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How PK Fertiliser 0-5-5 + Calcium compares
Full guide and scienceGround rock mineral PK 0-5-5 (this product)
- Ten nutrients from one bag: P, K, Ca, Mg, S, Fe, Mn, Cu, B, Zn
- Released by root acids and soil biology over months, so it builds a reserve rather than a spike
- Zero nitrogen, so you set the leaf-to-flower balance yourself
- No salt load, so no root scorch at the stated rates and nothing to wash out in a wet spring
Soluble PK booster (superphosphate, muriate of potash, liquid PK)
- Water-soluble and fast, but it leaches on light soils and locks up on heavy ones
- Muriate of potash carries chloride, which potatoes, soft fruit and many other crops dislike
- Usually no calcium, no magnesium, no iron and no trace elements
- High salt index, so rates have to be watched to avoid scorch
Customer highlight
Used in conjunction with other Dr Forest late bloom NPK ammendments this gives your plants everything they need, the best results I have ever achieved.
Editorial highlight from marketplace and store reviews — separate from the full review list below.
Made by Growers · Backed by Science
Organic PK fertiliser 0-5-5: phosphorus, potassium and calcium from ground rock, with no nitrogen
0-5-5 NPK 11.4% Calcium (CaO) Zero Nitrogen 3.3% Iron (Fe₂O₃) Compostable Packaging Packed in Stockport
This organic PK fertiliser is a finely ground blend of natural igneous and sedimentary rock, quarried, crushed and sieved, that supplies phosphorus, potassium, calcium and magnesium with no nitrogen at all. The nutrients sit inside the mineral crystals and come out slowly, as plant roots and soil microbes produce the mild acids that weather rock. That makes it a feed for building reserves in the soil rather than a quick hit. Dr Forest packs it by hand in small batches in Stockport, Greater Manchester, and it goes out in compostable packaging.
The zero-nitrogen analysis is the point of it. Most general feeds push leaf growth whether you want it or not. This one lets you raise phosphorus, potassium, calcium and magnesium levels for flowering and fruiting, for autumn soil preparation, or for beds where nitrogen is already plentiful, without a flush of soft growth on top.
5.1%P₂O₅ Phosphorus
5.0%K₂O Potassium
11.4%CaO Calcium
3.1%MgO Magnesium
What a phosphorus and potassium fertiliser like this is used for
- Flowering and fruiting without a nitrogen push — phosphorus and potassium for the reproductive stage, with none of the nitrogen that would send energy back into leaf
- Root establishment at planting — phosphorus for root cell division and calcium for the cell walls of new root tips; work it in before or at planting
- Autumn and winter soil preparation — apply after harvest to build mineral reserves for next season; ground rock does not leach over winter the way soluble salts do
- Calcium and magnesium top-up — 11.4% CaO and 3.1% MgO in one product, useful on sandy, acidic or heavily cropped soils
- Fruit trees, orchards, vines and ornamentals — perennials that want a strong root system and a full set of flowers
- Lawns in autumn — root reserves and cold tolerance without late-season top growth
Why ground rock rather than a soluble PK booster?
Ground rock mineral PK 0-5-5 (this product)
- Ten nutrients from one bag: P, K, Ca, Mg, S, Fe, Mn, Cu, B, Zn
- Released by root acids and soil biology over months, so it builds a reserve rather than a spike
- Zero nitrogen, so you set the leaf-to-flower balance yourself
- No salt load, so no root scorch at the stated rates and nothing to wash out in a wet spring
Soluble PK booster (superphosphate, muriate of potash, liquid PK)
- Water-soluble and fast, but it leaches on light soils and locks up on heavy ones
- Muriate of potash carries chloride, which potatoes, soft fruit and many other crops dislike
- Usually no calcium, no magnesium, no iron and no trace elements
- High salt index, so rates have to be watched to avoid scorch
Packed in Stockport
Dr Forest is a small-batch organic fertiliser business in Stockport, Greater Manchester. This is a mined mineral product with no animal inputs, weighed and packed by hand at our workshop and supplied in compostable packaging. If you are choosing between a mineral PK and a high-phosphorus organic blend, what is a bloom fertiliser? sets out the difference. Made by Growers. Backed by Science.
The Science
The science of mineral PK: how ground rock feeds a plant
What this product actually is
It is rock, ground fine. A blend of igneous rock (the source of most of the potassium, magnesium and iron) and sedimentary rock (the main source of phosphorus and calcium), quarried, crushed, sieved and blended. No acid treatment, no chemical processing. The nutrients are held in the crystal lattices of minerals such as feldspar, mica, apatite and calcite, and they only become plant-available when those minerals dissolve.
That dissolution is what everything else on this page depends on. Harley and Gilkes (2000) reviewed the geochemistry of silicate rock powders and found that release depends on how far the soil solution is from equilibrium with the mineral, on the surface area exposed (which is why the powder is ground fine), and on biological activity in the root zone, which supplies the acids that attack mineral surfaces [1]. Swoboda, Döring and Hamer (2022) reached a similar conclusion: slow and inconsistent weathering held rock powders back historically, and results in any trial come down to the rock, the grind, the rate and the soil [2]. Best to be straight about that. Ground rock builds a reserve over seasons, and how fast depends on the soil it lands in.
Full mineral analysis
| Nutrient | Declared as | Analysis |
|---|---|---|
| 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% |
Two things stand out. Calcium is the largest single nutrient, more than twice the phosphorus or potassium, so this behaves partly as a gentle calcium fertiliser and partly as a PK feed. And the figures are totals held in the rock, not water-soluble fractions, which is why the application rates are generous by the standards of soluble feeds.
Soluble feeds are spent in weeks. Ground rock is still there next year.
Mechanisms of action
01
Roots dissolve rock phosphate with their own acids
Phosphorus in sedimentary rock is mostly calcium phosphate, and it does not dissolve readily on its own. Hinsinger (2001), reviewing phosphorus bioavailability in the rhizosphere in Plant and Soil, described how roots change the chemistry of the soil around them, releasing protons and organic anions such as citrate and malate that dissolve sparingly soluble calcium phosphates, and noted that the response varies with the plant's nutrient status and is more pronounced under phosphorus deficiency [3]. In practice, phosphorus is released where roots are actively growing and asking for it.
02
Potassium comes out of feldspar and mica slowly
Manning (2010), in Agronomy for Sustainable Development, reviewed the silicate minerals that hold potassium. Feldspars dissolve slowly; micas and nepheline considerably faster, and nepheline about a hundred times faster than potassium feldspar. Soil organisms and plant roots speed all of them up [4]. The potassium here is a reserve that soil biology draws on over the season, a different thing from the near-instant availability of sulphate of potash.
03
Potassium governs water and sugar movement
Wang and colleagues (2013), reviewing potassium in the International Journal of Molecular Sciences, list its roles in enzyme activation, protein synthesis, photosynthesis, osmoregulation, stomatal movement and phloem transport, and its part in drought tolerance and crop quality [5]. It is the nutrient most closely tied to how fruit fills and how a plant copes with a dry spell, which is why a potassium reserve going into flowering matters.
04
Calcium builds the cell wall
White and Broadley (2003), in Annals of Botany, describe calcium as required for structural roles in the cell wall and membranes [6]. Standard texts add that it is barely mobile in the phloem, so fast-growing tissues that transpire little, such as fruit and shoot tips, need a continuous supply from the soil [7]. At 11.4% CaO this product delivers that supply steadily as the rock weathers. If blossom end rot is the problem you are chasing, read blossom end rot in tomatoes first; it is usually a watering issue rather than a calcium shortage.
05
Magnesium and sulphur, so the secondary nutrients are not forgotten
Magnesium (3.1% MgO) is the central atom of every chlorophyll molecule; sulphur (4.2% SO₃) goes into the amino acids cysteine and methionine and so into every protein [7]. Both are commonly short on sandy, acidic or heavily cropped soils. Getting them in the same bag as phosphorus, potassium and calcium is a practical advantage, because they are the nutrients most often left out of a feeding plan.
06
Iron: plenty in the bag, slow to become available
At 3.3% Fe₂O₃ this blend carries far more iron than a typical PK feed, but iron oxide is one of the least soluble things in soil. Colombo and colleagues (2014), in the Journal of Soils and Sediments, note that soils usually hold abundant total iron while the available fraction is very low, especially at high pH, and that plants and microbes mobilise it by releasing organic acids and siderophores [8]. Treat the iron here as a long-term contribution to the soil pool. It will not fix chlorosis this season; for a quick correction, a chelated liquid such as Liquid Micronutrients is the right tool.
What the field evidence says
Much of the trial work on rock powders is on basalt in cereals, so read across to a garden blend with care. Kelland and colleagues (2020) grew sorghum in soil amended with crushed basalt at 10 kg per m² and recorded a 21% (± 9.4%) yield increase after 120 days with no P or K fertiliser added [9]. That rate is well above anything on this page, and sorghum is not a tomato. Van Straaten (2006) argued that rock and mineral fertilisers are most useful on weathered, acidic, nutrient-depleted soils and when combined with organic matter [10]. The sensible reading for a UK garden: ground rock works slowly, and best in a living soil with organic matter to drive the weathering. So pair it with compost or Seaweed Extract Powder and keep the soil covered.
Scientific references
- Harley, A.D. & Gilkes, R.J. (2000). Factors influencing the release of plant nutrient elements from silicate rock powders: a geochemical overview. Nutrient Cycling in Agroecosystems, 56, 11–36.
- Swoboda, P., Döring, T.F. & Hamer, M. (2022). Remineralizing soils? The agricultural usage of silicate rock powders: A review. Science of the Total Environment, 807, 150976.
- Hinsinger, P. (2001). Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review. Plant and Soil, 237(2), 173–195.
- Manning, D.A.C. (2010). Mineral sources of potassium for plant nutrition. A review. Agronomy for Sustainable Development, 30(2), 281–294.
- Wang, M., Zheng, Q., Shen, Q. & Guo, S. (2013). The critical role of potassium in plant stress response. International Journal of Molecular Sciences, 14(4), 7370–7390.
- White, P.J. & Broadley, M.R. (2003). Calcium in plants. Annals of Botany, 92(4), 487–511.
- Marschner, P. (ed.) (2012). Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London.
- Colombo, C., Palumbo, G., He, J.-Z., Pinton, R. & Cesco, S. (2014). Review on iron availability in soil: interaction of Fe minerals, plants, and microbes. Journal of Soils and Sediments, 14(3), 538–548.
- Kelland, M.E., Wade, P.W., Lewis, A.L. et al. (2020). Increased yield and CO₂ sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil. Global Change Biology, 26(6), 3658–3676.
- van Straaten, P. (2006). Farming with rocks and minerals: challenges and opportunities. Anais da Academia Brasileira de Ciências, 78(4), 731–747.
Figures describe the published literature on ground rock minerals and plant nutrition. They are not product-specific yield guarantees.
How to Use
How to use organic PK mineral fertiliser: application rates and guide
Ground mineral powder, applied 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, so there is no risk of root burn at recommended rates.
Give it time
Ground rock releases over months. Where you can, incorporate it 2–4 weeks before planting so weathering has started, and pair it with a nitrogen feed during vegetative growth, because this one contains none.
Application rates
Garden beds, borders and vegetable plots
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 and established shrubs
Measure the canopy from the trunk out to the drip line and apply at the bed rate across that area. 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 and turf
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
- 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).
- 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.
- 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.
- 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.
- Water after application. A thorough watering starts the weathering process and stops the fine powder 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 Organic All Purpose Fertiliser 4-4-4 during vegetative growth, then switch to this PK mineral fertiliser at flowering for nitrogen-free reproductive support. Use alongside Seaweed Extract Powder for biostimulant activity. Combine with Yorkshire Polyhalite for additional slow-release K, Ca, Mg and S. For liquid foliar feeding during fruiting, add Amino Chelated Calcium for chelated calcium delivery.
Storage
Keep the bag sealed and dry. Ground rock does not degrade, but damp powder cakes and is harder to spread. Keep out of reach of children and pets, as with any fertiliser.
Questions & Answers
Frequently asked questions about organic PK fertiliser
A PK fertiliser supplies phosphorus (P) and potassium (K) without nitrogen (N). The three numbers are the NPK analysis: 0% nitrogen, 5.1% phosphorus as P₂O₅ and 5.0% potassium as K₂O. This one is a ground rock mineral rather than a processed salt, and it also carries 11.4% calcium (CaO), 3.1% magnesium (MgO), 4.2% sulphur (SO₃), 3.3% iron (Fe₂O₃) and traces of manganese, copper, boron and zinc.
By design. Nitrogen drives leaf and stem growth. During flowering and fruiting, more nitrogen than the plant needs pulls energy into leaf, and in autumn it pushes soft growth at the wrong time of year. A zero-nitrogen feed lets you build phosphorus, potassium, calcium and magnesium whenever you like. During vegetative growth, pair it with a feed that does contain nitrogen, such as All Purpose 4-4-4.
Not in the sense most people mean. A PK booster is usually a soluble liquid or salt that gives a fast hit of phosphorus and potassium over a couple of weeks. This is ground rock. It releases over months as roots and soil microbes weather it, and it is far better suited to building a reserve in beds, borders and around fruit trees than to a quick pre-flower push. If you want a fast, soluble potassium feed for the last weeks of a crop, Organic Liquid Potassium Feed is the one.
Gradually. Two things drive release: organic acids from plant roots, and weathering by soil bacteria and fungi. That is slower than a water-soluble feed but it lasts across a season and beyond. For the best chance of seeing an effect on this year's crop, apply 2–4 weeks before planting so weathering has started by the time roots arrive.
Yes. It suits vegetables, fruit trees, soft fruit, vines, roses, ornamentals, lawns and turf. It is most useful for flowering and fruiting crops during the reproductive stage, and for any bed where nitrogen is already adequate. For tomatoes in pots and growing bags it is a background reserve worked into the mix at planting; the Organic Tomato Fertiliser 3-4-6 does the regular feeding.
The calcium and magnesium have a mild liming effect over time, so it will gradually nudge acidic soils upward. For most vegetables and flowers, which prefer a pH of 6.0–7.0, that is helpful. The effect is gentle compared with garden lime. On already alkaline soil it is not the product to use for a big calcium correction; use Liquid Gypsum, which adds calcium without raising pH.
It is ground rock, with no synthetic additives and no soluble salts, so there is nothing to scorch a lawn or contaminate a pond. As with any fertiliser, keep the bag out of reach, wear gloves when scattering by hand, and water it in so it is not left on the surface as loose dust.
It is made from natural rock with no chemical treatment and no synthetic additives, so it fits organic, no-dig and living-soil gardening. It is a mined mineral with no animal inputs, so it is suitable for vegan growing. We do not hold organic certification, so we describe it as made with organic ingredients rather than certified.
They overlap but do different jobs. Polyhalite is a single Yorkshire-mined mineral with 14% K₂O, 17% CaO, 6% MgO and 48% SO₃, and no phosphorus; it is the stronger potassium and sulphur source. Rock Phosphate is 31% P₂O₅ and 46% CaO with no potassium; it is the concentrated phosphorus source. This PK 0-5-5 sits between them: less of each, but phosphorus, potassium, calcium, magnesium, sulphur and iron in one application.
The rock is quarried, crushed and sieved, then weighed and packed by hand in small batches at our workshop in Stockport, Greater Manchester. It is supplied in compostable packaging.
This one goes on before or at planting, or as a mid-season top dressing, at 100–300 g per m². Use the lower rate on reasonably fertile soil and the higher rate for new beds or hard-cropped ground. It's ground rock and releases over months, so put it down a few weeks before plants start flowering rather than waiting for the first buds. It also suits autumn, after harvest, to build reserves for next season, and lawns in autumn, because it adds no nitrogen to push late soft growth.
For a slow, season-long reserve, scatter this ground-rock PK at 100–300 g per m² and work it into the top few centimetres. It also brings calcium (11.4% CaO), magnesium (3.1% MgO) and sulphur, with no salt load and nothing to wash out over winter. If plants need a quick lift this season, use a soluble feed alongside it, such as Bloom Booster 2-10-5, and add compost: soil life is what weathers the rock.
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