Question 10.3: Suppose a clay is measured to have the following cations (in......
Suppose a clay is measured to have the following cations (in meq/100 g):
Ca^{2+} = 16.2, Mg^{2+} = 4.4, K^{+} = 01, Na^{+} = 1.6, and H^{+} = 10.2
What is its percent base saturation?
At the pH of the CEC measurement, the total CEC = 16.2 + 4.4 + 0.1 + 1.6 +
10.2 = 32.5 meq/100 g. The 22.3 meq/100 g of base exchange capacity due to Ca^{2+}, Mg^{2+}, K^{+}, and Na^{+} represents 68.6% of the total CEC. The 10.2 meq/100 g of exchangeable H^{+} is 31.4% of the total CEC. Assume that there is no pH-dependent surface charge. Then
CEC_{pH=7} = 32.5 meq/100 g, and percent base saturation = 68.6%
Percent base saturation is related to the soil pH as follows:
. The higher the percent base saturation, the higher will be the pH (more sites have been vacated by H^{+} and occupied by metal cations).
. The lower the percent base saturation, the lower the pH (more sites are
occupied by H^{+} and unavailable to metal cations).
Leaching of soils reduces base saturation but does not change CEC. Therefore, soil leaching tends to increase soil acidity.