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Competency Area 5: Soil pH and Liming

PO 38. Describe how CEC, soil texture, exchangeable acidity, and soil organic matter affect lime requirements.

Lime requirements increase with CEC, and a soil's CEC increases with organic matter content and clay content.  Thus, clay soils with high organic matter content require more lime for a similar pH change than sandy soils with low organic matter.

In other words, high CEC soils tend to be well-buffered, requiring more lime to change the pH; while sandy soils are poorly-buffered, requiring less lime per unit pH change.  Also, because of the greater buffering, the soil pH will decrease slower on high CEC soils than on poorly-buffered sandy soils.

In reduced-tillage systems, acidifying effects of N are concentrated at the soil surface.  This is why two samples are required for pH testing: 0-1" and 0-6" (or 0-8").

  • If the normal "plow-depth" soil sample calls for lime, apply as recommended.
  • If the normal "plow-depth" soil sample does not call for lime, check the pH in the surface soil sample.  If the pH is less than 6.0, apply 1 ton limestone/acre.