When a farmer or gardener submits a soil sample to a laboratory for analysis, two common measurements are taken: soil pH and buffer pH. Understanding the difference between the two can make a big difference in your crop yields.
The pH reading is determined by mixing a 1:1 slurry of soil and water, and measures acidity (pH less than 7.0) or alkalinity (pH greater than 7.0) of the soil. In mineral soils (less than 20 percent 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. When initial soil pH is below these ranges, limestone is recommended to increase soil pH.
Soil pH is the trigger for determining when limestone is needed on mineral soils, but it does not tell you how much limestone is needed to increase pH to the recommended range. For that, they use a test called buffer pH.
The amount of limestone needed to raise soil pH depends on the soil’s resistance to pH change. Often this concept is referred to as "buffer capacity." Generally, the higher the amount of clay, organic matter content, and cation exchange capacity of the soil, the higher the buffer capacity.
The higher buffer capacity translates into higher rates of limestone needed to achieve the desired pH. The measurement of buffer pH has commonly been done with a test called the Shoemaker, McLean and Pratt or SMP1. In recent years, the SMP1 test has been replaces with a new test, the Sikora test, which uses less hazardous chemicals to report the same information.
The term "buffer pH" is often confusing. The best way to explain it is through an example. Let’s say we have two soil samples, A and B, and both have a pH measured at 4.7. Sample A has a buffer pH of 6.3, while sample B has a buffer pH of 6.5. The higher buffer pH value on Sample B indicates a soil will be relatively easier to change the pH, as compared to Sample A. Conversely, a low buffer pH value, like the one in Sample A, indicates the soil is resistant to pH change.
If the farmer is trying to change the pH of these fields to 6.5, it will take approximately 3.8 tons of lime per acre to change Sample B’s field, whereas it will take 5.6 tons of lime to change the pH of Sample A’s field. The difference is because Field A has more clay, organic matter or a higher cation exchange capacity, which makes it more difficult to change pH. Not understanding this, of course, can lead to applying the wrong amount of lime, and lead to poor yields over time.
Jeff Burbrink is an Extension educator in agriculture and natural resources. Write to him at 17746 E. C.R. 34, Goshen, IN 46528; call 533-0554; or fax 533-0254.