This is called your Area of Interest, or AOI. You need to complete this step before any soil data becomes available.
Next, click the Soil Map tab. This generates a color-coded map of all the soil types within your outlined area, along with a legend and brief descriptions of each soil. The Soil Data Explorer tab lets you dig deeper, filtering the data by specific uses like building foundations, septic systems, farming, or drainage. Finally, the Shopping Cart tab lets you download or print a custom report of everything you’ve selected.
If you want something quicker, the SoilWeb app from UC Davis (available for iOS and Android) uses your phone’s GPS to pull up soil information for wherever you’re standing. It draws from the same NRCS dataset but skips the steps of drawing an area on a map.
What the Map Units Mean
Each colored polygon on a soil map represents a “map unit,” which is a defined area where the soil shares similar characteristics. Every map unit gets a label, typically a short code like “BaB” or “CeC2.” These codes pack a lot of information into a few characters.
The letters at the beginning refer to the soil series name. A soil series is the most specific level of soil classification, like a species name in biology. For example, “Ba” might stand for “Bango” or “Bayboro.” The letter after the series abbreviation indicates slope class: A is nearly flat (0 to 2 percent), B is gently sloping (2 to 6 percent), C is moderately sloping (6 to 12 percent), and so on up through steeper grades. A trailing number like “2” indicates the degree of erosion, with 2 meaning moderately eroded.
So “CeC2” tells you: this is the “Ce” soil series, on a moderate slope, with moderate erosion. When you click on that map unit in the Web Soil Survey, you’ll get a full description including the soil’s texture, how well it drains, how deep it goes before hitting rock or a water table, and what it formed from.
Reading the Soil Description
Each map unit comes with a written description that tells you what the soil actually looks and behaves like. The most important properties to look for depend on why you’re checking, but a few show up in almost every description.
Texture describes the mix of sand, silt, and clay particles. A “sandy loam” is coarse and drains fast. A “silty clay loam” holds water longer and can get sticky when wet. Texture affects everything from how well a garden grows to whether a basement stays dry. The USDA classifies soil into named texture classes based on the percentage of each particle size, and these names appear directly in the map unit descriptions.
Drainage class tells you how quickly water moves through the soil. Terms range from “excessively drained” (water disappears fast, common in sandy soils) to “very poorly drained” (water sits at or near the surface for long periods). If you’re evaluating land for a septic system or a building site, drainage class is one of the first things to check.
Depth to restrictive layer indicates how far down you’ll hit bedrock, hardpan, or another layer that roots and water can’t easily penetrate. A soil described as “shallow” might have rock within 20 inches of the surface, which limits what you can plant and what you can build.
Hydrologic soil group (A through D) rates how much rainfall runs off versus soaking in. Group A soils absorb the most water. Group D soils shed the most, which matters for flood risk and stormwater planning.
Using the Soil Data Explorer
The raw soil map tells you what’s in the ground. The Soil Data Explorer tab in Web Soil Survey tells you what that means for specific uses. This is where the tool becomes genuinely practical.
The Explorer organizes interpretations into categories. Under “Suitabilities and Limitations for Use,” you can check ratings for things like dwellings with basements, roads and streets, septic tank absorption fields, and various crop types. The system rates each soil as “not limited,” “somewhat limited,” or “very limited” for that use, and explains what specific property creates the limitation.
Under “Soil Properties and Qualities,” you can pull up detailed physical and chemical data: available water capacity (how much moisture the soil can store for plants), organic matter content, pH, permeability rates, and shrink-swell potential (how much the soil expands and contracts with moisture changes, which can crack foundations).
For agricultural purposes, the “Land Classification” section assigns capability classes from I (best farmland, few limitations) through VIII (unsuitable for cultivation). Most productive cropland in the U.S. falls in classes I through III.
Understanding Map Scale and Accuracy
Soil maps are not precise to the square foot. Most county soil surveys in the U.S. were mapped at a scale of 1:20,000, meaning one centimeter on the map equals 200 meters on the ground. At that resolution, the smallest area that gets its own polygon is roughly one to three acres. Anything smaller than that gets lumped into whatever surrounds it.
This has real consequences. If your property is a half-acre lot, the soil map might show one soil type across the entire thing, but the actual soil could vary significantly from one corner to another. The boundaries between map units are approximate lines, not surveyed edges. In reality, soils blend gradually from one type to another rather than changing abruptly at a drawn line.
Higher-detail maps do exist for some areas. First-order surveys at 1:12,000 scale provide finer resolution and are sometimes available for research stations, specific project areas, or urban zones. At the other end, regional planning maps at scales of 1:100,000 or smaller are only useful for broad generalizations about soil patterns across large areas.
For any decision with significant financial stakes, like buying land, placing a building, or installing a septic system, the soil map is a starting point. On-site testing by a soil scientist or engineer confirms what the map suggests.
Reading a Paper Soil Survey
Before everything moved online, soil data came in large printed books published county by county. Many landowners, libraries, and county offices still have these, and they contain the same core information as the Web Soil Survey in a different format.
A paper soil survey has three main sections. The front matter describes the county’s climate, geology, and general soil patterns. The middle contains aerial photo sheets with soil boundaries drawn on them, each polygon labeled with the same kind of map unit codes described above. The back section has detailed tables listing every property for every soil in the county: texture by depth, permeability, pH, erosion potential, suitability ratings, and more.
To use a paper survey, find your property on the aerial photo sheets (a table of contents matches sheet numbers to geographic areas), identify the map unit labels overlapping your land, then look those labels up in the description and table sections. The process is slower than the digital version but the data is identical, though the online version may reflect more recent updates through the NRCS annual soils refresh cycle.
Common Color Schemes on Soil Maps
Colors on soil maps don’t follow a single universal standard, so always check the legend. In the Web Soil Survey, each map unit gets assigned a distinct color simply to make adjacent units visually distinguishable. The color itself doesn’t encode a property.
Thematic maps are different. When you generate an interpretive map through the Soil Data Explorer (say, a map of drainage class or farmland classification), the colors do carry meaning. Greens typically indicate favorable conditions, yellows are moderate, and reds flag limitations. These maps include a color legend that spells out exactly what each shade represents. If you’re printing or sharing a soil map, always include the legend, because the colors are meaningless without it.