Why use lime?
Because peat, which is used in potting soils, is acidic (pH is about 3,7 – 4), lime is added to raise the pH to the right level. For many plants, the best pH is between 5,5 and 6,2. The lime is usually Dolomite lime, Calcium Carbonate (CaCO3).
After mixing, the CO3 (Carbonate) will take some H+ ions and make H2CO3 (which will separate in H2O and CO2, later on).
This means that adding Dolomite lime, will raise the pH and in addition Calcium is added.
How much to use?
The effect of lime on pH is not linear; to bring the pH from 4 to 5 will need a lot more lime than to bring it from 5 to 6, why? Because the pH is the negative logarithm of the concentration of H+ ions.
Also there are big differences between the various raw materials. For example black peat needs much more Dolomite to reach a pH of 6 than a young white peat does.
Furthermore there are different types of Dolomite lime, the most important thing is to know the Acid Binding Value, after you know this, testing should take place to find out the exact effect of the Dolomite on the mix.
Where does it come from?
This limestone is created by organisms that live in the sea .The limestone therefore is mainly Calcium carbonate and regularly you find fossils (like shells) in the limestone.
Limestone can be found in tropical areas (created by coral), but also in Northern Europe like in Limburg, the Ardennes and also the cliffs of Dover are made of limestone.
Important to us, who are in horticulture, is to know what other elements there are in limestone, because we don’t want to face problems with salts we don’t like (like Sodium and Chloride).
If you have any questions feel free to send an e-mail.