TSSH Part 631
Development of Use-Specific Local Interpretations
Soil interpretations are an important aspect of the soil survey program. Interpreting the suitability or capacity of a soil for specific uses can help on a small scale for broad planning purposes, such as development of zoning ordinances, land use planning, and targeting for preservation or mitigation. Interpretations can also be important at a large scale with site-specific planning and design for such purposes as septic system design and wastewater disposal. Soil surveys include basic soil interpretations for map units as a whole that were derived using national criteria or rules. Where needed, use- specific local interpretations can be developed based on criteria set up at the local level. These interpretations may be used on a broad State, county, or municipal scale to create maps for planning purposes or at site-specific scale.
Creating State and local interpretations (631.01)
State and local interpretations and their related criteria are for use within a local area or region (National Soil Survey Handbook [NSSH], part 617. Deviations from the nationally supported standard interpretations and their related criteria are documented, and the interpretations are renamed by the State. The development of interpretations follows the procedure in NSSH part 617.10. The offices in which the local, State, or regional interpretations are created retain the criteria and performance documentation. Guidelines for submitting new interpretations or changes to standard interpretations are spelled out in NSSH part 617.04. Submission of regional, State, or local interpretations to the National Leader for Soil Survey Interpretations will ensure that these developments are shared with potential users. Soil interpretations must meet the requirements outlined in NSSH part 617.10. The interpretations should be supported by field observations, research (laboratory and field), and other documentation.
Creating use-specific local interpretations (631.02)
Local interpretations can be created when a specific use requires criteria and soil property data other than those used for a standard national interpretation or when unique local properties, criteria, or categories are more useful for a specific location than those of the standard interpretation.
One of the most common examples is where the local county ordinance criteria require specifications for placement of septic tank filter fields that differ from those of the national NRCS NASIS rules, resulting in differences in suitability pronouncement between local regulations and NRCS ratings. In order to be in agreement with the local ordinance, NRCS soil scientists typically will create a local interpretation that agrees with the local criteria. The soil scientist should make sure that the local criteria are sound and should discuss any differences in professional opinion with local experts.
As another example, the national criteria or NASIS rules for the “paths and trails” interpretation result in a more severe rating if the soil is shallow to bedrock. In Hawaii, however, there are Lithic Folists over pahoehoe, which is a smooth, ropy-surfaced lava that is more comfortable to walk on than the slick sapric soil material overlying it. As a trail is developed and used, the sapric material is quickly volatilized and the pahoehoe is exposed, thereby providing a high-quality and stable trail. A set of local “paths and trails” interpretation rules was developed in which a shallow depth to pahoehoe was considered.
Local interpretations can be produced during soil survey activities for USDA farm programs or for Federal, State, or local forms of government with which there is a memorandum of understanding or a cooperative agreement.
Additional information (631.03)
For information on interpretations in NASIS, access the online help. You can create a local interpretation table by creating your own query based on soil properties recorded in NASIS. This table can then be linked to special data for a survey area to create large-scale planning maps, or the information can be used at site-specific scale to determine suitability or capacity of the soil for a specific use.