Understanding Buffer pH on a Soil Test

Soil testing is an essential practice for farmers and growers to determine the fertility and health of their soil. Two important measurements on a standard soil test are soil pH and buffer pH While related, these tests provide different information that together give a complete picture of the soil’s acidity and liming needs In this article, we will explain what buffer pH is, how it differs from regular soil pH, and how to use buffer pH results from a soil test report.

What is Buffer pH?

Buffer pH specifically measures the reserve acidity in the soil. This refers to acidic ions attached to the soil particles that have not yet been released into the soil solution. Soils contain mineral compounds like clays, organic matter, and oxides that can absorb and release hydrogen (H+) and aluminum (Al3+) ions, which determine acidity. Buffer pH provides an index of the soil’s capacity to resist changes in pH by neutralizing added acid or base.

Soils with a lower buffer pH have less ability to resist pH change and often need more frequent applications of lime to raise pH. Heavy clay soils typically have a higher buffering capacity and require more lime to change pH than sandy soils, for example. Think of buffer pH as assessing the soil’s “acid bank account”—the amount of acidity stored and available to be released.

How Buffer pH Differs from Regular Soil pH

Regular soil pH is a measure of the active acidity—the hydrogen ion concentration already present in the moist soil solution that plant roots are exposed to. Soil pH directly affects nutrient availability and soil biology. Most plants grow best within a pH range of 6.0-7.0, though some prefer more acidic or alkaline conditions.

While soil pH measures current acidity, buffer pH evaluates reserve acidity Buffer pH will always be higher (less acidic) than soil pH, since the buffer test measures potential rather than active acidity. For example, a soil sample could have a pH of 55 (acidic) and a buffer pH of 6.2.

Why Buffer pH Matters

Together, the soil pH and buffer pH results on a soil test report provide a complete picture of the soil’s overall acidity status. Soil pH indicates if the soil is currently at the right pH for the crop to be grown Buffer pH predicts how much lime is needed to raise the pH to the desired level for optimum plant growth and productivity

Soil pH change is buffered by the reserve acidity measured by buffer pH. Soils with a lower buffer pH will require less lime to increase soil pH than soils with a higher buffer pH and more acid reserves. Accurately measuring buffer pH allows lime recommendations to be finely tuned for each field.

How to Use Buffer pH Results

When interpreting your soil test report, first look at the soil pH result. If the current pH is in the optimal range for your crop, lime is likely not needed regardless of buffer pH. For example, a soil with a pH of 6.2 may not need lime for wheat or soybeans, even if the buffer pH is quite low.

However, if the soil pH is below the ideal range, consult the buffer pH to determine the appropriate lime application rate to raise the pH. Soils with a lower buffer pH require less lime to change pH than those with a higher buffer pH. An accurate, field-specific lime recommendation is one of the biggest benefits of testing buffer pH.

Here are some guidelines for understanding buffer pH results:

• Buffer pH less than 6.2 indicates a low buffering capacity and lime requirement. Sandy soils often fall in this range.

• Buffer pH of 6.2-6.8 is moderate. These soils need moderate lime applications to change pH.

• Buffer pH above 6.8 represents a higher reserve acidity and lime demand. Heavier clay soils commonly have a high buffer pH.

Matching lime applications to the buffer pH prevents over-liming sandy soils or under-liming heavy soils. Consider retesting buffer pH every 2-3 years to monitor any changes in acidity reserves.

Additional Soil Acidity Tests

Some labs offer additional advanced soil acidity tests beyond the standard soil pH and buffer pH. These can provide further insight but come at a higher cost:

• Lime requirement test: Directly measures how much lime is needed to reach a target pH.

• Exchangeable acidity: Quantifies acidic cations on soil particle exchange sites.

• Base saturation percentage: Indicates nutrient cation ratios on the exchange complex.

• Cation exchange capacity (CEC): Measures the soil’s total exchange capacity.

Talk with your soil lab about whether these extra tests would be useful for your soils and cropping system.

Get the Most from Your Soil Test

Useful pH Ranges of Selected Biological Buffers Chart (25 °C, 0.1 M)