Ask an Agronomist
Evaluate Soil pH and Buffer pH To Determine Lime Needs
May 20, 2010
Question: What’s the difference between soil pH and buffer pH? My soil has a pH of 5.1 and a buffer pH of 6.7.
Answer: Soil pH is the measure of active acidity. Active acidity is soil solution hydrogen (H+), and levels can change quickly. Buffer pH is a measure of several things: active acidity, salt replaceable acidity and residual acidity. There are many different buffer solutions that laboratories can use, including Woodruff, Adams & Evan, Mehlich, Shoemaker, McLean, and Pratt. The Shoemaker, McLean, Pratt (SMP) buffer is one that is widely used and has a pH of 7.5. The laboratory will mix water with your soil and take a soil pH reading, then mix SMP buffer solution with the soil and take a SMP buffer pH reading. The more residual acidity in a sample, the more the SMP buffer solution is lowered from 7.5. Both your water pH of 5.1 and buffer pH of 6.7 are indicating that you have free H+ in soil solution and residual acidity that will need neutralized. I would recommend contacting the laboratory or a local agronomist that can help you determine the quantity of lime and type of lime to apply. There are also charts created for the SMP buffer that will help you determine how much lime to apply.
High-yielding growers hold soil pH constant, and don't let it swing up and down.
It's easier to maintain a good soil pH from the surface than to fix an extremely acid one after you get into strip-till.
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