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How Lime Works

The purpose of liming is to increase soil pH.  pH is a measure of H+ ions.  The lower the pH, the more H+ ions there are.  Increasing pH is done by removing (or neutralising) these H+ ions.  H+ ions are removed when they react with a suitable anion (an alkaline or base).  Carbonate (CO3) is the alkaline or base that is the active ingredient in lime.

 

The Neutralising Value of a lime is based on the percentage of calcium carbonate.  A NV of 100 means the lime contains the same amount of neutralising anions as pure calcium carbonate.   A good quality agricultural  lime will have a NV of at least 90.

 

The next step is to get the negative anions in the lime to react with the positive H+ ions in the soil.

 

H+ ions come into contact with the surface of lime particles.  It breaks the bond of calcium carbonate and turns into calcium ions, water and carbon dioxide.  None of these elements are reactive and they surround the rest of the lime particle.  These form a barrier between the lime particle and the rest of the H+ ions in the soil.  The rest of the lime cannot react with more H+ ions as there is a physical barrier of neutral material between them.

 

This is why particle size is important - you want the best surface area to volume ratio possible, so a larger percentage of the lime can react with the acid in the soil.