Clear Up Confusion About Soil Test Reports

Soil pH and buffer pH both are important numbers on any soil test report.

Published on: Feb 3, 2012

Some farmers simply ignore the column on their soil test results that says 'buffer pH.' Many others just don't understand what it is there for, or why the number is different than the number in the soil pH column. With soil pH being an important factor in determining how many nutrients are available to the plant in any one season, and with lime and fertilizer nutrients being an important piece of the input cost puzzle for 2012, Jeff Phillips believes it's important to understand what both numbers mean.

Phillips, Extension ag educator in Tippecanoe County, one was a key player in operating the Purdue University soil testing lab. Purdue no longer operates the lab, but leaves that function to private labs across the state. However, the experience Phillips gained there, and in working for Dave Mengel, former Extension specialist in soil fertility at Purdue, helps him be able to advise farmers today on how to interpret soil test results, and develop recommendations that make sense form the findings.

In an upcoming issue of Indiana Prairie Farmer, Phillips will use examples to explain the difference between soil pH and the buffer pH, also known as the lime index. Both are actually important if you're trying to manage pH levels in your fields economically.

The soil pH which most people recognize is simply the water pH of soil. It's what the plant sees when tis roots begin exploring their surroundings. Nutrient availability for several key nutrients needed by the plant are affected, either being more available or less available, based upon the pH.

The buffer pH comes into play in fields which need lime. If the soil pH isn't in the 6.0 to 6.5 range, then it probably needs lime. Primarily, it needs a source of calcium in a form that can counteract acidity. That means ag lime, wither whether high-calcium ag lime, or dolomitic ag lime, which contains more magnesium than regular ag lime.

The higher the cation exchange capacity and organic matter of a soil, the more lime it takes to offset the acidity. That's because the soil has the capacity to absorb and hold onto more particles that contribute to acidity. The bottom line is more lime will be needed to reach a specific soil pH level. On the other hand, the lower the CEC and organic matter, the less lime that will be need to be applied to affect soil pH.

Phillips observes, however, that soils with lower CEC and organic matter need lime more often. They will need to be monitored for pH changes, which will happen more quickly than in heavier, clay soils where pH doesn't change as rapidly.