Professor CC provides a great breakdown on the Liming Agents.
"Here's the deal on liming agents.
Calcium Carbonate Amendments
These are the 3 major forms of Calcium Carbonate (CaCO3)
Limestone - (this is what chemical reps refer to as Agricultural Lime) and will usually have a Calcium Carbonate level between 83 - 95% depending on which specific mine the product is coming out of. Limestone (Agricultural Lime) will also have a small amount of Magnesium (Mg) - about 3 - 5% depending again on which mine is the source
Calcitic Lime - this is pure Calcium Carbonate - usually around 95% and contains no traceable amount of Magnesium
Oyster Shell Powder - another pure form of Calcium Carbonate. This product is NOT from oyster shells from Happy Hour at Red Lobster. This is a specific product that is mined in the San Francisco Bay and has been since the 1920's. This is the product most often used by poultry producers, worm operations, etc. It carries a label showing 96% Calcium Carbonate
When figuring the amount of actual Calcium (Ca) when using the carbonate limes, take the total amount and multiply by 38.5% and that will (approximately) give you the actual Calcium levels.
Dolomite Lime - Calcium Magnesium Carbonate contains elemental Calcium (not Calcium Carbonate) so the numbers on the product will reflect the actual Calcium levels. The Magnesium Carbonate component is tightly bound to the elemental Calcium resulting in a much longer time period required for the Calcium to become available.
Gypsum - Calcium Sulfate Dihydrate contains both Sulfur Oxide and elemental Calcium. When broken down by microbes the sulfur releases Hydrogen (H) which will lower the pH (if necessary) and also contains Calcium which can raise the pH (again if necessary) - see Base Saturation and CeC
A typical liming agent used by organic farmers is something like this:
2x Calcium Carbonate
1x Dolomite Lime
Mix and add at the same rate if you were using a single agent.
At HomeDepot last night I looked at their products in the nursery section.
Soil Sweet - Dolomite Lime and it was less than $6.00 for 25 lbs.
Super Sweet - Limestone at around the same price
Gypsum - $8.95 for 50 lbs.
Choose your poison.
Prilled vs. Pelletized
Prilled soil amendments are coming and will eventually replace the powdered versions due to a finding by the Labor Department as it relates to worker safety. It will come to the retail market soon enough.
Prilled (as Dignan pointed out) is simply any agent that is coated with a clay thereby reducing the dust issue(s) which makes it far easier to apply to several hundred/thousand acres. The clay used for prilling processes is easily removed by water.
Pelletized is the same thing but the coating is applied thicker and a different clay is used which gives the farmer a longer source as it takes a couple of years to breakdown to the point where the actual amendment is NOW available to the soil biology to breakdown and use. You will never find this at a retail nursery."