Reading Soil Survey Maps for Prairie Nature Observation

A soil survey map won't tell you where birds are today. It tells you something more lasting: where the ground is likely to support short grass or taller grass, where water collects, where salts build up, and where the land is sandy, clayey or shallow. Because prairie birds choose habitat largely by vegetation structure and water, the soil map works as a forecast of habitat. For the glaciated plains of northeastern Montana around Medicine Lake, it can turn a flat-looking landscape into a readable pattern of uplands, swales, wet depressions and salty flats.

The Short Answer

To read a soil survey map for nature observation, find the map unit polygon for the area you care about and look up its symbol in the map legend. Then check four properties in the survey's descriptions or tables: drainage class, flooding or ponding, salinity or alkalinity, and texture or depth. Those four properties predict the plant community and whether water will stand there. That in turn suggests which groups of birds the site may suit, such as waterbirds in ponded depressions, shorebirds on wet or saline margins, and grassland songbirds on upland grass. Treat the result as a hypothesis to check in the field, because current land use, grazing, fire and recent precipitation can override what the soil would otherwise support.

Two Kinds of Soil Maps and When to Use Each

Soil maps come at very different scales, and the scale decides what questions a map can answer.

General (small-scale) soil maps: Statewide or regional sheets, such as a general soil map of Montana, group soils into broad associations covering large areas. Use them to understand the overall setting, for example glacial till plains versus river terraces versus sandier uplands. They are too coarse to locate an individual wetland or field.
Detailed county soil surveys: A county survey maps individual polygons over aerial photography and describes each map unit in detail. Published surveys exist for northeastern Montana counties, including Valley County to the west of the Medicine Lake area. Use these for site-level planning: which part of a section is a wet swale, which slope is shallow and gravelly, which flat is saline.
Digital soil data: The same detailed survey information is now distributed digitally by the USDA Natural Resources Conservation Service, most commonly through its Web Soil Survey application. Digital versions let you draw an area of interest and generate property maps, such as drainage class or ponding frequency, without paging through a printed volume.

Decoding the Map Itself

Map unit symbols

Each polygon carries a symbol, usually numbers, letters or both. The symbol has no meaning on its own; it points to an entry in the survey legend. In many surveys a trailing capital letter indicates a slope class, with letters later in the alphabet meaning steeper ground. Conventions vary between surveys and publication years, so always read the legend of the survey you are using rather than assuming a pattern.

Map unit names

The name tells you how many soils the polygon contains and how they are arranged. This matters for birding because a single polygon can hold both upland and wetland ground.

Consociation: Named for one dominant soil, for example a single soil series name followed by a slope range. Expect fairly uniform ground.
Complex: Two or more soils in a pattern too intricate to map separately, such as a well-drained upland soil intermixed with a poorly drained depression soil. On the glaciated plains this often describes pothole country, with small wetlands scattered through grassland.
Association: Two or more soils that occur in a predictable, larger pattern that simply wasn't separated at the mapping scale.
Undifferentiated group: Soils lumped together because they would be used or managed the same way.
Miscellaneous areas: Units such as water, badlands or rock outcrop that are not really soils but still shape habitat.

Map units also contain small inclusions of other soils that are not named. A polygon described as well drained may still hold a wet spot or two, so the map points you to the likely character of a place, not every square meter of it.

The Soil Properties That Matter Most for Habitat

Soil property	What the survey tells you	What it suggests about habitat
Drainage class	Ranges from excessively drained to very poorly drained	Poorly and very poorly drained soils mark wetlands and wet meadows; well-drained soils mark upland grass
Ponding and flooding frequency	How often water stands on the surface or overflows from streams	Ponded depressions may hold water seasonally or most years; flooded bottomlands support shrubs and trees along drainages
Salinity and sodicity	Whether salts or sodium accumulate in the root zone	Salt-affected soils carry sparser, salt-tolerant vegetation and can form open flats along wetland edges
Texture	Sand, loam or clay content of the surface and subsoil	Sandy soils often hold taller bunchgrasses and resist heavy grazing differently; clayey soils may crack, pond or carry shorter cover
Depth to bedrock or gravel	Whether the root zone is shallow	Shallow or gravelly ridges tend toward short, sparse grass and more bare ground
Slope	Gradient class of the map unit	Steeper breaks and coulees create varied cover and shelter compared with level plains

Ecological sites: the bridge from soil to vegetation

Many map units are linked to an ecological site, sometimes called a range site in older surveys. An ecological site groups soils that produce similar native plant communities, and its name usually hints at its character. Names along the lines of Loamy, Sandy, Clayey, Shallow, Saline Lowland or Wetland are typical of the northern plains. Ecological site descriptions explain the expected reference plant community and how it shifts with grazing or disturbance. For a birder, this is the most direct way to translate a soil polygon into an expected vegetation height and density.

Wildlife habitat interpretations

Older printed county surveys often include a wildlife habitat table. It rates each map unit's potential to support habitat elements such as grasses, shrubs and shallow water areas, and then rates its overall potential for broad categories like openland, wetland and rangeland wildlife. These ratings describe potential under suitable management, not present-day bird use, but they are a useful shortcut when you are screening a large area.

Putting It Together: An Illustrative Reading

Here is a hypothetical example of how the pieces combine. It does not describe a specific site. Imagine a polygon on the glacial till plain labeled as a complex of a well-drained loamy upland soil and a very poorly drained depression soil, with gentle slopes. The ecological site for the upland component is a loamy grassland site, and the depression component is rated as frequently ponded.

Read that as rolling grassland dotted with small basins that hold water in wet springs.
Expect mixed-grass cover on the rises, with vegetation height depending on how the land is grazed or hayed.
Expect basin margins to shift from wet to dry through the season, exposing mud as water recedes.
If a neighboring polygon is a saline lowland unit, look for a band of sparse, salt-tolerant vegetation and pale crusted ground between the grass and the open water.
That combination of open water, exposed margins, salty flats and upland grass would suggest a site worth scanning for waterbirds, shorebirds and grassland songbirds. Whether those birds are actually present depends on the year and season.

Preparing and Carrying Out a Field Visit

Before you go

Define your area of interest and pull the detailed county soil survey for it, digitally or in print.
Generate or note the map units, then look up drainage class, ponding, salinity and ecological site for each one.
Compare the soil polygons with recent aerial imagery. Imagery shows what soil maps cannot: whether the ground is now cropland, hayland, grazed pasture or native grass, and whether basins currently hold water.
Mark a few contrasting map units you want to compare, for example a sandy upland, a loamy upland, a saline lowland and a ponded depression.
Identify who manages each piece of ground. Much of the prairie around Medicine Lake is private land, and the national wildlife refuge, state lands and other public parcels each have their own rules. Confirm current access, seasonal closures and permitted activities with the managing agency or landowner before you visit. A soil map shows nothing about ownership or access.
Plan to observe from public roads, designated trails, parking areas and viewpoints wherever possible.

In the field

Stop at each target map unit and note what you actually see: vegetation height and density, bare ground, standing water, mud margins, salt crust, shrubs.
Compare that with what the soil survey predicted. Differences often reveal land-use history, recent weather or an unmapped inclusion.
Scan with binoculars or a spotting scope from a distance instead of walking toward birds or into cover.
Record the map unit symbol alongside each observation so your notes can be matched to soil properties later.
Note the date, time, weather and water conditions, since the same polygon can look entirely different from one season or year to the next.

Avoiding disturbance and habitat damage

The soil properties that make a site interesting for birds also make it easy to damage. Poorly drained and clayey soils rut deeply under vehicles when wet, salt crusts break and erode under foot traffic, and wetland margins hold the nests of ground-nesting birds during the breeding season.

Stay on established roads and trails. Never drive onto wet, ponded or saline ground the survey flags as poorly drained.
Keep out of wetland edges and dense grass during the nesting season, when ground nests are hard to see and easy to step on.
Watch for signs that you are too close: repeated alarm calls, birds circling overhead, a bird feigning injury to lead you away, or adults flushing from the same spot. If you see any of these, back away slowly and observe from farther off.
Do not use playback or approach nests, broods or roosting flocks.
Leave gates as you find them and respect posted closures and private property.
If a site was closed, inaccessible or unsuitable, record that rather than looking for a way around it.

What Soil Maps Cannot Tell You

Soil surveys describe the land's long-term potential, mapped at a fixed point in time. They do not show whether a field has been plowed, how heavily it is grazed, whether a basin is full or dry this year, or whether birds are using it this week. Use the soil map to choose where to look and to understand why a place looks the way it does, and let field observation decide what is actually there.