• Rock Phosphate Fertiliser — pile of fine ochre-tan rock phosphate powder
  • Rock Phosphate Fertiliser — 150g kraft pouch with Dr Forest Micronised Rock Phosphate 0-31-0 label
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Dr Forest

Rock Phosphate Fertiliser UK | 31% P · 46% Ca | Soft Micronised

Rock Phosphate Fertiliser UK | 31% P · 46% Ca | Soft Micronised

£6.99 GBP
£6.99 GBP
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  • ✓Handcrafted and dispatched from Stockport
  • ✓Made with organic ingredients, no slaughterhouse by-products

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Organic Micro Cal-Phos — micronised rock phosphate with 31% phosphorus & 46% calcium. The bonemeal-free alternative.

31% Total P₂O₅ Up to 20% Available P 46% Calcium (CaO) Micronised Powder Bonemeal-Free EU Organic CE-004003

Phosphorus and calcium are the two minerals most closely tied to fundamental plant health. Phosphorus drives every energy-intensive process in the plant — root formation, cell division, flower initiation, fruit development, and the conversion of sunlight into sugars. Calcium is the structural mineral, responsible for cell wall rigidity, membrane integrity, and fruit firmness. Get these two right and almost everything else follows.

Organic Micro Cal-Phos supplies both in a single concentrated mineral powder, sourced from natural stone quarries and micronised to solution grade — far finer than conventional rock phosphate. At 31% phosphorus (P₂O₅) and 46% calcium (CaO), this is a high-analysis source of both nutrients in a naturally co-occurring, organically certified form. Critically, it is derived entirely from mineral rock — no bonemeal, no slaughterhouse by-products. The same nutrients that bonemeal provides, from a completely plant-based and mineral source, in a more concentrated and faster-acting form.

31%Total P₂O₅
20%Plant-Available P₂O₅
46%Calcium (CaO)
0-31-0NPK Profile

What to use it for

  • Transplanting — worked into the planting hole or soil mix, it delivers phosphorus exactly where new roots will grow, giving transplants the energy to establish quickly with strong root systems
  • Flowering and fruit set — phosphorus demand peaks at flower initiation and during fruit fill; applications in early bud stage support maximum fruit set and reduce flower drop
  • Root crops — carrots, parsnips, beetroot, potatoes, and sweet potatoes respond strongly to phosphorus at sowing or planting, producing larger, denser roots with improved storage quality
  • Seedling establishment — young seedlings have limited root systems and limited ability to access soil phosphorus; incorporating into seed compost gives seedlings the energy they need before roots are established
  • Correcting phosphorus deficiency — purple or reddish discolouration of leaf undersides and stems, delayed flowering, and poor fruit set are the most common visible indicators
  • Calcium-deficient soils — 46% CaO makes this one of the highest-calcium mineral inputs available; useful where calcium supply is limiting fruit quality or contributing to physiological disorders
  • Soil building — worked into new beds before planting, it builds long-term phosphorus and calcium reserves in a slow-release mineral form

The bonemeal-free phosphorus solution

Micro Cal-Phos (This Product) — 100% Mineral

  • Source: natural stone quarry (mineral rock phosphate)
  • No slaughterhouse by-products of any kind
  • Phosphorus: 31% P₂O₅ — high total analysis; up to 20% plant-available
  • Calcium: 46% CaO — simultaneously high
  • Micronised to solution grade for rapid availability
  • Suitable for vegans, vegetarians, and organic growers
  • No BSE or animal disease transmission risk
  • Consistent analysis from mineral source

Bonemeal — Slaughterhouse By-Product

  • Source: animal bones from slaughterhouse waste
  • Incompatible with plant-based growing philosophies
  • Phosphorus: typically 10–20% (lower analysis)
  • Calcium: typically 20–30%
  • Coarser particle size — slower to become available
  • Not suitable for vegan or plant-based growers
  • Variable analysis depending on animal source
  • Requires careful handling due to pathogen risk

The science of phosphorus & calcium — and why micronised rock phosphate outperforms conventional sources

Source and processing — EU Certified Organic CE-004003-2022

Organic Micro Cal-Phos is a natural limestone phosphate extracted from quarry by mechanical means only — crushing, classification, and sieving. No chemical treatment, no acid processing, no synthetic additives at any stage. This preserves the full mineral matrix exactly as it occurs in nature and is the basis for its EU organic certification under reference CE-004003-2022. The analysis is: Total P₂O₅ 29–31%, Calcium (CaO) 46%, with trace amounts of MgO (0.20%), K₂O (0.05%), and Fe₂O₃ — all naturally co-occurring in the mineral deposit.

Understanding total vs plant-available phosphorus

Total P₂O₅ (31%) represents all the phosphorus present in the mineral, including fractions tightly bound in the crystal structure that will only become available over months to years through weathering, root exudate activity, and microbial organic acids. P₂O₅ soluble in 2% formic acid (11.3–20%) is the EU standard test for plant-available phosphorus in organic fertilisers. Formic acid mimics the organic acids produced by plant roots and mycorrhizal fungi. This fraction — up to 20% of total P₂O₅ — is what becomes available during the growing season. It is substantially higher than conventional coarse rock phosphate (typically 3–7%) due to the micronised particle size increasing the reactive surface area.

Phosphorus — the energy currency of the plant

  • Present in ATP — the molecule that powers every metabolic reaction in all living cells
  • Drives root development, cell division, flower formation, and seed and fruit filling
  • Forms the backbone of DNA and RNA
  • Structural component of phospholipid cell membranes
  • Central role in activation of enzymes involved in sugar metabolism
  • Deficiency produces: stunted growth, poor roots, delayed flowering, diminished yield

Calcium — the structural mineral

  • Cross-links pectin chains in the middle lamella — the glue between adjacent cells
  • Stabilises plasma membranes, governing which ions can pass in and out of cells
  • Essential for root tip formation and extension
  • Acts as a second messenger in stress response and hormonal signalling pathways
  • Deficiency produces: weak tissue, soft fruit, blossom end rot, tip burn, bitter pit
  • Cannot be remobilised from old tissue — must be continuously supplied to growing points

Why micronised outperforms conventional rock phosphate

01

Dramatically Increased Surface Area

Conventional rock phosphate uses particles of 100–500 microns or larger. Micronisation to solution grade increases the reactive surface area exposed to soil water and root exudates by orders of magnitude. More surface area means faster dissolution, more phosphorus released in the first growing season, and higher plant-available P at any given application rate.

02

First-Season Phosphorus Availability

Coarse rock phosphate releases phosphorus over years, making it a long-term soil amendment but poorly suited to situations where phosphorus is needed promptly. Micronised rock phosphate provides measurably higher phosphorus uptake in the first growing season — at transplanting, during flowering, for seedlings, or to correct an active deficiency — while still building long-term mineral reserves.

03

Liquid Application Becomes Practical

The solution-grade particle size makes it possible to disperse this powder in water and apply it as a soil drench — something not feasible with granular rock phosphate. Liquid application delivers phosphorus directly to the active root zone for rapid uptake, rather than relying on slow top-down dissolution from the soil surface.

04

Calcium & Phosphorus Co-Delivered

In ionic solution, calcium and phosphate react spontaneously to form insoluble tricalcium phosphate — which is why liquid fertilisers cannot normally contain both at high concentrations. In a dry mineral powder, they co-exist stably because there is no aqueous medium to drive the reaction. Micro Cal-Phos preserves both nutrients at very high analysis in a single product, delivering them simultaneously to the root zone where soil microbes and root exudates convert both into plant-available forms.

Scientific References

  1. Vance, C.P., Uhde-Stone, C. & Allan, D.L. (2003). Phosphorus acquisition and use: critical adaptations by plants for securing a non-renewable resource. New Phytologist, 157(3), 423–447.
  2. White, P.J. & Broadley, M.R. (2003). Calcium in plants. Annals of Botany, 92(4), 487–511.
  3. Hammond, J.P. et al. (2009). Regulatory responses of root morphology and gene expression to inadequate phosphorus. Plant, Cell & Environment, 32(5), 617–632.
  4. Chien, S.H., Prochnow, L.I. & Cantarella, H. (2009). Recent developments of fertiliser production and use to improve nutrient efficiency and minimise environmental impacts. Advances in Agronomy, 102, 267–322.

How to use Micro Cal-Phos — application rates, methods & timing guide

Pre-slurry before liquid application

When using as a soil drench, mix the powder into a small amount of water first to form a smooth slurry, then add to the full application volume and stir well. This ensures even distribution of mineral particles. Stir or shake before each application pass to prevent settling — this is a suspension, not a solution.

Application rates

Soil mix incorporation — before planting

Rate: 1.5–3 ml per litre of soil (~1.8–3.6 g/L) | Frequency: At preparation

Mix evenly through soil or potting compost before planting. Use 1.5 ml/L for already-fertile growing media or non-fruiting plants. Use 3 ml/L for phosphorus-deficient compost, new media with no existing reserves, or heavy-feeding fruiting crops like tomatoes and peppers.

Outdoor beds and borders

Rate: 50–150 g per m² | Frequency: Every 6–12 weeks during growing season

Rake lightly into the soil surface or apply before rain. Use 50 g/m² as a maintenance rate on established beds; up to 150 g/m² when building fertility in new or depleted ground or correcting visible phosphorus deficiency.

Top dressing — established containers

Rate: 0.2–0.5 g per litre of soil volume | Frequency: Every 6–8 weeks

Apply to the soil surface around the base of plants and water in thoroughly. For a standard 10 L pot, this is approximately 1.5–4.5 g total. Water the plant first, apply the top dressing, then water again to carry the mineral down to the root zone.

Liquid soil drench

Rate: 1–2 g per litre of water | Frequency: Every 3–4 weeks

Pre-slurry the measured powder in a small amount of warm water, then add to the full volume and stir thoroughly. Use 1 g/L for routine maintenance; 2 g/L for correcting active phosphorus or calcium deficiency. Apply to moist soil at the root zone — not to dry soil. Stir before each pass as particles will settle.

Trees and shrubs

Rate: 2–5 kg per tree | Frequency: Once annually in early spring

Apply around the drip line (outer edge of canopy), not tight against the trunk. Work lightly into the soil surface and water in well. For newly planted trees, incorporate 500 g–1 kg into the planting hole at planting time.

Transplant boost

Rate: 1–2 g mixed into the planting hole | Frequency: At planting

Work directly into the soil at root level before setting the plant. Delivers phosphorus exactly where new roots will grow first. Particularly effective for tomatoes, peppers, aubergines, courgettes, and all fruiting crops.

Measuring guide

At bulk density of 1.20 g/cm³: 1 ml ≈ 1.2 g. A level teaspoon (5 ml) ≈ 6 g. A level tablespoon (15 ml) ≈ 18 g. For liquid applications, weighing on a digital scale gives the most accurate results.

Mycorrhizal caution

High phosphorus availability suppresses mycorrhizal colonisation. If using alongside mycorrhizal fungi at transplanting, incorporate Micro Cal-Phos at the lower end of the range for the first 6–8 weeks, then resume normal rates once the fungal network is established. The two products are compatible and complementary over the full season — this caution applies only to the immediate post-inoculation window.

Works well combined with…

Micronised Gypsum (additional calcium sulphate for intensive fruiting crops), Fulvic Acid Powder (chelates phosphate ions and improves their mobility in the root zone), Mycorrhizal Fungi (the fungal network is highly effective at accessing and delivering soil phosphorus — inoculate at planting and allow to establish before increasing P inputs), and Sulphate of Potash (potassium and phosphorus work synergistically during reproductive growth).

Frequently asked questions about Micro Cal-Phos

Yes — and in terms of phosphorus and calcium analysis it is actually stronger than most bonemeal. Typical bonemeal contains 10–20% phosphorus and 20–30% calcium. Micro Cal-Phos delivers 31% phosphorus and 46% calcium (CaO) from a completely mineral, slaughterhouse-free source. In most UK garden soils with active biology, the availability from micronised rock phosphate is broadly comparable to bonemeal and in many situations faster — particularly when applied as a liquid drench or mixed into soil where root contact is immediate.
The raw material is the same — natural calcium phosphate mineral — but the particle size is very different. Standard rock phosphate is relatively coarsely ground (100–500 microns or larger) and dissolves very slowly, releasing phosphorus over years. Micro Cal-Phos is micronised to solution grade, dramatically increasing the surface area exposed to soil water, root exudates, and microbial organic acids. This means substantially faster phosphorus availability in the first growing season. The fine particle size also makes liquid application practical — which is not possible with coarser rock phosphate.
Purple or reddish-purple discolouration on the undersides of tomato leaves is a classic sign of phosphorus deficiency. This commonly occurs in early spring when compost is cold and phosphorus is temporarily immobile. For fastest correction, mix 1–2 g per litre and apply as a root zone drench. Ensure the growing medium is warm and moist — phosphorus uptake is reduced dramatically below 10°C. If plants are cold as well as deficient, moving them somewhere warmer and applying the drench simultaneously is the most effective intervention. Colour should improve in new growth over 7–14 days.
Micro Cal-Phos at 46% CaO is an effective calcium source for soil application. However, blossom end rot is more commonly caused by inconsistent watering (which disrupts calcium transport via the xylem) than by a lack of calcium in the growing medium. For addressing active blossom end rot quickly, Micronised Gypsum (calcium sulphate) is more effective because it delivers calcium in a water-soluble form for rapid uptake. Micro Cal-Phos is best used as a longer-term soil builder and for maintaining calcium levels across the season, particularly when incorporated before planting.
At high availability, yes — phosphorus suppresses mycorrhizal colonisation because the plant's incentive to invest in the fungal partnership is reduced. However, at recommended Micro Cal-Phos rates, and given this is a slow-release mineral source rather than highly soluble synthetic phosphate, the risk of complete suppression is low. If inoculating with mycorrhizal fungi at transplanting, use the lower end of the range for the first 6–8 weeks. Mycorrhizal fungi are in fact one of the primary mechanisms by which mineral rock phosphate becomes plant-available, so establishing a strong network actually improves this product's efficiency.
With some caution. Calcium phosphate does not have the strong liming effect of carbonate forms of calcium — the pH impact on already-acidic soil is generally minimal at garden rates. However, blueberries thrive at pH 4.5–5.5 where phosphorus is naturally more available, and significant repeated applications of any calcium mineral could gradually shift pH upward. For ericaceous plants, use the lower end of the range applied infrequently. Liquid application at 1 g/L as an occasional drench is safer than large incorporation rates.
Yes, completely. This is an EU organic-certified mineral fertiliser (CE-004003-2022) derived from natural stone quarry deposits. No synthetic additives, no industrial chemicals, no slaughterhouse-derived ingredients. Safe for all edible crops with no withholding period. Suitable for vegan and vegetarian growing practices and compatible with all certified organic production standards.
Not fully — it is a mineral suspension rather than a true solution. The micronised particles disperse very finely in water but do not dissolve to a clear liquid like soluble fertiliser salts. The resulting suspension is opaque and particles will settle if left undisturbed. Stir or shake before each application pass. Pre-mix the powder in a small amount of water first to break up clumps, then add to the full volume. At 1–2 g per litre, the suspension is fine enough to apply through most watering cans and garden sprayers.

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