Most Abundant Soils

Steve Custer
Last Modified 02 January 2010

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How do I Interpret the Limitations?

The properites of soils summarized in soil units mapped by the Natural Resources Conservation Service can be used to asess limitations for septic systems.  The Soil Units map is not shown because the map cannot be presented in a readable fashion.  Those interested in soil unit names should look at the soil survey for Gallatin County available at the Gallalatin County office of the Natural Resource Conservation Service.  There are ten themes that relate to limintations:
  1. Septic Limitation Index
  2. Number of Severe Limitations
  3. Flooding Limitation
  4. Depth To Bedrock Limitation
  5. Ponding Limitation
  6. Wetness (Depth To Shallow Water Table) Limitation
  7. Percolation Limitation
  8. Filtration Limitation
  9. Slope Limitation
  10. Large Stones Limitation

There are seven additional soil properties that may be of interest:

Property Desricptions

  1. Hydrologic Group
  2. Steel Corrosion
  3. Concrete Corrosion
  4. Frost Action
  5. Soil SAR
  6. Shrink-Swell Potential from 0-60 inches
  7. Hydric Soils

Example Questions the Database Can Answer

1.  Where are the areas where septic limitations are most severe?
              The severity index is a score. The score was built by assigning a 3 to severe, a 2 to
              moderate and a 1 to slight and a 0 to everything else.   The scores for all
              restrictive features are added to produce the total score in SEVINDX.  There are eight
              are eight restrictive features so the maximum possible score is 3 X 8 = 24.
              The highest score in the
              LWQD is 17.
 2.  How many severe ratings are present in each soil map unit?
              In this theme the number of severe ratings for the eight restrictive features is tallied
              for each soil map unit and reported.  The largest number of severe limitations is four.
 3.  What are the septic limitations from the perspective of flooding?
 4.  What are the septic limitations from the perspective of depth to bedrock?
 5.  What are the septic limitations from the perspective of ponding water?
 6.  What are the septic limitations from the perspective of depth to high water table?
 7.  What are the septic limitations from the perspective of percolation rate for drain fields?
 8.  What are the septic limitations from the perspective of filtration of microbes?
 9.  What are the septic limitations from the perspective of slope?
10.  What are the septic limitations from the perspective of large stones (coarse fragments)?
11.  Where are the areas with hydric soils?
12.  Where are the high and low runoff areas?
            This data is useful for runoff analysis with the NRCS Curve Number approach.
13.  Where are the areas with high potential for steel corrosion which might impact underground pipes?
14.  Where are the areas with high potential for concrete corrosion which might impact concrete septic tanks?
15.  Where are the areas with high potential for frost action?
16.  Where are the areas with high SAR where drainfield soils may be sensitive to sodium from water softeners?
17.  Where are the areas with shrink-swell clays which may be sensitive to sodium from water softeners?
18.  What soil units exist in the area I am interested in?

This theme focuses on most abundant soils mapped by the NRCS and US Forest Service. Each soil map unit may contain more than one soil.   The most limiting soil was selected by examining each soil in a map unit for severe ratings in the order listed in U.S.  Natural Resources Conservation Service, 1996, National Soil Survey Handbook,  Part 620 NRCS 620-46  Soil Interpretations Rating Guides, Table 620-17 Septic Tank Absorption Fields The first soil that produced a severe listing was used for all further limitations.  For example, if there were three soils and none had severe flooding limitations, but one had depth to bedrock less than 40 inches, that soil became the soil used to define the most limiting restrictions.  Likewise,  if none of the three soils had a severe flooding limitation or depth to bedrock limitation, but one had a ponding limitation, the soil with the ponding limitation was selected to represent the most limiting conditions regarding installation and operation of septic systems.  The choice of soil was independent of the area the soil component represents in the map unit.  As a result the most limiting soil may represent as little as ten percent of the mapped unit.    The examples above illustrate questions that can be explored with this database.  The limitations are sometimes surmountable with sound engineering practice, but such work can be expensive and can substantially influence the development potential of the site from a cost perspective.  In all cases, the user must be aware that site specific work may reveal other limitations or show that the map unit has areas without the indicated limitations.  These themes simply alert the user to problems which may be present based on second and third order soil surveys.


There are a variety of sources for information regarding septic systems in general and  the limitation criteria and properties in particular.  A glossary of attributes has been prepared to help you with interpretation.

About the Data

Mstabnd.shp Metadata
The limitations are based upon Natural Resources and Conservation Service (NRCS) data for the most abundant soil mapped at a scale of 1:24,000 during a level second order soil survey and Gallatin National Forest data for the most abundant soil mapped at a scale of 1:62.500 during a third order survey. The data are projected in UTM meters NAD83.   The limitations were developed from NRCS 620-46 Part 620 Soil Interpretations Rating Guides (Table 620-17).    The most abundant soil may represent from 33 to 100 percent of the land area in the mapping unit since many of the soils represent a complex. The limitations for the most abundant soil differ from that for the most limiting soil.  This data should not be used for site assessment but is useful for planning and  generalized assessment of  areas larger than10-20 acres.

The theme for lowland agricultural soils was created from soil tables in Rolfes, T., J. Brooker, B. Duncan, T. Keck, and R. Simms, 1998,  SSURGO Soil Survey Geographic Database, Gallatin County Area, Montana:  USDA-NRCS, Bozeman, Montana.  The theme for the upland forest soils was developed from Davis, C.E., and Shovic, H.F., 1984 Soils survey of Gallatin national Forest Area, Southwestern Montana:  Interim Draft Report: USDA-Forest Service, Gallatin National Forest; and from Davis, C.E., and Shovic, H.F., 1996, Soil Survey of Gallatin national Forest, Montana: USDA-Forest Service, Gallatin National Forest, Bozeman, Montana. The soils in the Big Sky area have been extrapolated from US Forest Service soil data; more detailed surveys have been conducted in this area that are not yet available digitally, but are available through the Gallatin NRCS office. The Gallatin National Forest soils were not originally interpreted for septic limitations, but such interpretations can be made as shown by Christner, W.T., Jr., 1999, Septic Interpretations from a Third Order Soil Survey:  Master of Science in Land Resources and Environmental Science, Montana State University, Bozeman, Montana.  Each soil unit may contain as many as three soils.  The most abundant soil was selected,  septic system limitation criteria were applied to the soil properties,  and severe, moderate, slight, or no data were assigned to each property for each most abundant soil.  The most abundant soil in each unit covers from 33-100 percent of the soil unit's area.

A list of attributes and their definitions for the attribute table has been prepared as has a glossary of soil terms.

Direct links to various Natural Resource Conservation Service manuls can be found via this link.