SOIL AND WATER CHEMISTRY PROBLEMS, SOLUTIONS AND FACTS (PART 2)

SOIL

Low pH Soil:

Add CaCO3 (lime) to neutralize the acidity, which will increase the soil pH (reaction 5 above).  Even though CaCO3 is a salt, it acts as a base in an acid soil.  Consequently CaCO3 causes the following events to occur in an acid soil, most of which take place simultaneously:

  1. Acidity is neutralized
  2. Base saturation (Ca and Mg) of the soil increases
  3. Ratios of basic cations adsorbed and in solution change
  4. Soil pH increases (as soon as the CO2 dissipates away), which in turn affects the solubility of most of the plant nutrients in a soil
  5. Toxic concentration of Al3+, Mn2+, and possible other ions, are neutralized (or otherwise inactivated)
  6. Acid weathering of primary and secondary minerals is curtailed by the decreased concentration of H+
  7. pH-dependent cation exchange complex (CEC) (i.e., negative charge) increases, adsorbing Ca2+ and Mg2+ from which it is hydrolyzed (mobilized) for ready uptake by plants or movement to lower depths in the profile
  8. pH-dependent anion exchange capacity (AEC, i.e., positive charges) decreases, forcing previously adsorbed anions such as SO42- into solution
  9. Dinitrogen fixation increases
  10. Nitrogen mineralization from plant residues and soil organic matter increases
  11. Electrolyte concentration increases with dissolution of lime, and where CEC dominates over AEC, the electrolyte disappears from solution as CO2 volatilize
  12. Hydroxyl ion (OH) concentration increases with dissolution of lime, and where AEC dominates over CEC, the increased OH ion concentration neutralizes + charges, forcing SO42- into solution

High pH Soil:

Add acid (H2SO4 or another acid source) to dissolve the free lime (CaCO3) that will lower the pH in the soil and ultimately produce Ca2+ and SO42- ions (gypsum) (reaction 4 and possibly 3 above).  Ionized calcium is now able to attract or flocculate the soil colloids thus increasing water infiltration.