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Soils - Part 4: Soil pH

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Defining pH

The definition of pH is: The negative logarithm of the hydrogen ion activity (concentration) in equivalents per liter [eq./l] or written as an equation:

 pH = -log[H+] = log 1/[H+]          (3)

The mathematical expression, pH of 7, is much easier to say than “a hydrogen ion concentration of 0.0000001 gram equivalents per liter” as shown in Equation 4.

 pH = 7.0 = -log(.0000001 eq.H+/1)          (4)

 pH = 5.0 = -log(.00001 eq.H+/1)          (5)

 pH = 9.0 = -log(.000000001 eq.H+/1)          (6)


As shown in Equation 5, there is 100 times more hydrogen present for pH 5 than for pH 7 and similarly for pH 9, there is 100 times less hydrogen than for pH 7. The pH scale, by definition, ranges from 1 to 14. In perspective, the pH of lime juice is about 2, while aqua ammonia is about 12.

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