Bitterroot Valley Soils: What Is Under Your Yard

What kind of soil is in the Bitterroot Valley?

Mostly gravelly, well drained soils formed in material washed out of the mountains, with smaller areas of fine, limey soils on older fan surfaces and wet soils along the river and old channels. The valley floor is covered by the NRCS soil survey called the Bitterroot Valley Area, Montana (survey MT645), which maps roughly 370,000 acres. By our tally of the survey's data in the NRCS Soil Data Access database, about two thirds of that mapped acreage is in soils whose family name includes the word skeletal, the soil scientist's term for ground dominated by gravel, cobbles or stones (the NRCS description of the Riverrun series, for example, gives 35 to 70 percent rock fragments in the layer that defines its family).

Underneath, the valley is a deep basin. The U.S. Geological Survey's study of Bitterroot geology and water describes it as a structural basin partly filled, at its deepest point, with more than 1,640 feet of Tertiary sediments, coarse along the valley edges and finer toward the middle, with about 40 feet of younger river alluvium under the flood plain and low terraces of the present Bitterroot River. Your yard's topsoil is a thin skin on top of all of that.

What are the most common soil series in the valley?

Ranked by the acreage of the major soil components in the MT645 survey data, the largest are Chereete, Perma, Riverrun, Whitlash, Losthorse, Farnuf, Quast, Holter, Sheafman, Burnt Fork and Wimper. Series named for local places, such as Victor, Hamilton and Burnt Fork, are on the list too. A soil series is a named kind of soil with a defined set of layers, and NRCS publishes an Official Soil Series Description for each one.

You do not need to memorize series names. What matters for a yard is the group a series falls into: gravelly outwash soils that drain fast, limey fan and bench soils that are alkaline below the surface, and bottomland soils with a seasonal water table. Each group behaves differently under a lawn, a tree or a patio, and the rest of this guide takes them one at a time.

Gravelly outwash fans: Chereete, Sheafman and Victor

The single largest series in the survey, Chereete, is described by NRCS as a very deep, excessively drained soil formed in outwash, alluvium or colluvium on outwash fans, drainageways and escarpments. Its type location is on the Hamilton North quadrangle in Ravalli County. The surface is a gravelly coarse sandy loam, slightly acid in the typical profile (pH 6.3), and it turns sandy and very gravelly within the first one to one and a half feet.

Sheafman and Victor are close relatives. NRCS describes Sheafman as excessively drained, formed in coarse gravelly outwash or colluvium derived from igneous rock, and Victor as somewhat excessively drained, formed in outwash derived from igneous rock, with a typical surface pH of 5.8 (moderately acid). Both have type locations on the Victor quadrangle.

In a yard, these soils mean fast drainage and very little water held in reserve. Lawns on them need shorter, more frequent irrigation, trees need deep, wide watering, and digging means sorting rock. Their typical profiles are slightly to moderately acid at the surface, which makes them friendlier to maples and other plants prone to iron chlorosis. Our guide to rocky, cobbly ground covers how to plant in them.

Limey benches and fan remnants: Burnt Fork, Woodchuck and Farnuf

On older, eroded fan surfaces the soils change character. NRCS describes Burnt Fork, whose type location is on the Corvallis quadrangle, as a very deep, well drained loam on fan remnants with a calcic horizon (a layer where lime has accumulated) starting 33 to 76 centimeters down, roughly 13 to 30 inches. Its native vegetation is listed as bluebunch wheatgrass, needle and thread and prairie Junegrass, and it is used for rangeland, irrigated pasture and hayland.

Woodchuck, with a type location in Ravalli County, is a moderately deep loam on fan remnants that is strongly effervescent (fizzes with acid because of lime) at the surface in the typical profile, and sits on a cemented silica and lime hardpan, a duripan, 50 to 100 centimeters down. Farnuf, also mapped widely in the survey, has lime masses in its subsoil and reaches pH 8.3 to 8.5 in the lower layers of its typical profile.

For a homeowner, these are the soils most likely to give yellow leaves on maples, apples and other lime-sensitive plants, and the place where a planting hole can hit a layer that roots and water struggle to get through. Our guide to alkaline soil in Montana explains how to plant for them, and a soil test will confirm the pH.

River bottoms and wet ground: Riverrun and Curlew

Close to the Bitterroot River and its old channels, water is the issue rather than rock or lime. NRCS describes Riverrun as moderately well drained, on flood plains, stream terraces, outwash fans and drainageways, with a seasonal high water table 100 to 150 centimeters (about 3 to 5 feet) down for extended periods in late spring or early summer. Its native vegetation includes bluebunch wheatgrass, basin wildrye, snowberry and woods rose, with groves of cottonwood and willow in places.

Curlew, with a type location on the Victor quadrangle, is very poorly drained, with a seasonal high water table 0 to 30 centimeters from the surface for extended periods, often in late spring or early summer. Yards on soils like this stay wet when everything else has dried, and are prone to standing water, waterlogged tree roots and heaved patios. Our guides on standing water in the lawn and French drains cover the fixes.

What about terrace soils like the Hamilton series?

Between the fans and the bottoms are stream terraces with finer, silty soils. NRCS describes the Hamilton series, with a type location on the Corvallis quadrangle, as well drained, with very little gravel and a zone of secondary lime starting 33 to 59 centimeters down. These are among the easiest soils in the valley to dig and garden, and they hold water better than the outwash gravels, but the lime below the surface still matters for deep-rooted, lime-sensitive trees.

Are Missoula Valley soils the same?

They are related but not the same, and they come from a separate NRCS survey, the Missoula County Area (MT638). Much of that survey is mountain and forest soil, but among the valley landforms the largest major components are Bigarm, a somewhat excessively drained loamy-skeletal soil on fans and terraces, and Moiese, an excessively drained sandy-skeletal soil, also on fans and terraces, with Newbar on flood plains.

Missoula has one soil setting the Bitterroot list does not lead with: lake plains. Grassvalley, a fine textured and well drained soil, is mapped there, along with fine-silty Auggie. Soils like these dig and drain very differently from cobble, so a neighborhood a mile from a gravel terrace can have entirely different ground. Look up your own address rather than assuming.

How do I find out what soil is on my property?

Use the NRCS Web Soil Survey. NRCS describes it as the single authoritative source of soil survey information, with soil maps and data online for more than 95 percent of the nation's counties. You use the Area of Interest tab to outline your lot, then the Soil Map tab to see the map units and their descriptions. Click a map unit name to read its typical profile, drainage class and depth to water or restrictive layers.

NRCS is clear that surveys are for general and planning use and that onsite investigation is needed for some purposes. On top of that, most residential lots have been graded, filled or topsoiled at some point, so the ground in a yard may not match the mapped soil. Use the map to know what to expect, then dig a few test holes and send a sample for testing before you spend money on plants or amendments.

Does soil type change what I should plant?

It changes almost everything: how often you irrigate, which trees will stay green, how deep a patio base needs to drain, and whether a rain garden will drain or turn into a pond. On fast-draining outwash soils, plan irrigation zones by plant water needs and use drip for beds and trees; our drip irrigation and irrigation zone design guides explain how. On limey benches, choose lime-tolerant species. On wet bottoms, choose plants that tolerate saturated roots in spring and fix drainage before building hardscape.

Our native plants guide is a good starting list, because plants that evolved here already cope with these soils. The Burnt Fork and Riverrun descriptions, for example, list bluebunch wheatgrass, snowberry and woods rose among their native vegetation.

Getting help with a difficult site

Soil is the first thing we look at when we walk a property for a new landscape, irrigation system or hardscape project, because it decides how everything else should be built. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide, and the soil work it describes feeds directly into our landscape design and installation and hardscaping services.

If you are planning a project in the Bitterroot or around Missoula and want it built for the ground it sits on, call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate.

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