Growing media
Calcium carbonate for potting soil.
A natural marine carbonate for pH adjustment and calcium supply in peat- and coir-based potting mixes, nursery substrates and blended container media.
Why carbonate is in the mix
Peat and coir start acidic. Carbonate moves them.
Soilless media built on peat, coir, bark or composted substrates start below the pH window most container crops want, and they hold little calcium of their own. A carbonate input does both jobs at once: it supplies alkalinity that lifts substrate pH and it contributes calcium to the fertility program.
- Composition
- 96.17% CaCO3
- Elemental calcium
- 38.51%
- AG-CAL sizing
- 0.2–0.5 mm
- OMRI Listed
- avw-23214
Representative analysis, ASTM C 25, Bowser-Morner report 124914A.
The basis for calcium contribution within the complete fertility program.
The grade used in media programs; blends run across mesh 20 to 325.
Crop fertilizer listing, for growers running qualifying organic programs.
What decides the rate
Four inputs, and none of them is the bag label.
A media rate is calculated from the substrate's own chemistry and the crop's requirements. The same product goes in at different rates for different mixes.
Starting substrate
Document the substrate pH, lime requirement, electrical conductivity and exchange capacity before calculating anything. Peat, coir, bark and composted components each behave differently.
Target pH window
Crop, cultivar and production cycle set the window. Container volume and crop length determine how long the substrate has to hold it.
Irrigation water
Water alkalinity pushes substrate pH over the crop cycle in its own right, so it belongs in the calculation alongside the carbonate rate.
Fertilizer program
Acidic and basic residual fertilizers move substrate pH in opposite directions. Evaluate the carbonate rate within the complete fertility program, not on its own.
Blending into production
Uniformity is a particle-size question.
Particle distribution
Sizing governs reaction timing and blend uniformity. Finer fractions react sooner and disperse more evenly; coarser fractions persist longer in the container. AG-CAL at 0.2–0.5 mm sits in the range used for media blending, and other distributions are available across mesh 20 to 325.
Mixing sequence and dispersion
Addition point, mixer type, batch size and mixing time determine whether the carbonate is evenly distributed or concentrated in pockets that give inconsistent pot-to-pot results.
Moisture and segregation
Substrate moisture at mixing affects dispersion, dust and the tendency of a dry mineral to segregate during handling, storage and filling.
Batch consistency
Grade specifications and analytical documentation define the chemistry and sizing behind each supplied batch, which is what makes a media recipe repeatable.
Material comparison
Carbonate choices in a container mix.
Dolomitic limestone is the default in many media recipes because it carries magnesium as well as calcium. Whether that is an advantage depends on what the rest of the program already supplies.
| Decision factor | Oolitic aragonite | Calcitic limestone | Dolomitic limestone |
|---|---|---|---|
| Chemistry | Natural marine CaCO3, aragonite polymorph | Calcitic CaCO3, calcite polymorph | Calcium magnesium carbonate |
| pH contribution | Supplies alkalinity; condition-dependent | Supplies alkalinity; condition-dependent | Supplies alkalinity; condition-dependent |
| Calcium basis | 38.51% elemental calcium | Grade-dependent | Grade-dependent |
| Magnesium | Not a magnesium source | Not a magnesium source | Supplies magnesium as well as calcium |
| Selection control | Substrate test, grade data and a crop trial | Supplier data and a crop trial | Substrate magnesium status and a crop trial |
Application path
Bench blend before the production mixer.
Bench blend
Prepare representative small batches using the production substrate, irrigation water and fertilizer program.
Track
Measure substrate pH and electrical conductivity at defined intervals across the production cycle, alongside crop response.
Scale
Move the established rate and mixing sequence into the production blending program.
Questions
Frequently asked questions.
Why is calcium carbonate added to potting soil?
Soilless media built on peat, coir or bark start below the pH window most container crops want and hold little calcium. Calcium carbonate supplies alkalinity that raises substrate pH and contributes calcium to the fertility program at the same time.
How much should be added to a mix?
The rate comes from the substrate's lime requirement, the crop's target pH window, irrigation-water alkalinity and the fertilizer program — not from a single recommended figure. Bench-blend representative batches with your own substrate and water, then measure pH and electrical conductivity across the production cycle before scaling.
Which grade suits a container mix?
AG-CAL at 0.2–0.5 mm is the grade used in media programs. Where a recipe calls for a different reaction timing or blend uniformity, blends are produced across mesh 20 to 325.
Does it replace dolomitic lime?
It replaces the calcium and alkalinity that dolomitic lime supplies, but not the magnesium. If the substrate and fertility program depend on dolomitic limestone for magnesium, that magnesium has to come from somewhere else in the program.
Is it OMRI Listed for growing media?
The crop fertilizer listing is avw-23214. OMRI operates a listing program rather than a certification, and whether a specific operation may use a listed input rests with your certifier.
Will it change the electrical conductivity of the mix?
Carbonates are only slightly soluble in water, so the direct contribution to EC is small next to fertilizer salts. Measure pH and EC together through the crop cycle regardless — substrate pH movement changes nutrient availability, and that is what shows up in the crop.
Keep reading
Where to go next.
Discuss a media, crop and blending program.
Send the substrate analysis, crop, target pH window, batch volume and destination, and AragoCor will match a grade and quote against it.