Demystifying Buffer pH on Soil Tests

Soil testing provides key information to farmers, gardeners and landscapers for effective fertilizer and soil amendment management. Two important values on a standard soil test report are pH and buffer pH. While pH reveals the current acidity or alkalinity of the soil, buffer pH provides additional insight into how resistant the soil is to pH change. Understanding buffer pH empowers you to make informed liming decisions.

What is Buffer pH?

On a chemical level, buffer pH measures the reserve acidity in the soil. It indicates how much resistance the soil will have to changing its pH when acidic or alkaline substances are added.

Physically, buffer pH provides an estimate of the soil’s capacity to store or supply acid or base components. Soils with higher clay content and organic matter have a greater buffering capacity than sandy soils lower in organic matter.

For agriculture and gardening purposes knowing the buffer pH determines how effective a liming treatment will be at raising the soil pH. It provides a guide for how much lime needs to be added to reach the desired pH for specific crops.

How Buffer pH is Determined

Buffer pH is measured as part of a standard soil test. The soil sample is treated with a liquid solution that has an alkaline pH of 7.5. The pH of the soil sample is then measured again.

If the pH increases significantly, the soil has a low buffering capacity – it takes less lime to change the pH. If the pH remains similar, the soil has a high buffering capacity, meaning it strongly resists pH change.

The resulting buffer pH value indicates the amount of resistance the soil showed to changing pH when the alkaline solution was added A higher buffer pH reading correlates to higher reserve acidity in the soil

Buffer pH Vs. Standard pH

While standard pH reveals the current acidity or alkalinity of the soil buffer pH provides additional information

• Standard pH measures active acidity – the current pH that plants experience.

• Buffer pH indicates reserve acidity – the soil’s resistance to pH change.

• Standard pH can fluctuate seasonally.

• Buffer pH changes slowly over years.

• Standard pH determines if liming is currently needed.

• Buffer pH shows how much liming is required to reach target pH.

Both values guide liming decisions, but buffer pH provides crucial data for determining lime application rates.

Liming Application Rates

Agronomists use buffer pH, standard pH and target pH to calculate liming recommendations. Here’s how it works:

• The standard pH reveals if the soil is too acidic for the crop.

• The target pH is the ideal pH for the crop being grown.

• The buffer pH indicates how much the pH will resist change when lime is added.

• Agronomists combine these values with calibration data to estimate the tons of lime needed per acre to reach the target pH.

Soils with higher buffer pH require greater amounts of lime to change the pH. Sandy soils with lower buffer pH often require frequent, small lime applications. Heavier soils demonstrate a stabilizing effect.

Why Buffer pH Matters

Buffer pH provides key insights into soil chemistry and physics:

• Estimates the soil’s clay content and organic matter which influence nutrient holding capacity.

• Reveals the soil’s mineral reserve for resisting pH change.

• Indicates the reserve of acidity to neutralize when raising pH.

• Allows accurate liming recommendations to achieve desired pH.

• Helps anticipate the soil’s long term liming needs and frequency.

Knowing the buffer pH improves liming efficiency. Over-liming wastes money. Under-liming produces poor results. Buffer pH provides the missing data point for optimum lime application rates.

Buffer pH Ranges

Buffer pH ranges from 6.5 to 7.5 for most productive agricultural soils. Here are guidelines:

• 6.5 – 7.0 – Low reserves of acidity; small amounts of lime needed to change pH.

• 7.0 – 7.5 – Moderate to high reserves of acidity; higher lime rates needed.

• Above 7.5 – Very high buffering capacity slowing pH change.

• Below 6.5 – Possible issues with calcium, magnesium or potassium deficiency.

A lower buffer pH indicates the soil is nearing a critically acidic condition for crops.

Examples and Interpretation

Here are two sample scenarios to demonstrate how buffer pH guides liming recommendations:

Scenario 1

• Soil Type: Sandy loam
• Current pH: 5.2
• Target pH: 6.5 (for corn)
• Buffer pH: 6.4

The buffer pH is relatively low indicating this sandy soil has minimal acidity reserves. To raise the pH from 5.2 to the target of 6.5 will require about 2 tons of lime per acre. Frequent, small lime applications will be needed to maintain optimal pH.

Scenario 2

• Soil Type: Clay loam
• Current pH: 5.5
• Target pH: 6.8 (for alfalfa)
• Buffer pH: 7.2

The higher buffer pH shows this clayey soil can strongly resist pH changes. To increase the pH to the target 6.8 will require a heavy liming of about 6 tons of lime per acre. The high buffering capacity means this large lime application should keep the pH stable for many years.

Balancing pH Key for Nutrient Availability

Understanding buffer pH equips growers to make wise liming decisions for optimizing soil pH. Maintaining the ideal pH range unlocks the full potential of the soil to provide nutrients, health and vigorous growth for crops and gardens.