Soil Testing for Montana Lawns and Gardens

Do I really need a soil test?

Not always. MSU Extension's MontGuide on home garden soil testing (MT200705AG) notes that many gardeners skip testing and use standard fertilizer rates, and it recommends that if you are using standard rates and your plants look healthy, you keep doing what you are doing. MSU suggests testing when you want to reduce the risk of a deficiency or toxicity, or diagnose one.

The case for testing gets strong when plants are not thriving despite normal care. MSU points out that soil testing may be the only way to find abnormal nutrient levels, and that gardens can end up with excessive nutrients from heavy compost and fertilizer use, which wastes money and can harm plants and the environment. A test is also worth doing before a new lawn, a large planting or any project where you are about to buy amendments by the truckload.

How do I take a soil sample for a lawn or garden?

Representative sampling is the whole game. MSU recommends collecting a minimum of ten samples and mixing them, from your garden or from each 1,000 square feet of lawn. Remove mulch or lawn thatch before you sample. If one side of the lawn or garden looks or feels different from the other, submit separate samples, and the samples can go to the lab moist or dry.

For home gardens, lawns and trees, MSU says samples are generally a 6 inch deep core from the soil surface. Hand probes or augers work best; MSU advises avoiding shovels and spades because it is hard to take the same amount of soil from each depth with them, and suggests a bulb planter as an alternative for a 0 to 6 inch sample. Many Extension offices have probes they will lend, and tools should be cleaned between areas and stored away from fertilizer so they do not contaminate the sample.

When is the best time to test soil in Montana?

MSU says soils are ideally sampled each spring to best estimate what will be available in the growing season, but that fall can be more practical because the soil is dry and there is more time. Allow about one to two weeks for the lab, plus time to apply anything the results call for before planting or seeding.

Two timing details matter. If you sample in midsummer or fall, MSU advises waiting at least two months after fertilizing so the fertilizer has dissolved and been used. And fall nitrogen numbers can mislead: organic matter releases nitrogen over winter, while wet winters can leach nitrate out of shallow or sandy soils, which describes a lot of the gravelly ground in the Bitterroot (see our guide to Bitterroot Valley soils). Potassium, by contrast, tests highest and most accurately in spring just after the soil thaws.

Which soil tests should I ask the lab for?

At a minimum, MSU recommends testing for nitrogen, phosphorus, potassium, organic matter, soluble salts and pH. Choose a lab that takes part in the North American Proficiency Testing Program or the Agricultural Laboratory Proficiency Program, and send to the same lab every time, because MSU notes that results have varied quite a bit between laboratories. The MontGuide lists regional labs in its appendix, and your county Extension office can point you to one.

Ask specifically for the Olsen phosphorus test. MSU explains that Montana fertilizer guidelines are based on Olsen P, which works on neutral to alkaline soils, while the Bray test does not work well above pH 7 and its results do not convert easily. A lab that defaults to Bray or Mehlich will hand you numbers you cannot use with Montana guidelines.

What does soil pH mean for my yard?

pH runs from 0 to 14, with 7 neutral, below 7 acidic and above 7 alkaline. MSU's guide says conifers thrive between about 6 and 7.5, while vegetables, grasses and most ornamentals do best between 5.8 and 6.5, and that nitrogen, phosphorus, potassium and sulfur are most available between pH 6.5 and 8, with most micronutrients most available between 5 and 7. It also notes that eastern Montana surface soils typically run 7 to 8, while western Montana soils tend to be slightly more acidic.

Slightly more acidic is not the same as acidic everywhere. Some Bitterroot soils are limey below the surface, and a test is the only way to know your own number. If it comes back high, our guide to alkaline soil explains why MSU says lowering pH is difficult and expensive in Montana's well buffered soils, and what to do instead.

Why is phosphorus often low in Montana soils?

MSU's MontGuide states that Montana has among the lowest soil test phosphorus levels in the country, and that in most Montana soils high calcium ties up phosphorus, making it relatively unavailable to plants. Phosphorus also barely moves in soil, so it has to be where the roots are.

That is a good reason to test rather than guess, because the opposite problem also happens. Manure and heavy compost use can push phosphorus and potassium well past what plants need, and MSU advises periodic tests to track them. Its fertilizer tables give no phosphorus for lawns once Olsen P reaches the medium class of 12 to 16 ppm or higher.

What do organic matter, CEC and soluble salts tell me?

Organic matter is the partly decomposed plant and animal material in the soil. MSU considers 5 to 8 percent suitable for good tilth, water holding and erosion resistance, and notes that compost raises it, with plant compost adding much more organic matter per inch than manure compost. Bark, wood shavings and straw tie up nitrogen as they break down, so MSU suggests adding nitrogen when you work them in.

Cation exchange capacity (CEC) measures how well the soil holds nutrients; MSU calls anything above 10 meq/100 g adequate, and more organic matter raises it. Soluble salts, reported as EC, can cause water stress and nutrient imbalances; MSU says values above 4 may damage plants, seedlings are most sensitive, and salts should be sampled in a drier period because rain and irrigation lower the reading.

How do I turn soil test results into a fertilizer plan?

Most reports sort each nutrient into classes such as low, medium and high. Below the critical level, fertilizer helps; above it, only enough to replace what is lost or removed is needed. MSU warns that a lab's printed recommendation may not be based on Montana guidelines, or may be written for farm crops, so compare it with the MontGuide's tables for lawns, gardens and trees.

For lawns, MSU's standard rate without a test is about 3 pounds of nitrogen per 1,000 square feet a year, split into 1 pound applications around Memorial Day, Labor Day and Columbus Day. Our guide to Montana lawn fertilizer rates and timing explains how that schedule fits our climate. MSU also cautions against adding micronutrients unless a test shows a need, since excess can be harmful, and notes that iron shortage symptoms are common in the rose family, ash, apple, crabapple and some maples (our iron chlorosis guide covers that).

Should I test plant tissue instead of soil?

For a home yard, usually not. MSU reports that tissue samples in Montana have often given inconsistent results because of differences in sampling, handling, preparation and shipping, and because a couple of weeks can pass between sampling and applying fertilizer, by which time growth may already have suffered. MSU's recommendation is to find potential deficiencies by soil testing before the growing season. If you do want a tissue test, it suggests calling your local Extension agent for sampling instructions.

How do I convert ppm to pounds per acre?

Labs often report nutrients in parts per million, and Montana's nitrogen guidelines use pounds per acre. For a 0 to 6 inch sample, MSU's conversion is simply ppm times 2, so 6 ppm nitrate-N equals 12 pounds of nitrogen per acre. The conversion only works if the sampling depth is known, which is one more reason to sample at a consistent 6 inches and write the depth on the form. MSU also advises retesting every few years to monitor your fertility program.

Can a soil test explain weeds, moss or thin grass?

Sometimes. A test can show low nitrogen, a pH problem or high salts that weaken grass and let weeds in. But a lot of thin, weedy turf in western Montana comes from compaction, mowing too short, uneven irrigation or thatch, none of which shows up on a lab report. MSU also warns that herbicide damage, disease, insects, salinity and moisture stress can mimic nutrient deficiency symptoms.

When we look at a struggling lawn, we look at all of those together. Our core aeration and overseeding service deals with compaction, and the guides on common lawn weeds and lawn renovation explain what weeds and bare patches are telling you.

Putting the results to work

HJ Property Care & Tree Service is licensed for fertilizer, weed control and pesticide application, and our fertilizer and weed control programs are built on a seasonal schedule matched to the soil and the grass. If you have a soil test in hand, or want help deciding whether you need one, we are glad to look at it with you. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide.

Call (406) 215-7319 or request a free lawn care estimate for a property anywhere from Missoula and Lolo to Hamilton and Darby.

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