Soil testing and pH
Before you add lime or sulfur, a soil pH test shows how acidic or basic your soil is. Most plants prefer neutral soil, but some grow best in more acidic soil, for example blueberries, rhododendrons and azaleas. This guide covers when to test, how to collect and submit a sample, and how to read the result. It then covers how to raise or lower pH, and the mistakes that cause trouble along the way. Soil pH levels range from 0 to 14, with a reading below 7 indicating an acidic pH, 7 a neutral pH, and above 7 a basic pH.
When to do it
Test your soil every three to five years, and also whenever you make a change such as converting a lawn to a garden bed. It is good to test in spring before planting, or in fall after the garden has been harvested. The interval depends on what you grow: test annual crops, pastures and legumes every 2 to 3 years, and test Christmas trees, fruit and nut trees, berries and grapes every 3 to 5 years. The Clemson University Extension Service recommends soil sampling every year. For a no-till area, collect several representative cores less than 2 in (5 cm) deep to check the surface pH. If that shallow sample reads below 6.2, take a standard, full-depth sample for laboratory analysis. If your soil pH tests between 5.5 and 7.0, there is no need to adjust it.
What you need
A plastic bucket, a shovel or soil probe, and a paper bag or newspaper are the basic items for taking a soil sample. Use clean sampling tools. Do not use galvanized metal, brass or bronze tools for a sample that will be tested for micronutrients such as zinc. You will mix your cores in a clean plastic bucket. For the lab, bring a minimum of 2 cups of soil per composite sample in a clean jar or zip-lock bag. Some labs ask you to pack about 1 pint of soil, filling to the dotted line, into their sample bag. A local county Extension office can provide free soil sample bags and submission forms. Label each bag with your name, address, county and the type of lawn, crop or landscape plant. A home pH test kit or device is an option, but these generally do not give results as accurate as sending a sample to a soil testing service.
Step by step
- Step 1
Test your soil every three to five years, and test again whenever you make a big change, such as turning a lawn into a garden bed
Spring, before you plant, or fall, after you clear the garden, are both good times to sample.
- Step 2
A soil pH test tells you how acidic or basic your soil is
Soil pH runs on a scale from 0 to 14, where a reading below 7 is acidic, 7 is neutral, and above 7 is basic. That scale is logarithmic, so a soil pH of 6.0 is ten times more acidic than a pH of 7.0.
- Step 3
Most plants grow best in neutral soil, but a few, such as blueberries, rhododendrons, and azaleas, prefer more acidic soil
Azaleas, rhododendrons, blueberries, and conifers do best in acidic soil with a pH of 5.0 to 5.5, while centipedegrass prefers a pH of 5.5 to 6.0. Most vegetables, turfgrasses, and ornamentals do best in slightly acidic soil, with a pH of 6.0 to 6.5. The best pH range for a vegetable or flower garden on sandy soil is 5.8 to 6.3. Highbush blueberries do best at a pH of 5.0, so test your soil before you plant them.
- Step 4
Send your sample to a soil testing lab rather than rely only on a home pH kit or meter
Home soil pH test kits and meters generally give less accurate results than sending a sample to a lab.
- Step 5
Gather clean sampling tools before you start
You need a plastic bucket, a shovel or a soil probe, and a paper bag or newspaper to hold and dry the soil. Skip galvanized metal, brass, and bronze tools if you plan to test for micronutrients such as zinc.
- Step 6
Collect your soil cores from the right depth for what you are sampling
For a lawn, sample to a depth of 2 to 4 inches, and for a garden or bed, sample to a depth of 6 inches. Other guidance puts the ideal depth for most garden plants at about 6 to 8 inches. If you use no-till methods, sampling deeper than 2 inches can hide pH changes that have built up near the surface.
- Step 7
Collect soil from the area where the plant's roots will grow
Remove roots, grass, and mulch from the sample so your results are accurate.
- Step 8
Take several cores across the sampling area and combine them into one sample
Collect 8 to 10 or more cores, walking the area in a zigzag pattern and taking soil from 10 to 15 spots. Some guidance instead combines subsamples from 15 to 20 locations into a single sample. Mix the core samples together thoroughly in a clean plastic bucket.
- Step 9
Submit soil from areas with different fertilizing or liming histories, such as lawns, shrub beds, and vegetable gardens, as separate samples
Avoid taking soil from unusual spots, such as old manure piles, fertilizer bands, or fence lines. If you want to check a specific problem spot, take a separate sample from that area instead of mixing it into the general sample.
- Step 10
Let a wet sample air dry before you bag it
Spread the soil out on newspaper or a paper grocery bag and let it dry fully. The sample must not be excessively wet when it reaches the lab. If you are requesting a nitrogen test, keep the sample cool and send it to the lab right away.
- Step 11
Pack enough soil into a clean container and label it clearly
Bring at least 2 cups of soil per sample in a clean jar or zip-lock bag. Some labs instead ask for about 1 pint, packed into a soil sample bag and filled to the dotted line. Label the bag with your name, address, county, and the type of lawn, crop, or landscape plant. Name each sample and make a map, so lab results can be matched to the right spot in your yard. A local county Extension office can give you soil sample bags and submission forms at no cost.
- Step 12
Expect results about two weeks after you submit your sample
A copy of the soil analysis is usually emailed or mailed within seven to fourteen days. Giving an email address can get your results to you more quickly.
- Step 13
Look for phosphorus, potassium, calcium, magnesium, zinc, manganese, copper, and boron on a standard soil test report
Most labs do not recommend testing for nitrogen, since available nitrogen levels change quickly because nitrogen moves easily through the soil.
- Step 14
For a no-till area, check the surface pH with a separate, shallow sample
Collect several cores less than 2 inches deep and mix them in a clean bucket to check the surface pH. If that shallow sample tests below pH 6.2, take a standard, full-depth sample for lab analysis.
- Step 15
Check whether your results call for a pH change at all
If your soil pH tests between 5.5 and 7.0, you do not need to adjust it. When mineral soil pH drops below 5.5, aluminum toxicity can reduce plant growth. In acidic soil, potassium, calcium, and magnesium become less available, while aluminum, iron, and zinc become more available. In alkaline soil, iron, manganese, zinc, and boron are often in short supply.
- Step 16
To raise the pH of acidic soil, add a liming material such as calcium carbonate or dolomite
Test for the lime requirement before you apply any liming material, rather than guessing the amount. For lime to work well, mix it thoroughly into the top 6 to 8 inches of soil.
- Step 17
To lower the pH of alkaline soil, apply elemental sulfur, since sulfate forms of sulfur will not lower soil pH
In planted areas, apply no more than 7 lbs of elemental sulfur per 1,000 square feet to avoid burning plants.
Mistakes to avoid
Guessing at the amount of lime is a common error, because you should test for the lime requirement before applying any liming material. Gypsum and Epsom salts are not liming materials, and sulfate forms of sulfur will not lower soil pH. Problems from over-liming begin to appear any time soil pH is raised above about 6.3. Time is the best cure for over-limed soil, and there is no quick fix. The indiscriminate addition of lime or a fertilizer containing phosphorus may create nutrient imbalances. Lowering pH has its own limits: do not go below 5.0, because of the potential for aluminum toxicity. Excessive applications of aluminum sulfate may injure roots and cause aluminum accumulation, so avoid repeated applications without retesting. Adding sulfur at high rates or applying it too frequently can damage plants. Mixing samples from areas with different fertilizing or liming histories, or sampling unusual spots such as former manure piles, gives a poor picture of your soil. If blueberry soil pH is too high, it will cause chlorosis, a yellowing of the leaves. Finally, do not taste soil in an attempt to determine its pH.
How this page was made
- Written with AI assistance from the sources listed on this page. AI is never the source of a fact.