A soil map is a detailed geographical inventory of soil resources within a specified area. These maps are the product of extensive soil survey efforts, classifying the ground based on its physical, chemical, and biological properties. The resulting visualization provides a baseline for understanding the landscape beneath our feet.
This information is valuable for property owners, agricultural producers, and construction professionals. By detailing the soil’s inherent characteristics, these documents help users assess the land’s suitability for various uses, such as farming, building, or environmental protection. A soil map translates complex data into a structured report that informs land stewardship and planning.
Accessing and Defining Your Area of Interest
The process of obtaining soil information typically begins with the USDA Natural Resources Conservation Service (NRCS) Web Soil Survey. This online platform is the authoritative source for soil data across the United States. Users locate their property using a street address, geographic coordinates, or by selecting a state and county.
Once the location is found, the user must define an Area of Interest (AOI). This delineation is performed using on-screen tools to draw a precise boundary around the parcel of land. The AOI acts as a filter, instructing the system to generate a customized report containing only the soil data specific to that boundary.
The Web Soil Survey limits the size of a single AOI to 100,000 acres, which still allows for fine-scale analysis on most properties. After the area is delineated, the system displays the “Soil Map” tab. This map shows the boundaries of the different soil types found within the defined AOI, providing the visual context needed to interpret the underlying data.
Decoding the Map Unit Symbols
The soil map divides the landscape into irregular polygons, with each shape representing a specific soil type or combination of types. Inside each polygon is a unique alphanumeric code known as the Map Unit Symbol (MUSYM). These short symbols, such as “GuC” or “HaB,” prevent the map from being cluttered with long names.
To interpret the symbol, the user must consult the map legend or the accompanying report. This key links the abbreviated symbol to the full Map Unit Name, such as “Gilpin-Upshur silt loams, 8 to 15 percent slopes.” The map unit is a classification of a geographic area for which a common management strategy is suitable.
The Map Unit Name provides information in a standardized format. It includes the name of the dominant soil series, which is classified based on the soil’s profile and properties. The name also contains descriptive phases, such as the soil texture (“silt loam”) and the range of the surface slope (“8 to 15 percent slopes”). The map unit is not a perfect representation, as every unit contains a major soil component along with minor components that were too small to map separately.
Interpreting Key Soil Attributes
The detailed report associated with each map unit symbol provides quantitative and qualitative information about several attributes that govern the soil’s behavior. One of the most fundamental attributes is texture, which is defined by the relative proportions of sand, silt, and clay particles. Sandy soils drain quickly but retain less moisture for plant use.
Conversely, clay soils hold water tightly and have a greater capacity for nutrient retention, but their slow drainage can lead to waterlogging. A loam soil is a balanced mixture of the three particle sizes, offering favorable water holding and drainage characteristics. Understanding this texture classification is foundational for determining irrigation needs and suitability for various crops.
Another attribute is the drainage class, which describes the frequency and duration of wet periods under natural conditions. Classes range from excessively drained, where water is removed rapidly, to very poorly drained, where the soil is wet at shallow depths for long periods. This classification is based on observations of the water table and the presence of redoximorphic features, which are colors resulting from the oxidation and reduction of iron compounds in response to wetness.
The depth to a restrictive layer is a measurement that indicates how far down a root or a foundation can effectively penetrate. A restrictive layer is a continuous layer, such as bedrock, a cemented hardpan, or a dense glacial till, that significantly impedes the movement of water, air, or roots. If the report notes a shallow restrictive layer, it directly implies limitations for deep-rooted plants and certain types of construction.
Finally, the slope is reported as a percentage range, which has direct implications for erosion and land use. A higher percentage slope indicates a greater risk of soil loss, especially in combination with fine-textured or silty soils. This factor is also used to determine the feasibility of surface irrigation and the need for conservation practices like contour plowing.
Using Soil Map Data for Land Management
Interpreted soil attributes translate directly into actionable land management decisions across various sectors.
For agriculture and gardening, the data informs choices about crop selection and soil amendments. For example, a soil map indicating high clay content suggests a need for careful tillage to prevent compaction and may suit crops that tolerate slower drainage.
Soil reports also provide ratings for engineering and construction applications. Suitability for building foundations, road construction, and septic systems is assessed based on factors like the depth to a restrictive layer and the drainage class. Soils with very slow permeability or a shallow water table are generally rated poorly for conventional septic tank absorption fields.
The data is also used in environmental planning and erosion control. Combining information on texture, which affects erodibility, with the percentage of slope allows managers to quantify the risk of soil erosion. This assessment guides the implementation of protective measures, such as establishing permanent vegetation or constructing terraces.