Acidic Soils Suit Some Plants!

Soil acidity or alkalinity can be measured using pH. A pH of 7 is neutral. Values below 7 are considered acidic while those above, alkaline. Soil pH is an important characteristic for soil health because it affects many other soil characteristics, such as nutrient availability, cation exchange capacity, and base saturation, all of which in turn affect a soil’s capacity to provide and hold nutrients for plants.

Although the optimum soil pH for both plant growth and soil fertility is between 6 to 7, many plants can tolerate a soil pH as low as 5.5. Some like rhododendrons, azaleas, blueberries and others in related families prefer the soil pH to be 5.5 or lower. Sometimes, the soil needs to be acidified to a lower pH in order for plants to thrive.

As a garden lover, I have always admired those beautiful hydrangeas that come in many different varieties and colors. The bigleaf hydrangea (Hydrangea macrophylla) or mountain hydrangea (H. serrata) flowers open a beautiful blue when the soil pH is below 5. As the soil pH increases, the flowers turn pink or somewhere in between depending on how much higher the soil pH is. This is because the plant requires aluminum in the tissue of the hydrangea flowers in order for them to show their heavenly blue color. Although our soils in CT have plenty of aluminum, it is not available to the plants when soil pH is high.

Sky blue color in hydrangeas is due to alumimum. Photo by Haiying Tao, 2023.

The soil pH is not the same around the hydrangea plant leading some flowers to be more pinkish. Photo by Lisa Rivers.

Another plant that we commonly have in our gardens are blueberries, which provide both beauty and food for us. The ideal pH for blueberry plants is around 4.5 to 5.5, where iron is more available in the soil. When soil pH is high, iron availability is low and iron chlorosis can occur in blueberry plants, which can significantly reduce blueberry plant health and yield.

Blueberry with leaves exhibiting symptoms of iron chlorosis due to the soil pH being too high. Photo by DM Pettinelli.

Most soils in Connecticut naturally have a low pH but occasionally we may have to acidify our soil for specific plants. One of the cheapest ways to acidify soil is to apply sulfur. The soil bacteria will convert the sulfur to sulfate and lower the soil pH. To acidify soil for growing blueberry plants, iron sulfate, also called ferrous sulfate, is a good choice since it will also provide a soluble source of iron to the plants while also acidifying the soil. When acidifying the soil for blue hydrangea flowers, aluminum sulfate is a better choice because it provides the aluminum that the flowers need to turn blue.

Keep in mind that most of agricultural crops prefer a slightly acidic to near neutral soil pH (6.5 – 7.0). To learn the optimum range of soil pH for common crops, please refer to this publication by UConn Soil and Nutrient Analysis Laboratory: https://soiltesting.cahnr.uconn.edu/wp-content/uploads/sites/3514/2022/06/Plant-pH-Preferences-List.pdf. Many soils in CT have a soil pH lower than the optimum range for plants like vegetables and lawns. For better soil and plant health, the soil pH may need to be increased. Soil amendments, such as calcitic or dolomitic limestones, can be used to increase soil pH. Applying the right amount of limestone is key to increasing the soil pH to the target level for a particular plant while avoiding over-liming. Wood ashes and compost can also raise the soil pH.

The easiest way to determine if limestone or sulfur are necessary and how much to apply is by submitting a soil sample to the UConn Soil Nutrient Analysis Lab (www.soiltesting.cahnr.uconn.edu). Whenever collecting a soil sample, be sure to take a representative one. This is done by taking several subsamples from the top to about 6 inches down, placing these subsamples in a clean container, mixing them up and then scooping up a cup of soil. This would give you a more representative sample than if you just collected soil from one spot.

There are some home pH testing kits, and they vary in their accuracy. One might try collecting a sample, mixing it thoroughly and dividing it in half. Send half to a lab and test the other half yourself to see if the results are comparable.

Haiying Tao

Assistant Professor, PSLA

 

Soil pH – The Master Variable

The UConn Soil Nutrient Analysis Lab tests for and analyzes multiple soil parameters; but none as critical, and as often overlooked, as pH. Soil pH plays a crucial role in the growth of vegetation planted, as well as ground water quality. Before we start talking about soil pH, I think it is a good idea to try to define what exactly pH is, and how it is determined.

When most of us think of pH, a pool probably comes to mind. I remember growing up, watching my mother apply different chemicals to our pool, and impatiently wondering why I had to wait to go swimming. She would tell me that she was adjusting the pH of the water to ensure it was safe to swim in. The basic understanding is that pH is tells us how acidic, neutral, or alkaline something is. To get a little more technical, pH is the measurement of the activity of Hydrogen Ions (H⁺) in an aqueous solution. The equation for determining and quantifying pH is:

pH = -log₁₀ (a_H+)

(a_H+ = Hydrogen Ion Activity in Moles/L)

We express pH on a logarithmic scale of 0-14, where 0-6 is considered “acidic”, 7 is “neutral”, and 8-14 is “basic”.

(Image from: http://www.edu.pe.ca/gulfshore/Archives/ACIDSBAS/scipage.htm)

Mineral soil pH values generally range from 3.0 – 10.0. There are numerous factors that determine soil pH including climate, parent material, weathering, relief, and time. Texture and organic matter content also influence soil pH. Most Connecticut soils are naturally acidic. Nutrient availability is directly influenced by pH with most plants (with some exceptions) thriving at pH values between 6 and 7. A majority of nutrients are available within this range.

(Image from: http://www.pda.org.uk/pda_leaflets/24-soil-analysis-key-to-nutrient-management-planning/)

Our lab measures pH using an 1:1 soil-to-DI water ratio. The saturated soil paste is mixed, then is analyzed using a glass electrode and a pH meter. We calibrate our meter using 2 solutions with known pH values, 4 and 7. We use these values because we expect most Connecticut soils to fall within this range. Once the initial pH value is obtained, a buffering agent is added. In our lab we use the Modified Mehlich Buffer. A second pH reading is obtained, and from these two values plus crop information, we are able to make limestone and/or sulfur recommendations.

The Buffering Capacity of a soil is the resistance it has to change in pH. Soil buffering is controlled by its Cation-Exchange-Capacity, Aluminum content (in acidic soils), organic matter content, and texture. A soil with a lot of organic matter and clay will have a higher buffering capacity than one with little organic matter that is mostly sandy.

If the soil pH is lower than the target range for a particular plant, limestone would be recommended. Whether you use pelletized, ground or granular limestone, the application rate would be the same. Once the target pH is reached, a maintenance application of 50 lbs/1000 sq ft would be applied every other year to maintain it.

If the soil pH is higher than desired, sulfur recommendations are made. Typically only powdered sulfur is available locally but granular sulfur could be mail ordered. Aluminum sulfate can be substituted for sulfur and used at a higher rate. Check out this list of preferred pH ranges for many common plants.

Monitoring your soil pH is essential to ensure that it is falling within the range best suited for the vegetation you are growing. The Standard Nutrient Analysis performed at our lab gives you a pH value, a buffer pH value, a lime/sulfur recommendation, available micro & macro nutrient levels, and a fertilizer recommendation. For more information on pH, you can contact Dawn or myself (Joe) at the UConn Soil Nutrient Analysis Lab (www.soiltest.uconn.edu)!

Test, don’t guess!

Joe C.