Is Clay Soil Acidic Or Alkaline?

Clay soil can have either an acidic or alkaline pH, depending on various factors. The pH level refers to the acidity or alkalinity of the soil and is an important factor in plant growth and overall soil health. While some generalization can be made about clay soils tending to be more alkaline, a soil test is required to determine the exact pH level.

The pH scale ranges from 0 to 14, with 7 being neutral. Readings below 7 indicate acidic soil, while values above 7 denote alkaline soils. Most plants grow best within a pH range of 6 to 7.5. However, clay soils can easily become too acidic or alkaline for optimal plant health.

This article will examine the properties of clay soils and what makes them prone to pH extremes. We’ll look at how to test soil pH, along with ways to modify pH levels. Gardening considerations for acidic and alkaline clay soils will also be covered.

What Determines Soil pH?

Soil pH is determined by several key factors inherent to the soil’s formation and environment. Two major influences are the mineral content of the parent material from which the soil formed and the amount of rainfall and leaching that occurs.

The mineral content of the bedrock or sediment that develops into soil plays a role in its pH level. Some parent materials like limestone or shells contain minerals that make the soil more alkaline. Others like granite tend to weather into more acidic soils over time. The mix of cations (positively charged ions) in the parent material impact whether the resulting soil is acidic or basic (USDA NRCS, 2014).

Areas with heavy rainfall tend to have more acidic soils, while drier climates lead to more alkaline soils. As water moves through soil, it leaches basic cations like calcium, magnesium, and potassium out of the upper layers. This leaching makes the soil more acidic over time (Envirothon Pennsylvania, n.d.). The amount of rainfall and drainage affects the degree of leaching and pH change.

References:

USDA NRCS. (2014). Inherent Factors Affecting Soil pH [PDF file]. Retrieved from https://cropwatch.unl.edu/documents/USDA_NRCS_pH_guide_edit_6_3_14.pdf

Envirothon Pennsylvania. (n.d.). Inherent Factors Affecting Soil pH Soil pH Management [PDF file]. Retrieved from https://www.envirothonpa.org/documents/pH_guide.pdf

Clay Soil Properties

Clay soils have very small particles less than 0.002 mm in size (shreeramkaolin.com, 2023). The small size of clay particles gives clay soil some unique properties. One key property is the high cation exchange capacity (CEC) of clay soils (ahdb.org.uk, n.d.). CEC refers to the ability of soil to hold positively charged nutrients. The small size and plate-like structure of clay particles gives them a large surface area with many negative charges. This allows clay soils to bind and retain positively charged nutrients like calcium, magnesium and potassium.

Acidic Clay Soils

Acidic clay soils typically have a pH below 6.5. They are formed from acidic parent material like granite, and are more commonly found in areas with high rainfall amounts that leach calcium from the soil (Rivers Family Farm, 2024). The acidic conditions are partially due to the lack of calcium, which plays an important role in buffering against pH changes. Acidic clay soils tend to be lower in nutrients, and aluminum toxicity can become a concern below pH 5.5 (Penn State Extension).

Some key properties of acidic clay soils include (Quora, 2022):

• Formed from acidic parent material like granite



• Found in high rainfall areas that leach calcium
• Low in calcium and susceptible to aluminum toxicity
• Nutrient deficiencies more likely

When working with acidic clay soils, it is often necessary to add lime or other amendments to raise the pH. Certain nutrient deficiencies may need to be addressed as well. Careful management of acidic clay soils can allow healthy plant growth.

Alkaline Clay Soils

Alkaline clay soils are formed from calcareous or high calcium parent material typically found in arid and semi-arid regions. The high pH and calcium levels are due to the weathering of calcium carbonate minerals. Alkaline soils have a pH above 7.0 and can range to pH 10 or higher.

The main properties of alkaline clay soils include:

• High pH – pH levels above 7.0, usually between 8.5 to 10.
• High calcium carbonate content – Contains lime deposits from the weathering of parent material.
• Poor soil structure – Clay particles bind tightly together resulting in dense, heavy soils.
• Low infiltration rates – Water has difficulty penetrating the dense clay soil.
• Prone to compaction – Easily compacted from irrigation, rain, or traffic.
• High alkalinity – Excess sodium salts impair nutrient availability.
• Restricted root growth – Dense clay limits root system development.

Alkaline soils are also referred to as “caliche” or calciorphic soils in arid regions where calcium carbonate accumulates. They are one of the most difficult soils to manage for agriculture and gardening due to the poor drainage, hardness when dry, and crusting when wet.

Source: https://en.wikipedia.org/wiki/Alkali_soil

Determining Soil pH

There are a few methods available for testing your soil’s pH at home without expensive equipment. Simple dye indicator test strips or kits provide a quick and easy way to get a general sense of your soil’s acidity or alkalinity. These test strips contain dyes that change color depending on the pH. You simply take a soil sample, mix it with distilled water, and dip the test strip in to determine the approximate pH range based on the color change (source: https://www.thespruce.com/do-it-yourself-soil-ph-test-4125833).

For the most accurate pH readings, an electronic pH meter is recommended. These meters measure the electrical conductivity of the soil solution to determine the exact pH number. While more expensive than strips, they provide precise, quantitative results. To use a pH meter, take a representative soil sample, mix with distilled water, allow to settle, then insert the probe in the clear supernatant liquid (source: https://www.almanac.com/content/3-simple-diy-soil-tests).

No matter the testing method, it’s important to take samples from several areas of your garden and at various depths to get a complete understanding of your soil’s pH.

Modifying Soil pH

The pH of clay soils can be modified by adding soil amendments. To lower soil acidity, garden lime (calcium carbonate) can be added. According to Clemson University, garden lime is an effective way to raise the pH of acidic clay soils. It works by neutralizing acidity and increasing the availability of key plant nutrients like calcium and magnesium.

To increase soil acidity in alkaline clay soils, elemental sulfur can be added. As noted by Iowa State University Extension, sulfur oxidizes to form sulfuric acid which lowers pH. Sulfur should be thoroughly incorporated into the soil before planting. It’s important to test soil pH again after a few months to determine if additional applications of lime or sulfur are needed.

Plant Preferences

Most plants prefer slightly acidic soils with a pH between 6.0-6.5. This includes vegetables like potatoes, tomatoes, radishes, and leafy greens as well as most ornamental flowers like roses, lilies and impatiens. Acid Loving Plants – Shrubs​​ Popular acid-loving plants include: Azaleas. Rhododendrons. However, some plants need more acidic soils to properly absorb nutrients. Blueberries require very acidic soils with a pH below 5.5 to thrive.

Gardening Considerations

When gardening in clay soil, it’s important to match plants to the native soil pH. Most plants prefer a pH between 6.0-7.0, though some prefer more acidic or alkaline conditions. According to the University of Utah Extension, clay soils in Utah tend to be slightly alkaline with a pH between 7.4-8.4. Some plants that grow well in alkaline clay soil include lilacs, sagebrush, junipers, flowering pear, and Kentucky bluegrass.

If your soil test shows a high or low pH outside the ideal range for your desired plants, you may need to amend the soil to adjust the pH. To lower pH in alkaline clay soils, you can add elemental sulfur, pine needles, leaf mold, peat moss, or cottonseed meal. To raise pH in acidic clay, you can add lime or wood ashes. When amending soil, it’s best to mix in the additives thoroughly and test again after a few weeks to see if the pH has shifted to the target range. With proper amendments and plant selections, you can have a vibrant garden in clay soil.

For more details, see the references from Utah State University and The Spruce.

Conclusion

In summary, clay soil can be either acidic or alkaline based on its source material and environmental factors. The clay particles themselves do not determine the soil pH. Rather, the pH depends on the chemical composition of the parent material that weathered into clay and the conditions where the soil formed. While some types of clay tend to be more alkaline, such as those derived from limestone, clays can form under different conditions leading to variable pH levels. Acidic clay soils are typically found in humid regions where rainfall leaches basic minerals from the soil, while alkaline clays occur in drier areas. Gardeners and farmers need to test their own soil’s pH, rather than assume it is acidic or alkaline based on texture alone. With proper amendments, both acidic and alkaline clay soils can be modified to reach an ideal neutral pH range for growing most plants.