Alkaline Soil in Montana and How to Plant for It

What is alkaline soil?

Soil pH runs from 0 to 14, with 7 neutral. Anything above 7 is alkaline. MSU Extension's MontGuide on home soil testing says vegetables, grasses and most ornamentals do best between pH 5.8 and 6.5 and conifers between about 6 and 7.5, so a soil at 7.8 or 8.2 sits well above what many landscape plants prefer.

In Montana, alkaline soil often goes with lime, meaning calcium carbonate that has built up in the soil. NRCS soil descriptions call these layers calcic horizons and describe the soil as effervescent, because it fizzes when acid is dropped on it. Calcareous soils are strongly buffered, which is why their pH is so hard to change.

Is western Montana soil alkaline?

Some of it. MSU's MontGuide notes that surface soils in eastern Montana typically run pH 7 to 8 and that western Montana soils tend to be slightly more acidic. But that is a broad average, and the Bitterroot has both kinds within a few miles.

NRCS's soil survey of the Bitterroot Valley shows the split. The gravelly outwash soils that cover the most ground, such as Chereete and Victor, are slightly to moderately acid at the surface in their typical profiles. The soils on older eroded fan remnants are a different story: NRCS describes Burnt Fork with a lime layer starting 13 to 30 inches down and subsoil reaching pH 8.2, Woodchuck as fizzing right at the surface with lime layers at pH 8.2 to 8.4 above a cemented hardpan, and Farnuf with subsoil at pH 8.3 to 8.5. Our Bitterroot Valley soils guide explains how to find out which group your lot is in.

How do I know if my soil is alkaline?

A soil test is the only reliable answer. Take a proper composite sample, ask the lab for pH, soluble salts, organic matter and the Olsen phosphorus test, and you will know exactly where you stand. Our soil testing guide walks through MSU's sampling method. If you dig a planting hole and hit a pale, chalky layer, that is a strong hint too, so sample that layer as well as the surface.

The plants will also tell you. MSU's guide to ornamental tree problems describes leaf chlorosis as leaves going from light green early in the season to yellow later, often with veins staying bright green while the tissue between them yellows, and in advanced cases smaller leaves and browning edges. When several lime-sensitive plants in the same yard show it, the soil is the first suspect.

Why does alkaline soil make leaves turn yellow?

MSU Extension explains that some nutrients, such as iron, become less available to plants as soil pH rises. The iron is usually present in the soil; the plant simply cannot take enough of it up. MSU is careful to add that chlorosis is usually caused by several factors together, listing drought, overwatering, compacted soil, limited planting areas and poor drainage alongside pH.

That matters for the fix. A maple in alkaline soil that is also in a small bed, under a lawn sprinkler that runs every day, has three problems, not one. MSU's MontGuide also notes that iron deficiency is very common in the rose family and in ash, apple, crabapple and some maples, and that phosphorus is tied up by high calcium in most Montana soils. Our guide to iron chlorosis and yellow leaves covers diagnosis and treatment of individual trees.

Can I lower the pH of alkaline soil?

Not in any practical way across a whole yard. MSU's MontGuide says that because Montana's soils are well buffered, lowering pH is very difficult and generally expensive. Elemental sulfur is the usual method, but MSU cites lab results in which rates near 225 pounds of sulfur per 1,000 square feet only lowered pH from 7.9 to 7.5. Organic matter may lower pH over time, but MSU notes that many Montana and Wyoming manures are themselves alkaline and may not acidify soil much.

So we do not recommend chasing pH with amendments across a landscape. Compost is still worth adding to beds for organic matter, water holding and structure, and a small, contained bed for one special plant can be built with imported soil. But the durable answer is choosing plants that are comfortable at the pH you have.

Which trees and shrubs tolerate alkaline soil in Montana?

MSU's ornamental guide names linden, ash, boxelder, oak, elm, hawthorn and honeylocust as tolerant of alkaline soils, and apple, maple and poplar as more prone to chlorosis. MSU's soil testing MontGuide separately lists ash among the plants where iron deficiency is common, so if you plant ash in a limey spot, watch it; ash in our area also faces emerald ash borer concerns covered in our ash tree guide.

Many of the region's native shrubs and grasses grew up on these soils. NRCS lists bluebunch wheatgrass, needle and thread and prairie Junegrass as the native vegetation of the limey Burnt Fork soils, for example. Our native plants guide, our guides to shrubs for western Montana and xeriscaping, and our list of the best trees for western Montana are good places to build a plant list. Check each plant's soil preference before you buy.

What about lawns on alkaline soil?

Cool-season lawn grasses grow on a wide range of soils, and many lawns on limey benches look fine with good care. The common problems on alkaline ground are low available phosphorus, salts where drainage is poor, and the occasional yellow, iron-short patch. A soil test will tell you whether phosphorus or salts are actually an issue before you buy a product for them.

Most of the time, the practical levers are the same as anywhere: mow high, water deeply and less often, relieve compaction, and fertilize on a schedule. Our fertilizer and weed control programs are matched to the soil, and our guide to Kentucky bluegrass, fescue or a blend helps with seed choice.

How should I plant a tree in alkaline soil?

Follow MSU's management steps for chlorosis from the start. Water during drought, because MSU notes supplemental watering in dry spells reduces tree stress. Keep turf away from the base of the tree to cut competition for water, and mulch the root area with two to three inches of compost, as MSU recommends, to conserve moisture.

Equally important, avoid the stresses MSU lists alongside pH: do not plant in a tiny cutout, do not set a tree where lawn sprinklers will keep the root zone wet, and break up compacted soil before planting. If the planting hole hits a cemented layer, like the duripan NRCS describes in the Woodchuck soils, drainage below the root ball matters even more. Our guide on how to plant a tree in Montana covers the planting steps.

Is alkaline soil the same as salty soil?

No, though a soil can be both, and the two are managed differently. pH measures acidity or alkalinity. Soluble salts, shown as EC on a soil report, are a separate measurement. MSU Extension says high soluble salts can cause water stress and nutrient imbalances, that values above 4 may damage plants, and that seedlings are more sensitive than established plants. It also notes that certain fertilizers, amendments and manure can add salts in localized areas.

So a white crust, poor seed germination or leaf burn is a reason to test EC as well as pH, and to sample during a dry period, because rain or irrigation lowers the reading. Lime in the soil is not itself a salt problem. Wet bottoms and spots where de-icing salt drains are worth checking first; our guide to de-icing salt damage covers the second.

Should I use iron products or acid fertilizer?

They treat the symptom, not the soil. MSU's MontGuide advises against adding micronutrients unless a soil test shows a need, because excess can be harmful, and it suggests placing metal micronutrients in a band below the surface or on the leaves rather than on the soil surface, where they do little. For a valuable tree already showing chlorosis, ask an ISA Certified Arborist to look at it; our tree health assessments are a place to start.

Designing a landscape that fits the soil

When we design a yard on alkaline ground, we start with plants that like it, put irrigation on zones that match each plant group, and save the lime-sensitive favorites for spots where the soil has been tested and prepared. That is how our landscape design and installation work approaches every site, and it is a theme throughout our Montana Landscaping, Lawn & Irrigation Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the yard with you and build a plant list that suits your soil.

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