A soil test is an essential tool for farmers and growers to understand the fertility and chemistry of their soils. Two important measurements on a standard soil test are soil pH and buffer pH. These values provide different insights into the soil. While soil pH reveals the active soil acidity buffer pH indicates the soil’s capacity to resist changes in pH. In this article we will explore what buffer pH is, how it differs from soil pH, and how to use buffer pH to make informed crop management decisions.
What is Buffer pH?
Buffer pH is an index that measures the ability of soil to resist changes in pH, also called the buffering capacity More specifically, it estimates the amount of reserve acidity or alkalinity present in the soil This reserve acidity/alkalinity neutralizes inputs like fertilizers and lime that would shift the pH up or down.
Soils with higher clay and organic matter content generally have a greater buffering capacity and resist pH change more than sandy soils lower in organic matter, The buffer pH indicates the soil’s ability to maintain a stable pH when acids or bases are added,
How Buffer pH is Determined
To measure buffer pH, the lab adds a small amount of calcium hydroxide, a basic compound, to the soil sample. This causes the pH to increase. The higher the buffer pH reading, the less the pH changes because more acidity in the soil sample neutralized the added calcium hydroxide.
Soils with more reserve acidity have lower buffer pH values, while soils with more reserve bases to neutralize inputs have higher buffer pH values. The buffer reaction occurs because of the exchange between hydrogen and aluminum ions on the clay and organic matter. Therefore, buffer pH gives an indirect measure of the exchange capacity of the soil.
Difference Between Soil pH and Buffer pH
While soil pH simply measures the active acidity or alkalinity of the soil solution, buffer pH evaluates the soil’s resistance to pH change.
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Soil pH is like measuring the current level of a reservoir, while buffer pH estimates the total size of the reservoir.
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Soil pH indicates the immediate soil acidity that plants experience, while buffer pH predicts how stable the pH will remain when lime or fertilizer is added.
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Soil pH is useful for determining if the current pH falls within the ideal range for crops. Buffer pH helps calculate how much lime is needed to raise or lower the pH to reach the target level.
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Soil pH can change significantly over months or years, but buffer pH changes very slowly as the soil exchange capacity changes.
Simply put, soil pH provides a snapshot of current soil acidity, while buffer pH evaluates the soil’s capacity to withstand pH fluctuations. Both values provide important information to make crop management decisions.
Using Buffer pH to Guide Liming
One of the main uses of buffer pH is determining lime requirements to raise soil pH. The lower the buffer pH, the more lime is needed to increase the pH because there is more reserve acidity to neutralize.
Here is an example lime requirement calculation based on buffer pH:
- Soil pH: 5.8
- Buffer pH: 6.2
- Target crop: Alfalfa (target pH 6.5-7.0)
With a soil pH of 5.8, liming is recommended because the pH is below alfalfa’s ideal range. Next, we look up the lime requirement based on the buffer pH reading of 6.2. According to standard recommendations, a buffer pH of 6.2 needs about 4 tons of ECCE lime per acre to reach pH 6.8. Converting ECCE tons to actual limestone tons depends on the ECCE rating of the available lime source.
The lower the buffer pH, the more lime is required to raise the pH. A small change in buffer pH equals a significant change in liming requirement. For example, a buffer pH of 6.0 would need about 6 tons of ECCE lime per acre to reach the target, compared to 4 tons at a buffer pH of 6.2.
Guiding Fertilizer Application Based on Buffer pH
Buffer pH also provides guidance on fertilizer management. Soils with lower buffer pH are more prone to fertilizer induced acidification. When nitrogen fertilizers like ammonium sulfate are added, soil acidity increases over time.
Soils with less ability to buffer changes require more attentive pH management. These soils need lime applied more frequently to counteract acidification from fertilizer additions. Monitoring soil pH trends over time is important, especially for soils with lower buffer pH values.
Soil test pH and liming: Common questions and answers
By: Dan Kaiser, Extension nutrient management specialistWhen it comes to soil tests, I commonly receive questions about pH and liming. Below are some common questions I get and my answers.
Soil Health Webinar Episode 2 – BufferIndex
FAQ
What is a good pH buffer for soil?
Soil pH, determined in a 1:1 slurry of soil:water, is a measure of the acidity (pH less than 7.0) or alkalinity (pH greater than 7.0) of the soil. In mineral soils (less than 20% organic matter) corn and soybean grow best when soil pH is in the range 6.0 to 6.5. Slightly higher pH, 6.5 to 7.0, is suggested for alfalfa.
What should buffer pH be?
| Buffers | Useful pH Range | pKa (at 37 °C) |
|---|---|---|
| MES | 5.5–6.7 | 5.97 |
| Bis-Tris | 5.8–7.2 | 6.36 |
| ADA | 6.0–7.2 | 6.46 |
| ACES | 6.1–7.5 | 6.54 |
What is a buffer on a pH scale?
A buffer is a solution that can resist pH change upon the addition of an acidic or basic components. It is able to neutralize small amounts of added acid or base, thus maintaining the pH of the solution relatively stable.
Is a 7.0 pH in soil good?
A pH of 6.5 is just about right for most home gardens since most plants thrive in the 6.0 to 7.0 (slightly acidic to neutral) range.
What is a buffer pH test?
Nutrient availability may be hindered if soil pH is not within the optimum range. A buffer pH (BpH) test is used to determine lime rate requirements. The amount of lime needed to increase soil pH to a desirable level can be estimated by mixing a buffer solution (with a known pH) to soil and then measuring the change in pH.
How does buffer pH affect soil pH?
In general, the change in buffer pH determines how much lime is needed to change the pH to the desired level (based on the crops being grown). The more reserve acidity, the lower the buffer index and the more lime required to increase the soil pH. Soil pH is a measure of the active acidity or the hydrogen ion concentration in solution.
What is the difference between water pH and buffer pH?
The water pH test is used to measure active soil acidity, while the buffer pH test measures the soil’s “reserve acidity” or pH buffer capacity, which is its ability to resist pH changes. While it is important to know the soil’s active acidity, it is only a portion, and usually small, of total soil acidity.
How does buffer pH affect total acidity?
Relating Buffer pH to Total Acidity Buffer pH correlates to your soil’s total acidity – the amount of acid molecules bound to the soil particles plus any free hydrogen ions in the soil water. Soils with higher buffer pH can store more acidity on particle surfaces before the soil water pH shifts down.
What is a good buffer pH?
Most productive agricultural soils will fall between 6.0 and 7.0 buffer pH, with an average around 6.5. Here are some general guidelines for interpreting your soil’s buffer pH: Below 6.0 – The soil has very low acid buffering capacity and is vulnerable to acidification. Limit acidifying fertilizers and closely monitor pH trends.
What is a low buffer pH?
Soils that take less lime to neutralize and raise the pH have a lower buffer pH, indicating they have less ability to resist pH changes up or down. Most productive soils in the U.S. fall between 6.0 and 7.0 buffer pH. Relating Buffer pH to Total Acidity