• Organic Calcium Carbonate — pile of fine pale calcium carbonate powder
  • Organic Calcium Carbonate — 150g kraft pouch with dr forest Micro-Cal-Carb 54% Calcium, Micronised Calcium Carbonate
  • Organic Calcium Carbonate — back of kraft pouch with Ingredients and Directions for use panel
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Dr Forest

Garden Calcium Carbonate | Lime Alternative

Garden Calcium Carbonate | Lime Alternative

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£7.00 GBP
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Micro Cal-Carb — 96% pure micronised calcium carbonate from natural limestone

54.1% CaO Calcium 96.2% CaCO₃ Purity Micronised to 63µ pH Corrector 91%+ Carbonic Solubility EU Organic Compliant

The simplest and most concentrated calcium source available: 96.2% pure calcium carbonate from quarry-extracted natural limestone, mechanically crushed and micronised to 63 microns. No chemical processing, no additives, no fillers. At 54.1% CaO, this delivers more calcium per gram than gypsum (23% Ca), bone meal (~20% Ca), or any liquid calcium product on the market. The micronised particle size means it reacts rapidly in soil — far faster than agricultural lime — making it effective as both a calcium source and a fast-acting pH corrector for acidic soils.

The carbonic solubility exceeds 91% — meaning over 91% of the calcium carbonate dissolves in the weak carbonic acid that naturally occurs in soil water. This is the measure that matters for plant availability: it tells you how much of the product will actually release calcium into the soil solution under normal growing conditions, not just in a laboratory acid bath.

54.1%CaO Calcium
96.2%CaCO₃ Purity
63µMicronised
>91%Carbonic Solubility

Full mineral analysis

Component Content
Calcium carbonate (CaCO₃) 96.20%
Calcium (CaO) 54.10%
Silica (SiO₂) 1.20%
Iron (Fe₂O₃) 0.71%
Magnesium (MgO) 0.48%
Potassium (K₂O) 0.05%
Manganese (MnO) 0.01%
Sodium (Na₂O) 0.01%

Neutralising value: 54  |  Carbonic solubility: >91%  |  95% passes 63µ sieve  |  EU organic compliant

What Micro Cal-Carb is used for

  • Concentrated calcium source — at 54.1% CaO, this is the highest-concentration calcium product in the Dr Forest range; ideal when soil tests show calcium deficiency or when large calcium additions are needed without adding other nutrients
  • pH correction for acidic soils — calcium carbonate neutralises soil acidity; the micronised particle size and 91%+ carbonic solubility mean it acts far faster than coarse agricultural lime, correcting pH within weeks rather than months
  • Foliar and soil drench calcium — the 63µ micronised powder suspends in water for foliar spray or root drench application, delivering calcium directly where it is needed for cell wall construction
  • Soil mix amendment — incorporate into potting mixes, composts, and growing media to buffer pH and provide slow-release calcium throughout the growing season
  • Mushroom cultivation — calcium carbonate is used in mushroom substrates as a pH buffer to maintain the alkaline conditions that favour mycelial growth while suppressing contaminant organisms
  • Fruit trees, orchards, vines, and ornamentals — suitable for all crops; particularly valuable where calcium demand is high (tomatoes, peppers, brassicas, apples) or where soil pH is below 6.0

Why micronised limestone instead of agricultural lime?

Micronised Calcium Carbonate — Micro Cal-Carb

  • 96.2% CaCO₃ purity — minimal impurities
  • Micronised to 63µ — reacts rapidly in soil, effective within weeks
  • 91%+ carbonic solubility — genuinely plant-available
  • Fine enough to suspend in water for foliar spray or drench
  • Can be incorporated into soil mixes at precise rates
  • No chemical processing — quarry-extracted and mechanically ground

Standard Agricultural Lime

  • Coarse particle size — takes 6–12 months to fully react
  • Variable purity — often 70–85% CaCO₃ with clay and silica fillers
  • Cannot be suspended in water for liquid application
  • Too coarse for precise soil mix formulation
  • Slow pH correction — may take a full season to reach target
Handcrafted in Stockport

Every Dr Forest product is made by hand in small batches at our workshop in Stockport, Greater Manchester. We source ingredients for quality, not cost.

The science of calcium carbonate: pH correction & calcium delivery

Why particle size determines everything

Calcium carbonate is the most common liming material in agriculture. But its effectiveness depends almost entirely on particle size. A coarse limestone chip can sit in soil for years without fully dissolving. The same mineral ground to 63 microns has an enormously greater surface area exposed to soil acids, root exudates, and microbial activity — accelerating dissolution from months to weeks. This is why micronised limestone acts as both a rapid pH corrector and a plant-available calcium source, while coarse ag-lime is essentially a long-term soil amendment with minimal short-term benefit.

01

The Dissolution Chemistry

Calcium carbonate dissolves in acid: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. In soil, the acid comes from three sources: carbonic acid (CO₂ dissolved in soil water), organic acids from root exudates, and organic acids from microbial metabolism. The reaction consumes hydrogen ions (H⁺) — which is precisely how it raises pH. Each molecule of CaCO₃ that dissolves removes two hydrogen ions from the soil solution and releases one calcium ion. The 91%+ carbonic solubility of this product confirms that over 91% of the CaCO₃ dissolves under these natural soil conditions.

02

Calcium — Cell Walls, Signalling & Defence

Calcium cross-links pectin chains in cell walls, providing the structural rigidity that prevents fruit splitting, blossom end rot, tip burn, and bitter pit. It also functions as a secondary messenger in cell signalling — triggering defence responses to pathogen attack, regulating stomatal opening, and controlling pollen tube growth. Calcium is phloem-immobile: once deposited in a cell wall, it cannot be moved. Actively growing tissues need continuous external supply. Micronised CaCO₃ applied as a foliar spray or drench delivers calcium directly to where demand is highest.

03

pH and Nutrient Availability

Soil pH governs the availability of almost every plant nutrient. Below pH 6.0, phosphorus becomes increasingly locked up with aluminium and iron. Molybdenum availability drops sharply. Aluminium and manganese can reach toxic levels. Calcium and magnesium leach faster than they are replaced. Raising pH from 5.5 to 6.5 with calcium carbonate doesn't just add calcium — it unlocks the phosphorus, molybdenum, and other nutrients that were already present in the soil but chemically unavailable. This is often the single most cost-effective intervention in acidic soil management.

04

Surface Area & Reaction Speed

A 1 cm limestone chip has approximately 6 cm² of surface area. The same mass ground to 63µ particles has a surface area measured in thousands of square centimetres. Chemical reactions happen at surfaces — the more surface exposed to soil acids, the faster the CaCO₃ dissolves and the faster pH rises. Standard agricultural lime (2–4 mm particles) may take 6–12 months to fully react. Micronised limestone at 63µ achieves measurable pH correction within 2–4 weeks under active growing conditions.

05

96% Purity — Why It Matters

Lower-grade liming materials contain 70–85% CaCO₃ with the balance being clay, silica, and other inert fillers. These fillers contribute no calcium, no pH correction, and no agronomic benefit — they are dead weight. At 96.2% CaCO₃, this product delivers 54.1% CaO per kilogram applied. You need less product per square metre to achieve the same result, and the dosing calculations are more precise because almost everything in the bag is active ingredient.

06

Soil Biology & Calcium Carbonate

Most beneficial soil bacteria and fungi prefer a pH of 6.0–7.0. In acidic soils (pH <5.5), bacterial activity declines sharply while pathogenic fungi — particularly Fusarium and Pythium — thrive in the absence of bacterial competition. Correcting soil pH with calcium carbonate shifts the microbial balance in favour of beneficial organisms. The calcium itself also improves soil structure by flocculating clay particles, improving aggregation, aeration, and water infiltration.

Scientific References

  1. Marschner, P. (2012). Marschner's Mineral Nutrition of Higher Plants. 3rd ed. Academic Press.
  2. White, P.J. & Broadley, M.R. (2003). Calcium in plants. Annals of Botany, 92(4), 487–511.
  3. Holland, J.E. et al. (2018). Liming impacts on soils, crops and biodiversity in the UK. Soil Use and Management, 34, 504–519.
  4. Mengel, K. & Kirkby, E.A. (2001). Principles of Plant Nutrition. 5th ed. Kluwer Academic.

How to use Micro Cal-Carb: application rates & guide

Rate depends on soil test results

The correct application rate for pH correction depends on your current soil pH, target pH, soil type (clay vs sand), and buffering capacity. The rates below are general guidelines. For precise liming, have your soil tested and calculate the requirement based on the neutralising value (54) and your soil's lime requirement figure.

Application methods

Foliar spray or soil drench

Rate: 0.5–1 g per litre of water  |  Frequency: As required

Add the powder to water and stir vigorously. The micronised particles suspend readily at this concentration. Apply as a foliar spray or pour directly around the root zone. For foliar use, strain through a fine sieve before adding to a sprayer to prevent nozzle blockages. Delivers calcium directly to actively growing tissue.

Fruit trees & established shrubs

Rate: 0.5 kg per m²  |  Timing: Late winter / early spring, or after harvest

Scatter evenly under the canopy from trunk to drip line. Lightly rake into the top few centimetres of soil and water in. The micronised particles begin reacting within days.

Orchards

Rate: 0.6 kg per m²  |  Timing: Late autumn or early spring

Broadcast over the root zone. Work into the soil surface where possible. The higher rate reflects the greater calcium demand of productive orchard trees and the need for ongoing pH management.

Lawns & turf

Rate: 0.4 kg per m²  |  Timing: Autumn or early spring

Scatter evenly over the lawn surface. The fine powder settles into the turf canopy with watering or rain. Improves soil pH and calcium availability for root development and cold tolerance.

Soil mixes & growing media

Guideline: 1–3 g per litre of compost or growing medium

Incorporate thoroughly before planting. Buffers pH and provides slow-release calcium throughout the growing season. Start at the lower end for mixes that already contain lime; use the higher end for peat-based or coir-based media that tend to be acidic.

Step-by-step for liquid application

  1. Measure the powder. 0.5–1 g per litre. A level ½ teaspoon is approximately 1–1.5 g depending on how tightly packed.
  2. Add to warm water and stir vigorously. The micronised particles suspend readily but will settle if left standing. Maintain agitation.
  3. For foliar use, strain first. Pour through a fine sieve or muslin into the sprayer. Apply as a fine mist to both leaf surfaces.
  4. For root drench, pour directly. Apply around the root zone using a watering can. No straining needed.
  5. For dry application, scatter and rake in. Spread evenly, work into the top 2–5 cm of soil, and water thoroughly.
Works well combined with…

Use alongside Dr Forest Cal-Mag Supplement for combined calcium and magnesium delivery with boron. Pair with Yorkshire Polyhalite for a complete secondary nutrient package (Ca, Mg, K, S). For liquid foliar calcium during fruiting, add Cal-Mino amino acid chelated calcium. Do not mix with phosphate fertilisers in liquid form — calcium and phosphate precipitate when dissolved together.

Frequently asked questions

Same mineral — calcium carbonate — but fundamentally different particle size and purity. Agricultural lime is typically 2–4 mm particles at 70–85% purity. This product is micronised to 63µ (95% passes a 63-micron sieve) at 96.2% purity. The finer particle size means it reacts in weeks rather than months. The higher purity means more calcium per gram applied.
Gypsum (calcium sulphate) provides calcium and sulphur but does not raise soil pH — it is pH-neutral. Calcium carbonate provides calcium and does raise pH by neutralising soil acidity. Use gypsum when calcium is needed without pH change (e.g. in alkaline soils or to improve clay structure). Use Micro Cal-Carb when you need both calcium and pH correction.
Calcium carbonate is self-limiting. It dissolves in acid conditions but becomes increasingly insoluble as pH rises above 7.0. It is very difficult to over-lime with CaCO₃ — the reaction slows and effectively stops as the soil approaches neutral pH. This is a safer liming material than quicklime (CaO) or hydrated lime (Ca(OH)₂), which can raise pH rapidly and excessively.
The micronised particle size (63µ) means the calcium carbonate begins dissolving within days of contact with moist soil. Measurable pH change typically occurs within 2–4 weeks under active growing conditions. Coarse agricultural lime may take 6–12 months to achieve the same result.
Yes. At 0.5–1 g per litre, the micronised powder suspends in water for foliar application. Strain through a fine sieve before adding to a sprayer. This delivers calcium directly to leaves and developing fruit — useful for preventing blossom end rot, tip burn, and bitter pit where root uptake is insufficient.
As a dry amendment, yes — it mixes well with granular fertilisers and soil mixes. In liquid form, do not mix with phosphate fertilisers — dissolved calcium and phosphate react to form insoluble calcium phosphate. Also avoid mixing with acidic liquid feeds, as the acid will react with the carbonate. Compatible with most granular and dry organic fertilisers.
Yes. Calcium carbonate is widely used in mushroom substrates as a pH buffer. It maintains the alkaline conditions that favour mycelial growth while suppressing contaminant organisms. The micronised form mixes more uniformly through the substrate than coarser lime products.
This depends on your current soil pH, target pH, soil type, and organic matter content. Clay soils and high-organic-matter soils require more lime to shift pH than sandy soils. As a starting point: 200–400 g/m² will typically raise the pH of a light sandy soil by 0.5–1.0 units. For precise calculations, have your soil tested and use the neutralising value (54) to calculate the requirement.
Calcium carbonate is safe for all plants at recommended rates. However, acid-loving plants (blueberries, azaleas, rhododendrons, camellias) prefer a low pH and should not be limed. Do not apply to ericaceous or acid-loving species.
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