What Is Soil pH and How Is It Tested?

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What is soil pH and how is it tested? Soil pH is a number from 0 to 14 that tells you how acidic or alkaline your soil is, with 7 as neutral. It is measured in a laboratory or with a home kit, usually from a mixed sample taken across your bed or garden area. Most garden soil is not extreme, and the pH matters because it controls how easily plants can take up nutrients. The sources we opened agree that a soil test is the reliable way to learn your number, and that you should not add lime or sulfur without one.

Quick answer

Soil pH is the active acidity or alkalinity of the soil, on a scale from 0 to 14. Below 7 is acidic, above 7 is alkaline.
Purdue Extension states that “Every soil test should provide you with your soil’s pH, which measures the acidity or alkalinity in your soil.”
Lab testing reports a pH reading and, in most cases, a buffer pH that tells you how much lime is needed.
Home kits and meters give a quick reading, but borderline results are worth confirming with a lab before you change the soil.

What the person typing this wants to know

The person typing what is soil pH and how is it tested wants a plain definition of the number, a sense of whether their soil is in a good range, and a practical way to get the number. This article answers those three questions. It does not tell you how to change your pH step by step; the related guide on lowering pH in pots covers that for containers.

What soil pH actually measures

The pH scale is logarithmic, so each whole number is a tenfold change. Purdue Extension makes this point directly: “changing the pH from 7 to 6 is ten times” a change in acidity, which is why adjusting pH is slow and takes a measured approach.

The pH number does not describe how much nutrient a soil holds. It describes the environment that roots and soil microbes live in. Purdue notes that “Nutrients in the soil are most readily available to plants when the soil pH is” in a middle range, and that outside that range some nutrients become harder for plants to use even when they are present in the soil.

What range do garden plants need?

Most ornamental and vegetable plants grow well in slightly acidic to neutral soil, but the target depends on the plant. Purdue states that “most horticultural plants grow best when the soil pH is between 6.0 and 6.8 (slightly acidic).” Iowa State Extension gives a similar general range for Iowa soils, noting that pH “typically ranges from 5.5 to 7.5,” and that some plants, such as azalea or blueberries, prefer a lower pH of 5 to 5.5.

The useful takeaway is that a reading inside 6.0 to 6.8 is a reasonable general target, and a plant with a known preference should be checked against its own requirement. The sources do not describe a single ideal pH for every garden, so a number outside the general range is not automatically a problem.

Active pH and buffer pH

A lab report often shows two pH values, and they answer different questions. University of Maryland Extension explains that “the soil pH number is the actual pH reading and measures ‘active’ soil acidity,” while “the buffer pH is a measure of the ‘stored’ acidity.”

The buffer pH matters when you plan to raise pH. UMD states that “the buffer pH is important because it determines how much lime needs to be added to change soil pH,” and that “the lower the buffer pH (the closer it is to the actual soil pH), the more lime is required.” Iowa State says the same thing in its own words: buffer pH “is used only to determine how much lime to apply when raising pH,” and it is “different from the soil’s pH (do not confuse the two!).” Clay and organic-rich soils hold more stored acidity, so they need more lime to move the same amount.

How a soil pH test is done

A lab test depends on the sample. Purdue recommends collecting “several cores or slices” from one area, mixing them in a clean bucket, and submitting a single sample for that area. Purdue also advises sampling separately where soil looks different, such as a lawn versus a vegetable bed, and where some areas have been limed and others have not. The lab then returns a report with the pH and, usually, a buffer pH and a lime recommendation.

Purdue advises that you should “test your soil every three to five years,” and test sooner if you grow plants with specific pH needs. Collect the sample and submit it well before planting, so there is time to act on the result.

Home kits and meters

A home kit or electronic meter can give a quick estimate. Its limit is that it reads one spot, often with a small sample, and it may not report buffer pH. If the reading is near the edge of your target range, or you plan to add a large amount of lime or sulfur, send a sample to a lab. The lab result is the one to act on, because it includes the buffer value that tells you how much lime is needed.

How we chose

We limited this article to pages that define soil pH and describe how it is tested, and we used university extension guidance because it is written for growers and cites lab practice. We opened three pages on 2026-10-30: Purdue Extension publication HO-71-W, University of Maryland Extension’s guide to the soil test report, and Iowa State University Extension’s guide to soil test results. We chose these because each one states the pH definition and the sampling or report contents in plain language. We did not use forum posts, retail product pages or blog summaries.

What we could not verify

The sources do not give a single pH value that every garden plant needs. Each plant has its own range, and the sources we opened do not list every crop.
The sources do not say how accurate any particular home kit or meter is. We did not test any kit, and the sources we opened do not compare brands.
The sources do not give a specific lime or sulfur amount for your soil. That figure depends on the lab’s buffer pH and your target, and the report is where it should come from.
We did not find a source that gives a fixed schedule for how fast pH changes after lime is added, so this article does not state one.

What to do with your result

Read the pH number first. If it falls in the general 6.0 to 6.8 range, most vegetables and perennials should be fine. If it is well outside that range, compare it with the target for the plants you grow. Use the buffer pH on the report to judge how much lime might be needed, and follow the lab’s recommendation rather than a generic rate from a book. Purdue also warns that some gardens already sit near neutral or slightly alkaline, and in those soils added lime “will not help (and may hurt) nutrient availability.” Test before you add anything. If you are filling a new bed, our guide to filling raised beds explains how the soil layers go in, and our article on how much soil a raised bed needs covers volume.

FAQ

What is a good soil pH for most gardens?

Purdue Extension gives 6.0 to 6.8 as the range where most horticultural plants grow best. Some plants need a different range, so check the requirement for the crop you are growing.

Is a pH test the same as a soil test?

No. A pH test measures acidity alone. A full soil test usually includes pH and buffer pH, and may also report phosphorus, potassium, magnesium, calcium and organic matter. Purdue lists the common results as pH, phosphorus, potassium and calcium.

Why does my report show two pH numbers?

One is the active pH, which is the actual reading. The other is buffer pH, which measures stored acidity and is used to decide how much lime to apply. UMD and Iowa State both say the two should not be confused.

How often should I test my garden soil?

Purdue recommends testing every three to five years, and more often if you grow plants with specific pH or fertility needs.

Should I add lime if my soil is already neutral?

Not without a test. Purdue notes that many gardens have a pH already near neutral or slightly alkaline, where lime will not help and may hurt nutrient availability.

Bottom line

Soil pH is the acidity or alkalinity of your soil on a 0 to 14 scale, and the reliable way to learn it is a lab test that also reports buffer pH. Use a home kit for a rough reading, and use the lab result before adding lime or sulfur.

Sources

Purdue University Extension, “Collecting Soil Samples for Testing,” HO-71-W, https://www.extension.purdue.edu/extmedia/HO/HO-71-W.pdf (opened 2026-10-30)
University of Maryland Extension, “Understanding Your Soil Test Report,” https://www.extension.umd.edu/resource/understanding-your-soil-test-report (opened 2026-10-30)
Iowa State University Extension and Outreach, “How to Interpret Soil Test Results,” https://yardandgarden.extension.iastate.edu/how-to/how-interpret-soil-test-results (opened 2026-10-30)