Skip Navigation

Soil Survey Technical Note 4

Populating Map Unit Data: Taxonomic classes and map unit components


The purpose of this technical note is to increase awareness of current policy regarding populating data for map unit components in NASIS and to provide some background information and rationale for this policy.


The contact for this technical note is the National Leader for Soil Classification and Standards, National Soil Survey Center, Lincoln, NE.


Current policy, procedures, and database tools allow soil scientists to record soil properties, describe map unit composition, and provide interpretations with fewer constraints than in the past. One example is our policy allowing ranges of map unit component data to "extend beyond the established limits of the taxon from which the component gets its name" (NSSH 618.03).


Today we readily accept that a soil series is the lowest level of Soil Taxonomy and its properties can not extend beyond the limits of the family to which it belongs. This was not always so easily accepted. The 1965 NCSS conference proceedings includes a discussion of "guidelines for allowable tolerances in the stretching of family class limits by series class limits." The debate was whether the range in characteristics for a series must be within the limits of the family, or alternatively, should only the typical pedon itself have to fit within the family while its range could extend beyond the family limits. The first alternative was agreed to.

The decision to restrict series ranges to family limits has presented some difficulty ever since. We know that natural soil bodies commonly have ranges in properties that straddle one or more of the rigid taxonomic class boundaries. Over the years we have used devices such as taxadjuncts, variants, and "similar soils" to reconcile observations from natural bodies of soils with taxonomic limits. These concepts are awkward and not well understood beyond the soil survey community. In 1977 Dr. Marlin Cline wrote "At the lowest level of the system, we will have to acknowledge the differences between taxonomic soil series and mapping units that bear the same name and will probably have to rectify the confusion this causes. It is conceivable that soil families could become the lowest category of taxonomy, but some ingenious person may find a better solution."

Taxonomic Classes and Map Unit Components

The distinction between taxonomic classes and components of map units needs to be understood. We do not map taxonomic classes. We use conceptual landscape models to map natural bodies of soils. We then use our taxonomy to classify and name the soils we have mapped.

Taxonomic classes are defined by limits of key diagnostic properties. Their purpose is to group soils, which have formed from similar genetic processes and which respond similarly to use and management. The higher categories in our taxonomy reflect our global understanding of soils and are not specific to any one particular landscape. Taxonomic classes are useful because they provide the vehicle for organizing our knowledge of soil science and facilitating the transfer of information and technology from one location to another.

Components of map units (other than miscellaneous land types) describe the properties of natural bodies of soils in a particular landscape. They are correlated to an appropriate taxonomic class, which is then used as part of the map unit name.

Determining ranges for map unit component data should rely on two sources of information. First, the taxonomic class that the component is correlated to carries within its definition a wealth of information useful to populating the database. For example, even without collecting multiple samples for organic matter determination or installing piezometers to monitor water table levels, we are able to make reasonable estimates for these properties if we have correlated a map unit component as an Aquoll. Second, field and laboratory observations obtained within the specific landscape are used to confirm and further refine the estimated ranges.

How Have Things Changed?

Before the development of NASIS and the implementation of our current policy on this issue, the following points were true.

  • Estimated soil property ranges were stored nationally on the soil interpretation record (SIR). Data values were required to be no wider than family class limits.
  • Our soil database was populated locally with the SIR data and nationally generated interpretations.
  • Other soils known to occur in the map unit were described as either "similar" or "contrasting" soils in the manuscript text. They were not included in the database for the map unit.

In contrast, our current policy:

  • Requires map unit component data in NASIS to have a representative value within the taxonomic class limit of the soil being correlated. However, the low and high values are allowed to "extend beyond the established limits of the taxon from which the component gets its name, but only to the extent that interpretations do not change." (NSSH 618.03).
  • Permits ranges, where warranted, to extend beyond the technical definitions of terms in correlated names. For example, Alpha, silt loam could have an RV of 25% clay and a range of 20-30%, thereby including the low end of SiCl.
  • Accommodates what we generally think of as "similar" soils within the data for the map unit component and thereby more closely reflect the properties of natural soil bodies.
  • Includes contrasting soils in the database as components of minor extent.
  • Provides for interpretations generated from this locally tailored data.

Challenges & Conclusions

Our current policies and tools for correlation, data storage, and soil interpretation are improvements over previous concepts. To be utilized effectively however, we must:

  • Understand the roles of taxonomic units and map unit components in mapping, correlating, and interpreting soils.
  • Use care and judgment to limit the expansion of ranges to include only "similar" soils.
  • Achieve a balance in database complexity by providing our clients with the fewest number of components needed to adequately interpret the map unit, while still accurately reflecting map unit complexity.
  • Maintain and apply current standards defining taxonomic concepts to ensure effective correlation and facilitate transfer of information and technology.
  • Continue to work at achieving consistency among soil survey regions in the correlation process and in component data population.

One of the significant strengths of the US soil survey program has been our correlation process. We have effectively integrated a strong scientific foundation with policies, procedures, tools, and uniform standards for coordinating the work from individual-to-individual, survey area-to-survey area, and region-to-region. It is important that we strive to continue this important aspect of soil survey.

Further Reading

Cline, M.G. 1977. Historical highlights in soil genesis, morphology, and classification. Soil Science Society of America Proceedings 41:250-254.

Ditzler, C.A., R.J. Engel, and R.J. Ahrens. 2002. Soil taxonomy and soil survey p. 221-229 in H. Eswaran et. al. (ed.) Soil classification: a global desk reference. CRC Press, Boca Raton, FL.

Guthrie, RL 1982. The relationship between soil taxonomy and soil mapping. Soil Survey Horizons 3:5-9.

Nettleton, W.D., B.R. Brasher, and G. Borst. 1991. The taxadjunct problem. Science Society of America Journal 55:421-427.

Soil Survey Division. 1999. Report of the committee on the application of the new classification system to the 1965 NCSS conference in: Proceedings of the National Cooperative Soil Survey conferences and other soil survey documents 1963-1997. CD-ROM computer file. National Soil Survey Center, Lincoln, NE.

Webster, R. 1968. Fundamental objections to the 7th approximation. Soil Science 19:354-366.