Most plants have a preference for the degree of acidity or alkalinity in the soil, which is measured by a pH scale that ranges from 0 (extremely acidic) to 14 (extremely alkaline). Most garden soil across North America is naturally slightly acidic, falling in the 6.0 to 7.0 range and the vast majority of plants do fairly well in this range. But if you have soil that falls outside this range, you may find that you have trouble growing garden plants, vegetables, or even lawn grass.
There are some trees, shrubs, and flowers—such as hawthorne, lilac, and lavender—that prefer a more alkaline soil. There are also some acid-loving plants, such as azalea shrubs, spruce trees, and camellias, that like much more acidic soil.
A first step for new gardeners, or those seeking to grow new, unfamiliar plants, is to test the soil. It is possible to test soil pH by taking a soil sample and sending it away to a lab for analysis, or by using a pricey test kit you purchase. But a DIY soil test will give you immediate results and is easy to assemble and use. The whole process will take just 15 minutes or less. The results aren't as precise as a laboratory test, but will give you a ballpark idea if your soil is acidic or alkaline. An extreme reaction in either direction tells you to amend your soil to bring it into an ideal pH range for the plants you want to grow.
When to Test for Soil pH
It is a good idea to test the soil whenever you are planting a new garden bed or when growing a new plant variety that may have unique pH needs. Some experts recommend that gardeners test the soil every few years, especially when you have needed to amend the soil in the past. The materials used to adjust soil pH, such as elemental sulfur (used to lower soil pH), or lime (used to raise soil pH) will break down over time, and may need to be reapplied to keep the pH level at optimal levels.
Equipment / Tools
- Garden trowel
- 2 small plastic containers
- White household vinegar
- Baking soda
Collect a Soil Sample
Scoop some soil from the planting area into a clean plastic container. For best results take a blended soil sample. If you are planting a clearly defined garden space, then it's suitable to take your sample from a blend of samples taken from just that area. If you are planting a large area, such as an entire lawn, then it is best to take samples from many areas of the yard, then blend them together, and test a sample taken from that blended batch.
Testing a mixed sample will give you the most accurate results, since a single garden spot may give you uncharacteristic results. For example, a sample taken from an area near a pine shrub could give you non-typical acidic results, since pine needles tend to make soil acidic.
Test for Alkalinity
Add 1/2 cup of water to the soil sample and mix. Then, add 1/2 cup of vinegar. If the soil shows a visible bubbling or fizzing action, then it has an alkaline pH. The chemical reaction that you're seeing occurs when an acid (vinegar) comes into contact with something alkaline (soil). The more pronounced the fizzing action, the higher the pH.
Because most soils are naturally slightly acidic, any reaction at all with this test indicates that you probably have alkaline soil that may require an amendment to bring it to the pH range suitable for most plants.
Test for Acidity
Scoop another soil sample into a fresh container, add 1/2 cup of water, and mix. Then, add 1/2 cup of baking soda. If the soil bubbles or fizzes, the soil is acidic. The reaction you're seeing is the result of acidic soil coming into contact with an alkaline substance (baking soda). Again, the vigor of the action will give you some indication of how acidic your soil is.
A very small amount of fizzing is natural since most soils are slightly acidic to begin with. But a forceful reaction may indicate that you have very acidic soil. You will either need to amend the soil to raise the pH, or limit your plant selection to those species that like acidic soil.
Amend the Soil, if Necessary
If your tests show soil that is alkaline or very acidic, you can blend in amendments to bring the pH into a suitable range for the plants you want to grow. Wood ashes or agricultural lime will help raise the soil pH, making it less acidic and more alkaline. Pine needles or elemental sulfur are traditional amendments used to lower soil pH and make it more acidic.
But it can be quite difficult to gauge how much amendment is needed. At this point, you may want to invest in a laboratory test to determine the type of amendment required and the quantity needed. Your University Extension office usually offers services for testing soil, and the report you get will give you lots of detail on how to do it.
It is also possible to simply amend the soil yourself, starting with small amounts of acidifying or alkaline-building material. Test after each amendment, until you reach a fairly neutral soil pH.
Soil Testing Tips
- If you want a precise pH measurement, buy a soil test kit from your local home improvement store or university extension office. An extension office kit is likely to be the more expensive of the two options, but it will also provide you with more information. Expect your test results to include information about any mineral deficiencies that your soil has, along with recommendations on how you can make improvements.
- Soil amendment takes time, so make small changes and wait for them to take hold before making further amendments.
- You may have different types of soil in different parts of your yard, so consider testing each of your garden beds.
- Choose plants that will thrive in your soil. Hydrangeas and blueberries, for example, love acidic soil. Sometimes, it's just easier to work with the soil that you have than to fight it.
- Continue to test and tweak your soil over time. Maintaining healthy soil is an ongoing task.
- You don't have to spend money to improve your soil. There are free materials that you can add to your soil to improve pH, water retention, soil structure, and nutrient content. You can get free coffee grounds for your garden, use leaves in your garden, and even get free mulch.