“What’s the best way to ensure that my blue [bigleaf] hydrangea stays blue?” Question from Gaye of Saint Peters, Missouri
Answer: It all has to do with soil pH. There are two hydrangeas that have flowers whose color changes depending on whether the soil is acidic (3.5-6.8), alkaline (7.2-10), or neutral ( around 7). These are bigleaf hydrangeas (Hydrangea macrophylla, Zones 5-11) and Japanese mountain hydrangeas (Hydrangea serrata, Zones 6-9). If your flowers are on the pinker side then your soil is more alkaline, and if they are erring towards purple then your soil is more neutral. To achieve flowers with a bluer hue, you will need to lower your soil pH. There are several ways to do this.
When asked about soil and gardens, the most common question I receive is about how to determine soil pH. Garden professionals emphasize the need to measure soil pH because it can be so important if the soil pH errs towards the extremes–too acid or too alkaline. Soil pH testing is certainly useful when you are getting to know your soil, and it can be managed once identified. It can also help one devise a game plan for what to grow where.
If you want precision soil pH numbers, your best bet is to reach out to your local extension agent for professional testing (click here to learn more), but this can be costly, and precise numbers are not always needed. I recommend this route for gardeners living in areas where soils tend to be more extreme.
For home gardeners living in areas with more average loam soils, it is not necessary to get exact measurements of your soil pH from all the different spaces of your garden. Simple, home-grown tests will help determine if your soil is acidic, alkaline, or neutral. Once you know that, you will know what grows best in your soil, and you can work with it.
The Importance of Soil pH
pH is designated on a scale from 1 to 14; if you have a pH around 7 your soil is neutral, if your pH is less than 7 it is considered to be acidic, whereas a pH greater than 7 is alkaline. At the far extremes, hydrochloric acid (0 pH) has an extremely low pH, and caustic lye (12 pH) has an extremely high pH. Most plants grow very well in neutral or near-neutral soils, though some plants are specially adapted to more acid or alkaline soils.
For gardeners, soil pH can be an important determinant of what plants will thrive or struggle within a garden. As a general rule, most vegetables and flowers prefer neutral soils. Some plants, like camellia, blueberry, and rhododendrons, prefer more acid soils, while others, like crabapples, lilac, clematis, and sweet peas, tolerate more alkaline soils.
Some plants will grow in a variety of pH levels but show differences in how they grow. For example, bigleaf hydrangeas (Hydrangea macrophylla) will bloom blue when they are in soil that is more acidic and pink in more alkaline soil.
Collecting a Soil Sample
To test your soil at home, you are going to need to collect a sample. To do this, grab a trowel and dig three or four inches down. Scoop the soil into a bowl, making sure not to contaminate it by touching it. It’s a good idea to collect samples from three to five different areas of your garden to get a good idea of the average pH of the soil you will be working with. Mix them, and then perform the following tests.
Vinegar and Baking Soda DIY pH Test
(These tests are from the book Garden Alchemy: 80 Recipes and Concoctions for Organic Fertilizers, Plant Elixirs, Potting Mixes, Pest Deterrents, and More (Cool Springs Press, 2020). Reprinted with permission.)
This simple test for soil pH will tell you if you have alkaline, acidic, or neutral soil using household vinegar and baking soda. White vinegar (2.5 pH) is acidic, and sodium bicarbonate or baking soda (8.4 pH) is alkaline. When they are mixed, they fizz. With two samples of soil from the same place in your garden, adding these ingredients will help to determine your soil’s pH type.
2 small bowls
4 tablespoons soil
Distilled water (do not use tap water as distilled water has a neutral pH)
Add 2 tablespoons (30 ml) soil to a small bowl and mix it with 1 tablespoon (15 ml) of distilled water.
Add 1 tablespoon (15 ml) white vinegar in and stir. If the mixture fizzes, you have more alkaline soil.
Add 2 tablespoons (30 ml) soil from the same sample to a small bowl and mix it with 1 tablespoon (15 ml) of distilled water.
Add 1 tablespoon (15 ml) of baking soda and stir. If the mixture fizzes, you have more acid soil.
If it does not fizz for either test, you have neutral soil.
Once you know if you have acidic, alkaline, or neutral soil, congratulations! You now know what will grow best in your garden!
Managing Soil PH
Many folks want to take on the job of altering their soil pH if they are largely at one end of the scale or the other. There are various products one can use to manage soil pH. If you have acid soil, you can add liming agents, such as hydrated lime, in addition to more neutral soil amendments, such as Black Gold Earthworm Castings. If your soil is very alkaline, add more acid amendments, such as Black Gold Canadian Sphagnum Peat Moss, as well as acidifying fertilizers, such as elemental sulfur (click here to learn more about acidifying soil). For both liming or acidifying soil agents, be sure to follow the manufacturer’s guidelines for application.
Another practical option for managing soil pH is to only grow native plants that will naturally thrive in your soil, and reserve other plants less adapted to your soil to container gardens or raised beds (click here to learn more about Black Gold® Natural & Organic Raised Bed & Potting Mix). Either way, it always pays to amend your soil with great quality Black Gold Garden Compost Blend. Then celebrate your unique garden soil, and the plants that nature intended you to grow there.
“I have paid twice, and can’t get my local university extension to perform a soil test and return my results. Is there a simple way I can determine the most common concerns like PH, potassium, phosphorus and such?” Question from Douglas of Greer, South Carolina
Answer: Well, that’s not right. Extension service soil tests are the best. Try reaching out to the administrative staff in the Clemson University Horticulture department when they are available. I have found that the folks at Land Grant Universities, such as Clemson University, are very friendly and helpful. They will certainly help you get what you paid for or be reimbursed. See if you can’t get through to someone, even though the university is not currently open. The website lists some numbers and email addresses.
Unless you’re a professional grower, you should not have to test your soil more than once. A gardener can expect some changes in nutrients and organic matter, especially in vegetable gardens, but pH should not really change much unless you’re actively adding liming or acidifying products to adjust an extreme soil pH. There are home soil testing kits that work moderately well, such as the Rapitest Premium Soil Test Kit, but these won’t be as accurate as soil tests from an extension service.
Soil tests are most essential when garden plants show significant signs of nutrient deficiency. If they do not, simply feed your plants regularly, and add quality organic matter to your soil yearly, such as Black Gold Garden Compost Blend and Peat Moss.
“Is there any way to tell if you soil is acidic or alkaline, by the weeds that grow instead of a soil sample?” Question by Barbara of Youngstown, Ohio
Answer: This is an interesting question, but sadly the answer is, not really. Still, there are some great sources that can help you easily determine your soil’s pH.
Lots of research has been done on ecological “indicator” species, but many complex factors are at play in assigning true indicator species, such as climate, topography, soil pH, soil composition, competing plant species, etc. Weeds are never assigned as indicator species because weeds quickly adapt to new environments or changes in the environment.
According to a comprehensive research piece titled Weeds as Indicators: “Weed species or communities must not be used as [soil] indicators. The use of plants as indicators for soil types was sometimes successful (Ellenberg 1951; Meisel 1960; Hilbig 1967; Kuhn 1973; Weller 1978), but often without good results and therefore followed by the conclusion that weeds cannot be used as indicators.”
The Ohio State University Extension Service offers great soil testing services (Click here to learn how). I also encourage you to check out the USDA’s Web Soil Survey (Click here to view). They have “soil maps and data available online for more than 95 percent of the nation’s counties.” Pretty cool!
“What is the average pH range of your various gardening soils? I bought 2 varieties – the Waterhold, and the Natural & Organic Potting Mix + fertilizer.” Question from Michael of Scottsdale, Arizona.
Answer: Great question Michael! The right pH is essential for growing good potted crops and container specimens. That’s why Black Gold mixes are fortified with alkaline Dolomite Lime to naturally increase their pH to a target of 6.5–a close-to-neutral pH ideal for many garden plants. We then test our mixes to make sure their pH is within the correct range.