• PK Fertiliser 0-5-5 — pile of fine olive-grey powder with small grit
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Dr Forest

PK Fertiliser 0-5-5 + Calcium | Organic

PK Fertiliser 0-5-5 + Calcium | Organic

£13.99 GBP
£13.99 GBP
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  • ✓Free UK delivery on orders over £40
  • ✓Handcrafted and dispatched from Stockport
  • ✓Made with organic ingredients, no slaughterhouse by-products

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Organic PK mineral fertiliser — phosphorus, potassium, calcium & iron from natural rock

0-5-5 NPK 11.4% Calcium (CaO) 3.3% Iron (Fe₂O₃) Zero Nitrogen EU Organic Compliant Compostable Packaging

A ground mineral fertiliser blended from natural igneous and sedimentary rocks — quarry-extracted, mechanically crushed, and sieved. No chemical processing, no synthetic additives, no nitrogen. This delivers phosphorus for root development and energy, potassium for flowering and fruiting, calcium for cell wall strength, magnesium for photosynthesis, plus iron, sulphur, and trace elements — all from unprocessed mineral sources in a single application.

The zero-nitrogen formula is the key advantage. Most general-purpose fertilisers push vegetative growth whether you want it or not. This product lets you build phosphorus, potassium, calcium, and magnesium levels in the soil without adding any nitrogen — essential for flowering and fruiting stages, autumn soil preparation, and situations where nitrogen is already adequate or excessive.

5.1%P₂O₅ Phosphorus
5.0%K₂O Potassium
11.4%CaO Calcium
3.1%MgO Magnesium

Full mineral analysis

Nutrient Content
Phosphorus (P₂O₅) 5.10%
Potassium (K₂O) 5.00%
Calcium (CaO) 11.40%
Magnesium (MgO) 3.10%
Sulphur (SO₃) 4.20%
Iron (Fe₂O₃) 3.30%
Manganese (Mn) 0.05%
Copper (Cu) 0.01%
Boron (B) 0.01%
Zinc (Zn) 0.003%

Form: ground powder  |  EU organic compliant

What this mineral PK fertiliser is used for

  • Flowering and fruiting without nitrogen push — supplies the phosphorus and potassium that flowering and fruit-bearing plants need during reproductive stages, without the nitrogen that would divert energy into unwanted vegetative growth
  • Root development at planting — phosphorus drives root cell division; calcium builds the cell walls of new root tips; apply before or at planting to establish strong root systems from the start
  • Soil remineralisation — restores the full spectrum of minerals that intensive cropping depletes: Ca, Mg, K, P, S, Fe, Mn, B, Zn, Cu from natural rock sources that release nutrients gradually over the season
  • Autumn and winter soil preparation — apply after harvest or before spring planting to build mineral reserves without stimulating late-season growth; the slow-release mineral form won't leach over winter
  • Correcting calcium and magnesium deficiency — 11.4% CaO and 3.1% MgO provide substantial secondary nutrient loading; particularly valuable in acidic, sandy, or heavily cropped soils
  • Iron-rich soil amendment — 3.3% Fe₂O₃ is unusually high for a PK fertiliser; addresses iron chlorosis in alkaline soils and feeds iron-dependent soil biology
  • Fruit trees, orchards, vines, and ornamentals — the manufacturer's primary recommendation; the balanced P-K-Ca-Mg profile suits perennial crops that need strong root systems and abundant flowering

Why unprocessed mineral rock instead of synthetic PK?

Ground Mineral Rock — this product

  • 10 nutrients from natural rock sources in a single product
  • Slow-release: minerals weather gradually via root exudates and soil biology
  • Zero nitrogen — apply P, K, Ca, Mg without forcing vegetative growth
  • Builds long-term soil mineral capital, not just this season's crop
  • 3.3% iron oxide — most PK fertilisers contain none
  • No salt index — will not burn roots or damage soil biology

Synthetic PK (Superphosphate, MOP/SOP)

  • Water-soluble: fast-acting but prone to leaching and lock-up
  • Superphosphate is acidic near the root zone — can damage biology
  • MOP (muriate of potash) contains chloride — toxic to sensitive crops
  • No calcium, no magnesium, no iron, no trace elements
  • Single-purpose inputs requiring multiple separate products
  • High salt index — risk of root burn at higher rates
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. Supplied in compostable packaging.

The science of mineral rock nutrition: slow-release fertility from the earth

How ground rock minerals feed plants

This product is not a dissolved chemical fertiliser. It is finely ground natural rock — a blend of igneous and sedimentary minerals that have been quarry-extracted and mechanically processed. The nutrients are locked in mineral crystal lattices and released gradually through two natural mechanisms: dissolution by root exudates (organic acids secreted by plant roots) and weathering by soil microorganisms (bacteria and fungi that produce acids as metabolic by-products). This means the nutrients become plant-available at a rate governed by biological activity — not a flush-and-fade pattern like water-soluble salts.

01

Phosphorus — Root Energy & Reproductive Drive

Phosphorus is a component of ATP, the universal energy currency of living cells. Every reaction requiring energy — photosynthesis, cell division, sugar transport, nitrogen fixation — depends on phosphorus. It is particularly critical during root establishment (rapid cell division) and flowering (high energy demand). The citrate-soluble phosphorus in ground rock minerals is released by the same organic acids that plant roots produce naturally, making it available precisely where and when roots are actively growing.

02

Potassium — Water Regulation & Fruit Quality

Potassium regulates stomatal opening (water loss), activates over 60 enzymes, and governs the transport of sugars from leaves to fruit. Potassium-sufficient plants produce firmer, sweeter, better-coloured fruit with longer shelf life. Deficient plants wilt under mild drought, produce watery fruit, and show marginal leaf scorch. The potassium in mineral rock is released as the feldspar and mica components weather — a gradual, sustained supply that avoids the flush-and-crash of soluble potassium salts.

03

Calcium — Structural Integrity

At 11.4% CaO, this product delivers substantial calcium. Calcium cross-links pectin in cell walls, providing structural rigidity. It is immobile in the phloem — once deposited, it cannot be redistributed. Actively growing tissues (root tips, fruit, shoot tips) require continuous external supply. Rock-derived calcium releases steadily as the mineral weathers, maintaining availability throughout the growing season without the rapid leaching associated with soluble calcium sources like calcium nitrate.

04

Iron — The Hidden Bonus at 3.3% Fe₂O₃

Most PK fertilisers contain no iron. This product delivers 3.3% Fe₂O₃ — a significant quantity from the igneous rock component. Iron is essential for chlorophyll synthesis and is a component of cytochromes and ferredoxin in the photosynthetic and respiratory electron transport chains. Iron deficiency (lime-induced chlorosis) is the most common micronutrient problem in alkaline soils. The slow-release iron oxide form weathers gradually, providing sustained availability rather than the rapid oxidation and lock-up that occurs with soluble iron salts.

05

Magnesium & Sulphur — Photosynthesis & Protein

Magnesium (3.1% MgO) is the central ion in every chlorophyll molecule and activates over 300 enzymes. Sulphur (4.2% SO₃) is a component of the amino acids cysteine and methionine — essential for protein synthesis. Both are commonly deficient in sandy, acidic, or heavily cropped soils. Receiving them alongside P, K, and Ca from a single mineral source simplifies the nutrition programme and ensures secondary nutrients are not neglected while primary nutrients are addressed.

06

Why Zero Nitrogen Matters

Nitrogen drives vegetative growth — leaf expansion, stem elongation, chlorophyll production. During flowering and fruiting, excess nitrogen diverts the plant's resources away from reproductive development into unwanted leaf growth. It dilutes sugar concentration in fruit, delays ripening, and promotes soft, disease-susceptible tissue. A zero-nitrogen PK formula lets you build phosphorus, potassium, calcium, and magnesium reserves without any of these side effects — essential for pre-flower feeding, fruit fill, autumn soil preparation, and any situation where nitrogen is already sufficient.

Scientific References

  1. Marschner, P. (2012). Marschner's Mineral Nutrition of Higher Plants. 3rd ed. Academic Press.
  2. Harley, A.D. & Gilkes, R.J. (2000). Factors influencing the release of plant nutrient elements from silicate rock powders. Nutrient Cycling in Agroecosystems, 56, 11–36.
  3. White, P.J. & Broadley, M.R. (2003). Calcium in plants. Annals of Botany, 92(4), 487–511.
  4. Mengel, K. & Kirkby, E.A. (2001). Principles of Plant Nutrition. 5th ed. Kluwer Academic.

How to use organic PK mineral fertiliser: application rates & guide

Ground mineral powder — apply directly to soil

This is a fine ground powder, not a liquid. Scatter evenly over the soil surface and work in lightly with a rake or fork. Water after application to begin the weathering process. The minerals release gradually through root exudate activity and microbial weathering — there is no risk of root burn at recommended rates.

Application rates

Garden beds, borders & vegetable plots

Rate: 100–300 g per m²  |  Timing: Before planting, at planting, or as a mid-season top dress

Use the lower rate (100 g/m²) for maintenance on reasonably fertile soil. Use the higher rate (300 g/m²) for new beds, heavily cropped areas, or soils with known phosphorus, potassium, or calcium deficiency. Scatter evenly, rake in lightly, and water.

Fruit trees, orchard trees & established shrubs

Rate: 5–10 kg per plant  |  Timing: Late winter / early spring, or after harvest

Spread evenly under the canopy from the trunk to the drip line. Use the lower rate for young trees and the higher rate for mature, heavy-cropping specimens. Lightly fork into the top few centimetres of soil and water in.

Lawns & turf

Rate: 100–150 g per m²  |  Timing: Autumn or early spring

Scatter evenly over the lawn surface. The fine powder will settle into the turf canopy with watering or rain. Particularly valuable in autumn to build root reserves and improve cold tolerance without stimulating late-season top growth.

Step-by-step application

  1. Calculate the area to be treated. Measure the bed, border, or canopy spread in square metres. Multiply by the chosen rate (e.g. 200 g/m² × 5 m² = 1 kg).
  2. Weigh out the required amount. Use kitchen scales or a measuring jug — the powder has an apparent density of approximately 1.1 g/cm³, so 1 litre of product weighs roughly 1.1 kg.
  3. Scatter evenly over the soil surface. For small areas, sprinkle by hand wearing gloves. For larger areas, use a broadcast spreader or bucket with holes.
  4. Work in lightly. Rake or fork the powder into the top 2–5 cm of soil. This places the minerals in the root zone where biological weathering is most active.
  5. Water after application. A thorough watering starts the weathering process and prevents the fine powder from blowing away on exposed sites.
Best timing for this product

Autumn/winter: Apply after harvest or during winter soil prep to build mineral reserves for the following season. The slow-release minerals will not leach or stimulate unwanted growth. Pre-planting: Incorporate into beds 2–4 weeks before planting to allow initial weathering. Mid-season: Top dress around fruiting plants at the start of flowering to support fruit set and fill without adding nitrogen.

Works well combined with…

Pair with Dr Forest Veg 4-4-4 or All-Purpose 6-6-6 during vegetative growth, then switch to this PK mineral fertiliser at flowering for nitrogen-free reproductive support. Use alongside Seaweed Powder for biostimulant activity. Combine with Yorkshire Polyhalite for additional slow-release K, Ca, Mg, and S. For liquid foliar feeding during fruiting, add Cal-Mino for chelated calcium delivery.

Frequently asked questions

By design. Nitrogen drives vegetative growth — leaf expansion and stem elongation. During flowering and fruiting, excess nitrogen diverts energy away from reproductive development, dilutes fruit sugar, and promotes soft, disease-susceptible tissue. This zero-nitrogen formula lets you supply phosphorus, potassium, calcium, and magnesium without any of these side effects. Use it at flowering, for autumn soil prep, or whenever nitrogen is already adequate.
A blend of natural igneous and sedimentary rocks, quarry-extracted and mechanically processed — crushed, classified, sieved, and blended. No chemical treatment, no synthetic additives. The nutrients come from the mineral crystal structure of the rocks themselves. EU organic compliant.
Ground rock minerals release nutrients gradually through two mechanisms: dissolution by organic acids from root exudates, and weathering by soil microorganisms. This is slower than water-soluble fertilisers but provides sustained availability over weeks and months rather than a short-lived flush. For faster results, apply 2–4 weeks before planting to allow initial weathering to begin.
Yes. Suitable for vegetables, fruit trees, soft fruit, vines, ornamentals, roses, lawns, and turf. Particularly valuable for fruiting and flowering plants during reproductive stages, and for any crop where nitrogen is already sufficient or needs to be avoided.
The 3.3% Fe₂O₃ comes from the igneous rock component of the blend. Most PK fertilisers contain no iron at all. Iron is essential for chlorophyll synthesis and is commonly deficient in alkaline soils. The slow-release iron oxide form weathers gradually, providing sustained availability without the rapid lock-up that occurs with soluble iron salts.
Different strengths. Bone meal is a concentrated phosphorus source (~12% P) but contains no potassium, minimal calcium oxide, no magnesium, and no iron. It is also a slaughterhouse by-product — not suitable for plant-based growing systems. This mineral PK delivers phosphorus alongside potassium, calcium, magnesium, sulphur, iron, and trace elements from a single non-animal source. The trade-off is a lower phosphorus percentage per weight applied.
Three ideal windows: autumn after harvest (to build mineral reserves for next season), 2–4 weeks before spring planting (to allow initial weathering), and at the start of flowering (to support fruit set without nitrogen). Can be applied year-round — the slow-release mineral form will not leach or burn.
The calcium and magnesium content will have a mild liming effect over time, gradually raising pH in acidic soils. This is generally beneficial — most vegetables and flowers prefer a pH of 6.0–7.0. The effect is gentle and gradual compared to agricultural lime.
Yes. This product is supplied in compostable packaging.
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