Soil Survey Maps

The units delineated on a soil survey map depict land areas containing one or more soils. Map units are designed to reflect significant differences in use and management. The scale of the map controls the intensity and detail of the information. A typical county soil map at a scale of 1:24000 will have minimum size delineation of about 6 acres. This means different soil areas up to 6 acres in size cannot be shown. At a scale of 1:12000, areas as small as 1.5 acres can be placed on the map (see Table 1).

Map units placed on the aerial photograph encompass the continuum of soil conditions at the earth's surface but are named for the dominant soil or soils that are expected in that map unit area. Small soil areas below the scale limitation cannot be shown. In some complex soil patterns the proportion of included soils can be significant and contrasting. Clearly the scale will dictate how many of the contrasting soils can be delineated (Figure 1 & 2).

Map Unit Descriptions

Map unit descriptions provide a narrative overview of each soil map unit including how the unit fits on the landscape. Major use and management implications for the important soil components are reviewed. A vital part of map unit descriptions is the discussion of inclusions or the purity of the unit. This discussion highlights the included soils that could not be mapped due to scale or landscape complexity. The inclusions may be similar to the named soils or contrasting in properties. The review of included conditions is one of the most important but also most unused parts of a soil survey report (Figure 3).

Soil Series

Most soil maps are named for soil series or phases of soil series. Soil series represent the primary focus for organizing our knowledge of soil. North Carolina has nearly 475 series that are used in mapping. Each named series has an official description that defines its properties and its relationship to other soil series. Official series descriptions can be found at www.statlab.iasta te.edu/soils/osd. It is important to recognize that each series has a range in characteristics that can fall within its formal taxonomic classification.

The Goldsboro series has a range of texture in the Bt horizon that includes sandy clay loam, sandy loam, loam, or clay loam (Figure 4). In some of the lower Bt horizons it can be clay or sandy clay. Taxonomically the upper 20 inches of the Bt horizon should contain 18 - 35% clay size particles and less than 30% silt. The four textural classes listed above (sandy clay loam, sandy loam, loam, clay loam) fall in this range of clay.

Accurately recognizing a soil by series name certainly narrows the possible continuum of conditions. In most cases, however, the allowable range of characteristics within that series must be taken into account for a given land use interpretation. The allowable range of subsoil texture in a Goldsboro soil results in a range of estimated permeability (saturated hydraulic conductivity) of 0.6 - 2.0 inches per hour (Table 2). This is a wide range of water flow. It must also be understood that most of the specific physical and chemical properties that are quantified in a county soil survey report are estimates inferred from other morphological properties. Saturated hydraulic conductivity can be estimated from texture and bulk density (Figure 5). They are, however, excellent estimates when they are taken as a range and not as a specific value. The are intended to compare classes of soils and not as an indication of saturated hydraulic conductivity of a particular site.

County Level Soil Series

In each county soil survey report, a county specific soil profile description is provided for each soil (Figure 6). These descriptions represent conditions specific to that county and attempt to refine the total range of characteristics that would be found with the official series description. The Goldsboro series in Johnston County may not have the potential occurrence of clay in the lower Bt horizon that is allowed in the range of official characteristics for the series.

Using County Soil Survey Reports

As described in the previous sections soil data from this source must be understood and used appropriately. As resource data for large areas and for preliminary assessment and planning a soil survey report is valuable information. As the detail narrows to a particular one-acre lot, the information becomes a guide for specific on-site data. While the wide continuum of possible soil conditions is significantly refined by a review of the published information, it cannot and was not designed to accurately define a soil condition of a management unit the size of a drain field. A site specific morphological evaluation and interpretation is required.

The repetitive utilization of soil survey data in a given county coupled with the accumulation of site specific data or particular map units provides the basis for local refining of the composition of the map units. A sanitarian/soil scientist who has experienced many observations within given map units should use that data to actually refine the expectations for particular map units. This would improve the understanding and predictability of the map units for this particular land use interpretation.

Tool For Planning

Soil survey information provides a valuable tool for comparing soil conditions over broad areas. It can dramatically facilitate the planning and preparation for on-site investigation. Mapped information of this type can be effectively used to communicate the nature of soil differences across a county. In the context of general land use planning it provides an invaluable tool for basic and objective resource planning. When the decision making land unit becomes smaller than 4 or 5 acres, on-site investigation is clearly required. At the intensity of a single auger boring or a half-acre lot, caution must be raised on the use of this published information. On-site data is required when the focus is on a specific parcel of land.

Sources Of Information

A wide range of soil resource data can be found at the National Resources Conservation Services website (www.ncg.nrcs. usda.gov/soils.data.html).

This page created by Roland O. Coburn
Reasearch Tech I
on 4/19/00.