Question 10.4: Estimate the pH of the soil of Example 3....

The percent base saturation was measured to be 68.6%. At pH 7, the percent base saturation is about 100% (Rule of Thumb 2). Therefore, 68.6% represents a decrease of
100 – 68.6 = 31.4% from pH 7

A 5% decrease in base saturation causes about 0.1 decrease in pH (Rule of Thumb 4). Therefore, 31.4/5\approx  6 units of 5% decrease in base saturation, or 6\times 0.1 units decrease in pH. Therefore, 6\times 0.1=0.6  pH units decrease. Soil pH is around 7 – 0.6 = 6.4.
If a soil with a low base saturation is made into a slurry with water, the water pH will be acidic, around 4–5 or lower. Because a low base saturation means that many sites are occupied by H^{+}, acidic water results from exchangeable H^{+} being released from the soil into solution. Similarly, passing acidic water through soil causes H^{+} to exchange with soil-bound metal cations. The most weakly bound metals are displaced first and, if the pH is low enough, more strongly bound metals are displaced. At first, K^{+} and Na^{+} are removed, then Ca^{2+} and Mg^{2+} go into solution. Trivalent ions, such as Al^{3+}, are the last to leave the soil and go into solution.