Question

Question 4 (1 mark) Give the name of one soil mineral that can acquire a positive surface charge at biologically relevant soil pHs Part A: Use diagrams and chemical equations to explain the effect of soil acidification on the solubility of cations. Include in your answer t effect of acidification on cations that are bound via outer sphere processes (on cation exchange sites), inner sphere process (specific adsorption), and occluded cations.

Answer 1

Q4) Kaolinite

This can be understood by following images.
Creation of Clay Colloid Charge pH-dependent charge: on edges Crystal edge H* bound tightly, so the lower the pH, the less exchange there is (i.e., lower nutrient availability) OH ОН Н* Ht "ОН Si Н* Especially important in kaolinite, humus, where no internal charge imbalance

Cation Retention on Organic Matter Hydrogen Nutrients Increasing pH increases cation exchange capacity of organic matter Low pH, 4- 5 (acidic soil) Neutral pH, 7 ("sweet" soil)