Calcium carbonate is available as a powder or as a granular application. Powdered calcium carbonate works faster in the soil, but is more difficult to apply at a consistent rate. Conversely, pelletized forms are easier to apply, but take longer to break down and provide their benefits.
Calcium carbonate is an excellent product for raising the pH of soil. Most (not all!) plants do best in soils with a pH between 5.5 and 6.5. Calcium carbonate can be broadcast over and incorporated into soils in need of a dose of alkalinity.
For best and quickest results, it should be incorporated into the soil such as with a tiller, rather than just being allowed to sit on the soil’s surface. This should not just be done as a matter of course though; there are many regions where soils are typically alkaline and calcium carbonate should never be used.
If a soil analysis determines that a soil is deficient in calcium, but has a pH over 7.0, another source of calcium should be used, such as gypsum, which will not raise the soil pH but still provide the needed Ca.
One role of calcium carbonate known to many professionals, but that is not necessarily common knowledge to the average home gardener, is its ability to mitigate heavy metals in the soil.
This is usually only a prescribed course of action if a soil analysis indicates that heavy metals are present in the soil at levels that can be remediated; if the levels are too high, removal of the soil would be the likely solution.
Besides being potentially toxic for humans, heavy metals can be toxic to plants as well. Calcium carbonate does not eliminate the metals from the soil, but “ties them up” so that they are not as available for uptake by plants
One of the many unseen benefits of using calcium carbonate, as it relates to its ability to adjust soil pH, is its role in making nutrients available to plants.
All of the nutrients that a plant needs may be present in a soil, but they may be unavailable to plants if the pH is not conducive for the plant to easily “grab a hold” of. The majority of nutrients that a plant needs are most available to plants at the aforementioned 5.5 to 6.5 pH range.