By Alix Carpenter, Agronomy Specialist, University of Missouri Extension
While soil pH is an excellent indicator of soil acidity, it does not determine lime requirement to neutralize that acidity.
Measuring soil pH only measures the active acidity in the soil water. What also needs to be considered when developing a lime requirement is potential acidity, a function of soil clay and organic matter.
Four factors have a major impact on successful neutralization of soil acidity by lime: rate of lime applied, lime purity (relative to 100 percent calcium carbonate – the lime’s CCE [calcium carbonate equivalent]), fineness, and amount of incorporation into the soil.
When soil tests are performed, labs make lime recommendations based upon both the pH of the soil (the soil’s active acidity), and a measure of the soil’s structure or buffering capacity.
The buffering capacity of a soil is often expressed as the soil’s resistance to pH change. Buffering capacity increases as do clay and organic matter levels. Sandy soils are weakly buffered, requiring less lime to change the soil pH.
The reaction rate and degree of reactivity of lime increase as the liming material’s particle size decreases.
Three to four years after application, ag lime particles larger than 10- to 15-mesh will have dissolved little, while the majority of lime particles in the 50- to 60-mesh size will have dissolved.
The larger particles will have little effect on soil acidity (and pH), while the smaller particles will have rapidly neutralized soil acidity. Agricultural liming materials contain both coarse and fine materials.
Soil test results and recommendations generated by the University of Missouri Soil & Plant Testing Lab report lime recommendations in terms of ENM (effective neutralizing material).
The ENM value of a liming agent takes into account both the CCE and fineness of the lime.
Data on the purity and fineness of a particular lime material are printed semiannually in the Missouri Agricultural Liming Materials Report, a copy of which is available for review at local extension offices, or online at http://aes.missouri.edu/pfcs/aglime. These data are also available from individual quarries.