Agronomy Talk

Agronomy Talk: Soil pH vs. Buffer pH

Published on Friday, October 22, 2021

 

A soil’s pH is the most basic, yet most important value, for determining nutrient availability. Most crops such as corn, soybeans, wheat, and alfalfa prefer a soil pH in a range of 6.3 to 6.8. Soil testing labs will report pH in two ways, soil pH and buffer pH (Figure 1).

 

 

There is often confusion regarding these values. Soil pH is determined using a slurry method of onepart distilled water to one part soil. The soil pH measures the soil’s “active acidity,” aka hydrogen ion concentration in the soil solution, and is reported using a logarithmic scale. For example, a soil pH of 5.3 has ten times more acidity than a soil pH of 6.3. Furthermore, a soil pH of 4.3 has 100 times more acidity than a soil pH of 6.3.

 

 

One of the most important things to remember is soil pH will determine IF a liming source is needed to help correct (reduce) acidity. Buffer pH will determine the QUANTITY of liming material needed. Buffer pH measures the “reserve acidity” and determines the effectiveness a liming material will have to change the soil pH. It essentially measures the resistance of the soil to change pH. Buffer pH is determined by adding a solution with a pH of 7.5 to the soil sample, and then measuring how much the pH changes. The higher the pH after the buffer solution has been added tells us that the soil pH will adjust well and be easily corrected with a small amount of liming material. Conversely, if the buffer pH is close to the initial soil pH, that indicates a high buffering capacity and the need for greater amounts of liming material.

 

Soil type plays a significant role in how rapidly soils can acidify and how much liming material is needed to correct soil acidity. Sandy soils acidify quicker because they have a low buffering capacity, whereas loams and clays have higher buffering capacities and tend to resist pH changes better. Furthermore, it will take more liming material to raise the soil pH of clay soils than loamy soils. Likewise, it will take more liming material to raise the soil pH of loamy soils than sandy soils.

 

Lime Rate Recommendations

Some state universities base lime rate recommendations on measured soil pH, combined with soil type and cation exchange capacity (CEC) (Table 1). Essentially, the method illustrated in Table 1. accounts for buffering capacity of the soil. Many soil test labs will calculate lime recommendations using soil pH combined with buffer pH.

 

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Author: Chad Kalaher

Categories: Agronomy, Agronomy Talk

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Chad Kalaher
Chad Kalaher>

Chad Kalaher

Beck’s Hybrids team sales agronomist for 22 counties in NE ¼ of IL and 7 counties in NW IN. Raised on grain and livestock farm in southern IL. B.S. Agronomy 1995 – University of Illinois, M.S. Weed Science 1997 – North Carolina State University. Previous positions in seed industry as researc

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Beck’s Hybrids team sales agronomist for 22 counties in NE ¼ of IL and 7 counties in NW IN. Raised on grain and livestock farm in southern IL. B.S. Agronomy 1995 – University of Illinois, M.S. Weed Science 1997 – North Carolina State University. Previous positions in seed industry as research agronomist, district, and regional sales manager.

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