Soil Survey of Sweet Grass County Area, Montana - Soil Data Mart

18 Soil Survey
these organisms die, the nitrogen is released in
various forms and becomes available to plants.
Soils under forest plants tend to be cooler than
soils under grassland plants. Wet soils have less
oxygen available to microbes than drier soils. The
activity of microorganisms and animals is less
extensive in cooler, wetter soils. As a result, organic
matter is broken down more slowly and more organic
litter remains on the surface of the soil.
Topography
Topography, or relief, is determined by glaciation
and mountain formation and by the age and
resistance of geologic formations to erosion by wind
and water. Topography influences soil development
through its effect on drainage and runoff. The degree
of slope, shape of the land surface, and permeability
of the soil determine the rate of runoff, internal
drainage, and moisture content of the soil.
Relief features in Sweet Grass County have been
determined by glaciation, geological uplift, volcanic
action, and removal and deposition of materials by
wind and water. The slopes in Sweet Grass County
range from 1 percent on bottomlands and terraces to
over 60 percent on mountainsides and breaks along
drainageways.
The number and distinctness of soil horizons
generally decrease as slope increases. Soils on
steep slopes with rapid runoff have many
characteristics similar to those of soils formed in arid
climates. Soils on east- and north-facing slopes have
cooler temperatures than those on west- and southfacing
slopes. East- and north-facing slopes receive
less sunlight. As a result, the soils on these slopes
retain water longer and are cooler than soils on westand
south-facing slopes. The surface soil is darker
and the depth to lime is generally deeper on northfacing
slopes than on south-facing slopes. In some
parts of the survey area, these differences are
pronounced. Soils formed only on east- and northfacing
slopes include Vision soils. These soils differ
from the Winkler soils that formed in similar material
on south-facing slopes.
The soil horizons on gently sloping surfaces are
generally more distinct than soils formed in similar
parent material on steep and very steep surfaces.
Gently sloping soils absorb more moisture, and water
is retained in the soil to a greater depth. Soils on
steeper slopes generally have a thinner, lighter
colored surface layer and a shallower depth to lime
than soils formed on lesser slopes. Erosion caused
by the runoff on steeper soils also restricts the
formation of distinct soil horizons.
Parent Material
Parent material is the unconsolidated mass in
which a soil forms. It strongly affects the chemical
and mineralogical composition of the soil. The soils in
the Sweet Grass County Area formed in many
different kinds of parent materials. The major
materials are recent alluvium, glacial alluvium, mixed
alluvium and colluvium, soft bedrock, hard bedrock,
and volcanic mudflows.
Recent alluvium is water-deposited material on the
bottomlands and low terraces along the major
streams and rivers. These soils contain varying
amounts of sand, silt, and clay. Many of them contain
rounded gravel, cobbles, and stones. Soils formed on
these positions lack significant soil development; they
tend to be stratified. Many soils are still subject to
flooding. Havre and Ledger soils formed in alluvium
on bottomlands. Kobase and Korchea soils formed on
low stream terraces.
Glacial alluvium is material deposited by glacial
melt water. This material is on terraces and outwash
plains throughout the county. The deposits range
from Pleistocene to Miocene Ages. These soils
contain varying amounts of sand, silt, and clay with
large amounts of gravel and cobbles. The soils
formed in these materials generally have good
horizon development. Roy and Shawmut soils are
examples of soils formed in these materials.
Mixed alluvium and colluvium have been deposited
by the combined forces of gravity and water. They
consist of materials that have been moved downslope
from higher areas and redeposited on footslopes,
alluvial fans, and along drainageways. They contain
varying amounts of silt and clay with lesser amounts
of sand. Soils formed in these materials show varying
degrees of development, depending upon the other
soil-forming factors. Shambo and Yamacall soils are
examples of soils from these materials with slight
horizon development. Farnuf and Work soils show
good horizon development.
Material weathered from soft bedrock formations is
a major parent material in the uplands in this survey
area. Most of these materials are calcareous. The
soils formed in place, or they formed in material that
was locally reworked and transported by water and
wind. These deposits range in thickness from a few
inches to several feet over the underlying bedrock.
Most soils formed in this material have weakly