The lime you apply to a lawn is limestone or chalk. The main component is calcium carbonate. There are several types of lime. You can determine which one you need with a soil test.  

Lime with a high calcium content is referred to as calcitic lime and has the added benefit of adding calcium to the soil. Some limestone contains a significant amount of magnesium and is referred to as dolomitic lime. Dolomitic lime adds magnesium to the soil and could be used if soil tests indicate a magnesium deficiency. 

Why Use Lime? 

Lime is the most common way of changing pH of soil. Soil pH is a measure of a soil alkalinity or acidity. A soil is acidic, or "sour,"  if it has a pH below 7.0 (neutral). You may need to add lime to your soil if a soil test indicates a pH level below the optimum of 6.0 or 7.0.  
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