How can substances change the pH of soil?
When calcium carbonate or MgCO3 is added to soil, the CO3= ions combine with the naturally occuring H+ ions (derived from water equilibrium ) H2O at equilib with H+ plus OH- leaving the OH- ions in the water equilibrium to cause the soil to be more alkaline, or if the soil is originally acid containing excess H+ ions, the OH- ions will convert these excess H+ to water. In the case of soil being to alkali with excess OH- ions, the addition of Fe+2 and Al+3 ions cause just the opposite from the reaction above to happen. either of the ions will react with any OH- ions in the soil which is making it too alkaline and convert these loose OH- ions into insoluble compounds such as Fe(OH)2 or Al(OH)3 which binds up the OH-, leaving the H+ from the water equilibrium above to show up and increase the acidity of the soil.
