Most Abundant Soils
Last Modified 02 January 2010
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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
- Septic Limitation Index
- Number of Severe Limitations
- Flooding Limitation
- Depth To Bedrock Limitation
- Ponding Limitation
- Wetness (Depth To Shallow Water Table)
- Percolation Limitation
- Filtration Limitation
- Slope Limitation
- Large Stones Limitation
There are seven additional soil properties that may be of interest:
- Hydrologic Group
Potential from 0-60 inches
Example Questions the Database Can Answer
1. Where are the areas where septic limitations are most severe?
index is a score. The score was built by assigning a 3 to
severe, a 2 to
a 1 to slight and a 0 to everything else. The
scores for all
are added to produce the total score in SEVINDX.
There are eight
restrictive features so the maximum possible score is 3 X 8 =
score in the
2. How many severe ratings are present in each soil map
theme the number of severe ratings for the eight restrictive
features is tallied
soil map unit and reported. The largest number of severe
limitations is four.
3. What are the septic limitations from the perspective
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
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
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
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
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
The most limiting soil was selected by examining each soil in a map
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
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
Likewise, if none of the three soils had a severe flooding
or depth to bedrock limitation, but one had a ponding limitation, the
with the ponding limitation was selected to represent the most limiting
conditions regarding installation and operation of septic
The choice of soil was independent of the area the soil component
in the map unit. As a result the most limiting soil may represent
as little as ten percent of the mapped unit. The
above illustrate questions that can be explored with this
The limitations are sometimes surmountable with sound engineering
but such work can be expensive and can substantially influence the
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
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
and properties in particular. A glossary
attributes has been prepared to help you with interpretation.
About the Data
The limitations are based upon Natural Resources and Conservation
(NRCS) data for the most abundant soil mapped at a scale of 1:24,000
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
survey. The data are projected in UTM meters NAD83. The
were developed from NRCS 620-46 Part 620
Interpretations Rating Guides (Table 620-17). The
most abundant soil may represent from 33 to 100 percent of the land
in the mapping unit since many of the soils represent a complex. The
for the most abundant soil differ from that for the most limiting
This data should not be used for site assessment but is useful for
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,
SSURGO Soil Survey Geographic Database, Gallatin County Area,
USDA-NRCS, Bozeman, Montana. The theme for the upland forest
was developed from Davis, C.E., and Shovic, H.F., 1984 Soils survey of
Gallatin national Forest Area, Southwestern Montana: Interim
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
W.T., Jr., 1999, Septic Interpretations from a Third Order Soil
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,
system limitation criteria
were applied to
the soil properties, and severe, moderate, slight, or no data
assigned to each property for each most abundant soil. The most
soil in each unit covers from 33-100 percent of the soil unit's area.
A list of attributes and their
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.