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Determine buffer pH to pinpoint lime rate

One good reason for soil testing is to determine soil pH. Most people want pH in the 6.0 to 6.5 range for corn and soybeans. At this level, nutrients are more available to plant roots, and some herbicides are more effective.

The forgotten column on soil test reports is often the buffer pH. However, Jeff Phillips, Tippecanoe County Extension agricultural educator, says the buffer pH is crucial for figuring how much lime to apply. For example, a soil pH of 5.6 indicates you need lime. But it doesn’t tell you how much you should apply to get a reaction on your soil.

Key Points

Soil pH measures the pH the plant experiences at the present time.

The buffer pH, or lime index, is used to determine how much lime to apply.

The lower the buffer index, generally the more lime you will need up front.

Phillips worked for the Purdue University Soil Testing Laboratory. Then he worked for Dave Mengel, a former Purdue Extension soil fertility specialist. Phillips has evaluated tons of soil samples in his career. Today, he works with farmers to monitor changes in soil test levels over time.

The key to understanding where buffer pH fits is to start with simple definitions, Phillips says.


Soil pH: This is simply the water pH reading, measured after the soil is turned into a solution.

SMP solution: This is a solution that is at 7.5 pH, and is used to find out how much lime is needed.

Reserve acidity: Depending upon the soil’s cation exchange capacity, or CEC, and organic matter, it will have more or less acidity. To find reserve acidity, the soil solution for a sample is added to the SMP 7.5 base solution.

Buffer pH: This reading results from adding the 7.5 solution to the soil. It tells an agronomist how much lime will be needed to raise the soil pH to the proper level.

Lime index: This is another term for buffer pH.


Suppose the soil pH is 5.5. However, the buffer pH is 6.4. That’s how far the pH drops after the solution is mixed with the standard.

“That tells me the CEC and organic matter are probably fairly high, and there is a lot of reserve acidity which must be offset over time,” Phillips says. In fact, the CEC in this sample is 22.1, and the organic matter is 5.7%.

On the other hand, suppose the pH is the same 5.5 in another part of the field, but the buffer pH is 7.1

“Then I know I’ve got a low organic matter soil, probably a sandy soil,” Phillips says. “You’re going to need to apply less lime to raise the pH, but you probably need to apply lime more often.”

Figure lime rates while on the go

Maybe your consultant provides lime recommendations. If not, you can determine them on your own if you have the Purdue University Corn & Soybean Field Guide or the Tri-State Fertilizer Recommendations publication, Jeff Phillips says.

The new 2012 pocket field guide is now available. It contains a table that tells how much lime to apply based on the lime index to raise the pH to the desired level. You select the desired level.

Visit for ordering information for 2012 guides.


This article published in the March, 2012 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.