Dirt: Soil Data and Relationships – by Joe Homer, Soil Scientist

In NH, there are approximately 190 soil series and approximately 1,400 soil map units identified and mapped throughout the state. To correlate New Hampshire soil series into Bill Leak’s Forest Habitat Groups required a considerable amount of “lumping” to place 190 soil series into 20 or so of Leak’s groups. Common physical properties were used, primarily parent material, dominant soil texture, drainage class and depth to bedrock. These concepts have traditionally been used to group soils for the development of interpretations for a wide variety of uses. The user must recognize that some correlations from soil series to Forest Habitat Groups were straightforward while others required professional judgment based on experience having worked throughout NH, mapping soils, developing a variety of interpretations, collaborating with foresters, ecologists and other resource professionals.  

Original versions of the correlation between soil series and Forest Habitat Groups were done to facilitate collaborations between USDA-Natural Resources Conservation Service (NRCS) and the US Forest Service (USFS) in an effort to promote soil mapping in the White Mountain National Forest. During early discussions, it was recognized that the Forest Habitat Groups were developed and designed to work in the Bartlett Experimental Forest. It was acknowledged that with some “tweaking” the groupings could be used throughout the White Mountain National Forest. Further discussions acknowledged that with additional input and modification, done by experienced resource professionals, the groupings and concepts could be expanded outside of the WMNF. Correlations on a countywide basis were developed initially for Coös, then Grafton and Carroll counties. With interest in developing correlations for the remaining counties in NH, the original concepts using common physical properties, parent material, dominant soil texture, drainage class and depth to bedrock were used.

The close relationship between soil properties and vegetation in a natural, undisturbed (unmanaged) setting is well established. Going from the Massachusetts to Canadian border in NH, we see distinct changes in tree species and vegetation. The species we see growing on wet or moist compact glacial till (Forest Habitat Group 4) in Rockingham County along the Massachusetts border will often be different than those in Coös County along the Canadian border. As we move forward with field-testing and refinements of this data, it is critical that forestry professionals provide input to further fine tune forest types to soil groupings throughout NH. Further collaborations between soil scientists, foresters and wildlife biologists using this information will produce refined interpretations for the management of forest and wildlife species.

Soil Map Unit Concepts

During the mapping of soils on a countywide basis by the NRCS, map unit concepts that include slope, surface texture and stoniness are developed based on soil series. It is acknowledged and needs to be recognized that soil map units are not pure and do not contain 100% of the soil named in the map unit. In most NH soil map units, approximately 85% of the soil within any given delineation or polygon is the named soil or soils in the map unit name. A map unit description is written for each soil map unit, which describes inclusions within the map unit, names the inclusions, and identifies the properties and percentages of each inclusion. It is important for users of this information (foresters, wildlife biologists and landowners) to recognize that while looking at specific sites on the landscape, they may be on an inclusion within a map unit.

Kinds of Soil Map Units

On the landscape, soil areas differ in their size and shape, in degree of contrast with adjacent soils, and in geographic relationships. Four kinds of map units are used in NH countywide soil surveys to show these relationships: consociations, complexes, associations, and undifferentiated groups.

  • Consociations—Delineated areas are dominantly a single soil series. Generally, about 85% of the delineation is the soil series named for the map unit. Most of the remainder of the delineation consists of soil components so similar to the named soil that major interpretations are not affected significantly.
  • Complexes and associations—Consist of two or more dissimilar components occurring in a regularly repeating pattern. Complexes and associations are used in areas where the individual soil components cannot be mapped separately at a scale of mapping for the county. In both complexes and associations, the major components are sufficiently different in morphology or behavior that the map unit cannot be called a consociation. In each delineation of either a complex or an association, each major component is normally present, though their proportions may vary appreciably from one delineation to another. The total amount of inclusions in a map unit that are dissimilar to any of the major components does not exceed about 15 percent if limiting and 25 percent if non-limiting, and a single kind of dissimilar limiting inclusion generally does not exceed 10 percent if very contrasting.
  • Undifferentiated groups—Consist of two or more soil components that are not consistently associated geographically and, therefore, do not always occur together in the same map delineation. These soils are included as the same named map unit because use and management are the same or very similar for common uses. Generally, they are included together because some common feature such as steepness, stoniness, or flooding determines use and management. Every delineation has at least one of the major components and some may have all of them. The same principles regarding proportion of inclusions apply to undifferentiated groups as to consociations.

Soil Map Orders

In NH countywide NRCS soil surveys, two mapping Orders are used. Most of NH is mapped at the Order II level of intensity. Order II mapping is designed for general land use planning and includes a variety of interpretations for urban and agricultural uses (house lots, septic systems, roads, corn, hay, row crops, etc.) as well as forestry, recreation and wildlife. Some areas in NH (Coös, Grafton, Carroll and limited areas in Cheshire counties) are mapped at the Order III level of intensity in more remote areas where management for forestry, recreation and wildlife were the primary land uses and it was anticipated at the time of mapping that interpretations for urban and agricultural uses were not required. The introductory section of each NRCS county soil survey report provides information about mapping intensities within the county.

Soil Map Units – Minimum Delineation Size

At the start of an NRCS countywide soil survey, the project leader makes a decision on the minimum size delineation that will be applied throughout the county. In NH, this varies from county to county. The decision is based largely on intended uses for the mapping and interpretations, and the scale of imagery used during the mapping process. In Coös and Grafton counties, in areas mapped at the Order II level of intensity, the minimum size delineations are approximately 3 acres in size. Order III minimum delineations are approximately 40 acres in Coös County and 10 acres in Grafton. Throughout the rest of the state, minimum size delineations are generally 5 to 6 acres in size at the Order II level of mapping intensity.