Calcitic vs. dolomitic lime in growing media.

Peat is acidic, so almost every peat-based mix is limed, and many coir and bark blends are adjusted too. The choice most formulators make is between calcitic limestone, which is calcium carbonate, and dolomitic limestone, which carries magnesium carbonate as well. Both neutralize acidity. They differ in what else they supply, how fast they react and how long the effect lasts.

Calcium and magnesium

Dolomitic lime supplies magnesium with its calcium, which is why it is the default in many peat mixes where the fertilizer program supplies little magnesium. Calcitic lime supplies calcium and very little magnesium, so a mix limed with it needs magnesium from somewhere else, usually the fertilizer. Neither is better in the abstract; the choice follows the rest of the nutrient program.

Natural aragonite sits on the calcium side of that choice. AragoCor’s analysis is 96.17% CaCO₃ and 0.24% MgCO₃, representative analysis by ASTM C 25.

Reaction speed and residual lime

Calcitic limestone generally reacts faster than dolomitic limestone of the same fineness, so it moves substrate pH sooner but leaves less residual lime to hold it through a long crop. Dolomitic lime dissolves more slowly and buffers for longer. Many commercial mixes manage this through particle size, or blend the two: a fast fraction for the starting pH and a slower one for the crop cycle. Michigan State University Extension compares the limestone types used to raise substrate pH.

Aragonite adds a variable of its own: it is the more soluble calcium carbonate polymorph, by published reference solubility products. How that shows up in a particular peat or coir blend, at a particular particle size, is something to measure in an incubation test rather than assume.

Particle size is the main lever

Fine limestone reacts quickly because it has more surface; coarse limestone reacts slowly and may barely move pH within a short crop. Formulators set the release profile by choosing the distribution. AG-CAL is sized 0.2–0.5 mm as a published range, and custom classification is offered to customer specification.

Testing a new lime in a mix

• Incubate a moistened sample of the limed mix and track pH over the following days, to see how quickly the lime reacts and whether residual lime keeps raising pH.
• Compare against your current lime in the same peat or coir lot, at rates your formulator sets.
• Account for irrigation-water alkalinity, which keeps raising pH through the crop independently of the lime.
• Check the magnesium supply in the fertilizer program if moving from dolomitic to calcitic lime.

For organic mixes, the material’s OMRI crop fertilizer listing, avw-23214, applies to the ingredient rather than to a finished blend. The growing media page covers AG-CAL for substrates.