When soil samples are analyzed a pH test and lime recommendation are automatic. Usually, these aren't explained in detail for the farmer. If you're like most farmers, you assume a low pH test means you need to apply lime. But why?

It's a chemical equation. pH is a measurement of acidity, with lower pH numbers reflecting higher acidity. It is defined as the negative logarithm of the hydrogen ion (H+) concentration. A pH level of 5 has 10 times the H+ concentration as a pH of 6, and 100 times the H+ concentration of a pH 7 (which is neutral on the pH scale). Lime contains calcium, which ties up the H+ to raise the pH.

This basic soil pH measurement tells you whether the pH is adequate, but differs from buffer pH, which is used as a way to predict the amount of lime required to change the soil pH to a desired level. The reason basic pH and buffer pH vary is because soils vary in their response to lime. For example, sandy soils require less lime than soils with more organic matter or clay content. While lab methodology can vary, buffer pH can help adjust for these differences in soils.

Using a pH test to determine the amount of lime needed to raise soil acidity is not as straightforward as using a phosphate test to decide how much P to apply. Because it is a logarithmic relationship, it takes about 10 times the amount of lime to raise pH from 5 to 6 as it does to raise it from 6 to 7.

Tracy Blackmer, research director for the Iowa Soybean Association