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How Potassium Rich Fertiliser compares
Full guide and scienceMicro-K micronised potassium mineral, 11% K₂O
- One quarried rock, ground fine; no chemical processing and no chloride
- Releases as the particle surface dissolves, so it keeps feeding for weeks to months
- Suspends in water for foliar spray or drench; also broadcasts dry
- Low salt index, so no root scorch in pots and no salt build-up
- Carries the trace minerals of the parent rock alongside the potassium
Sulphate of potash or muriate of potash, 50–60% K₂O
- Around five times the potassium per gram
- Fully soluble: fast to act, and fast to leach in rain or heavy watering
- Higher salt index, so more care needed in small pots
- Muriate of potash brings chloride, which tomatoes, strawberries and potatoes dislike
- The right tool for a sharp deficiency or peak fruiting demand; pair it with a slow source for the rest of the season
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Made by Growers · Backed by Science
Potassium rich fertiliser: micronised mineral potash, 11% K₂O, for foliar, drench and soil use
11% K₂O Chloride-Free Micronised Mineral Foliar & Drench Vegan · No Additives Packed in Stockport
Micro-K is a potassium rich fertiliser made from a single quarried rock, ground fine enough to suspend in water, with 11% K₂O and no chloride. The potassium sits inside the rock rather than in a soluble salt, so it comes out gradually as the particles dissolve in soil moisture instead of arriving in one pulse and leaching away. Dr Forest packs it in small batches in Stockport, Greater Manchester, and it is the slow-release counterpart to our Sulphate of Potash (50% K₂O) and Liquid Potassium Feed (15% K₂O).
Three ways to use it: as a soil drench from the watering can, as a foliar potassium feed during fruiting, or broadcast dry on beds, borders and lawns to build a reserve. Nothing is added; it is rock, micronised and bagged, in sizes from 500 g to 9 kg.
11%K₂O potash
0.5–1 gPer litre, foliar or drench
0Chloride
500 g–9 kgFour pack sizes
What a high potassium fertiliser does in the garden
- Tomatoes, peppers, strawberries and other fruiting crops — potassium moves sugar from leaf to fruit, so it sets the sweetness, size and colour of what you pick
- Roses and flowering shrubs — the same sugar traffic feeds bud formation and petal colour through the summer
- Dry spells — potassium runs the stomata, the pores that decide how much water a leaf loses; a plant with enough of it shuts them faster and wilts later
- Autumn hardening — higher tissue potassium is linked to less chilling damage, which is why turf and orchard managers put potash on in autumn
- Lawns — a spring and autumn dressing for wear tolerance and winter survival, with no burn risk from soluble salts
- Building a reserve — a dry broadcast on tired beds adds potassium the soil holds and releases over the season
Why micronised rock potassium rather than a soluble potash salt?
Micro-K micronised potassium mineral, 11% K₂O
- One quarried rock, ground fine; no chemical processing and no chloride
- Releases as the particle surface dissolves, so it keeps feeding for weeks to months
- Suspends in water for foliar spray or drench; also broadcasts dry
- Low salt index, so no root scorch in pots and no salt build-up
- Carries the trace minerals of the parent rock alongside the potassium
Sulphate of potash or muriate of potash, 50–60% K₂O
- Around five times the potassium per gram
- Fully soluble: fast to act, and fast to leach in rain or heavy watering
- Higher salt index, so more care needed in small pots
- Muriate of potash brings chloride, which tomatoes, strawberries and potatoes dislike
- The right tool for a sharp deficiency or peak fruiting demand; pair it with a slow source for the rest of the season
Packed in Stockport
Dr Forest is a small-batch organic fertiliser company in Stockport, Greater Manchester, founded in 2020. Micro-K is a single quarried rock with no animal by-products and no additives, so it suits vegan and no-dig gardens as it is. The 1.5 kg, 3 kg and 9 kg sizes come in recyclable packaging. If you are deciding between potash types, read Polyhalite vs sulphate of potash: which potash should you use? Made by Growers. Backed by Science.
The Science
The science of potassium: the quality nutrient, and why the rock matters
What potassium does in a plant
Potassium is the most abundant cation in plant tissue, and unlike nitrogen or phosphorus it never becomes part of a protein, a membrane or a strand of DNA. It stays as a free ion, K⁺, and does its work by moving about: into guard cells to open a stoma, into the phloem to carry sugar, across a membrane to keep the charge balanced. Wang and colleagues (2013, International Journal of Molecular Sciences) list its jobs as enzyme activation, protein synthesis, photosynthesis, osmoregulation, stomatal movement, energy transfer, phloem transport, cation–anion balance and stress resistance.1
For a gardener the practical effect is on the things you can taste and see. Lester, Jifon and Makus (2010, Plant and Soil) call potassium the cation with the strongest influence on the quality attributes that decide fruit marketability and consumer preference, and note that soil supply often fails to keep up with fruit demand during development.2 That is the case for foliar potassium as a second route in. For the nutrient basics, see What is a fertiliser?
Composition and form
| Property | Value | Note |
|---|---|---|
| Potassium (K₂O) | 11% | Held in the mineral lattice, released by dissolution |
| Chloride | None | Suits chloride-sensitive crops |
| Form | Micronised powder | Suspends in water; settles on standing |
| Origin | Quarried rock | Ground and bagged, no chemical processing |
| Other minerals | Trace elements of the parent rock | Not declared individually |
Eleven per cent is modest next to a refined potash salt. For a rock source the number that matters is how fast it gives the potassium up, and that is decided by mineralogy, particle size and what roots and soil biology do to the particle surface.
A soluble salt tells you how much potassium you bought. A ground mineral tells you how long it will last.
Mechanisms of action
01
Potassium runs the stomata, and so the water budget
Humble and Raschke (1971, Plant Physiology) used electron-probe analysis on broad bean leaves and measured roughly 4 × 10⁻¹² gram-equivalents of K⁺ moving into each pair of guard cells as a stoma opened from 2 to 12 micrometres, enough, with its balancing anions, to account for the change in guard-cell volume.3 Wang et al. (2013) describe adequate potassium as essential to drought resistance through root elongation and membrane stability.1
02
Sugar transport and fruit quality
Potassium is needed to load sucrose into the phloem and move it from leaf to fruit; Wang et al. (2013) list phloem transport of photoassimilates among its core roles.1 In field melon trials, Lester, Jifon and Makus (2010) found that weekly foliar potassium during fruit development, as potassium sulphate or glycine-complexed potassium, gave the largest gains in firmness, sugars, ascorbic acid and beta-carotene, and that feeding through the leaf outperformed soil application for the same forms.2
03
Cold and stress tolerance
The same review reports that higher tissue potassium reduced chilling damage and increased cold resistance in the crops studied, and that maintaining adequate potassium is critical for drought resistance.1 This is the basis for the autumn potash dressing on lawns and fruit trees: get the tissue concentration up before the first frost.
04
How a ground mineral releases potassium
Harley and Gilkes (2000, Nutrient Cycling in Agroecosystems) reviewed silicate rock powders as fertilisers and set out the rules: dissolution happens at the mineral surface, mainly at defects; it is driven by the soil solution being out of equilibrium with the mineral as roots and leaching remove ions; root exudates and microbial acids speed it up; and release rates are best predicted per unit of surface area.4 Grinding a rock to micron size multiplies that surface area, which is the point of micronising. They are also candid that slow dissolution is the main limit on rock powders in agriculture, which is why we do not sell this as a substitute for a soluble salt when a plant is visibly short.
05
Not all potassium rocks are equal
Manning (2010, Agronomy for Sustainable Development) reviewed mineral potassium sources and found that nepheline dissolves around 100 times faster than potassium feldspar, so a rock's usefulness depends on which mineral carries the potassium, as much as on its K₂O figure.5 Mohammed and colleagues (2014, European Journal of Soil Science) grew leeks in sandy soil with microcline feldspar, biotite mica and nepheline syenite against potassium chloride: the soluble salt gave the fastest growth, biotite came next and weathered to vermiculite during the trial, while neither the feldspar nor the nepheline syenite grew the leeks any better than the untreated control, though tissue analysis showed the nepheline had given up some potassium.6 The honest reading: a ground mineral is a reserve and a slow feed, best used alongside a faster source at peak demand.
06
Foliar uptake, and the potassium–magnesium balance
Fernández and Eichert (2009, Critical Reviews in Plant Sciences) reviewed foliar fertilisation and concluded that stomata can significantly increase uptake and that relative humidity is a key factor, which is the case for spraying at dawn or dusk when the air is damp and the pores are open.7 One caution on any high-potassium programme: Xie, Cakmak and colleagues (2021, The Crop Journal) report that potassium suppresses magnesium uptake more strongly than the reverse, and that potassium-induced magnesium deficiency is widespread where potassium is applied heavily.8 Feed magnesium alongside if you are pushing potassium through fruiting.
Scientific references
- Wang, M., Zheng, Q., Shen, Q. and Guo, S. (2013). The critical role of potassium in plant stress response. International Journal of Molecular Sciences, 14(4), 7370–7390.
- Lester, G.E., Jifon, J.L. and Makus, D.J. (2010). Impact of potassium nutrition on postharvest fruit quality: melon (Cucumis melo L.) case study. Plant and Soil, 335(1), 117–131.
- Humble, G.D. and Raschke, K. (1971). Stomatal opening quantitatively related to potassium transport: evidence from electron probe analysis. Plant Physiology, 48(4), 447–453.
- Harley, A.D. and Gilkes, R.J. (2000). Factors influencing the release of plant nutrient elements from silicate rock powders: a geochemical overview. Nutrient Cycling in Agroecosystems, 56(1), 11–36.
- Manning, D.A.C. (2010). Mineral sources of potassium for plant nutrition. A review. Agronomy for Sustainable Development, 30(2), 281–294.
- Mohammed, S.M.O., Brandt, K., Gray, N.D., White, M.L. and Manning, D.A.C. (2014). Comparison of silicate minerals as sources of potassium for plant nutrition in sandy soil. European Journal of Soil Science, 65(5), 653–662.
- Fernández, V. and Eichert, T. (2009). Uptake of hydrophilic solutes through plant leaves: current state of knowledge and perspectives of foliar fertilization. Critical Reviews in Plant Sciences, 28(1–2), 36–68.
- Xie, K., Cakmak, I., Wang, S., Zhang, F. and Guo, S. (2021). Synergistic and antagonistic interactions between potassium and magnesium in higher plants. The Crop Journal, 9(2), 249–256.
These studies describe potassium and ground-mineral fertilisers as a class. They are not trials of this product and are not yield guarantees.
How to Use
How to use Micro-K: application rates and guide
Solution-grade micronised powder
Micro-K suspends in water for foliar spraying and soil drenching. Stir or shake well before and during application; as a mineral suspension it will settle over time. It can also be broadcast directly onto soil as a dry amendment for long-term potassium building. For a 10-litre watering can, 5–10 g is 1–2 level teaspoons.
A slow source, not an emergency fix
Rock-bound potassium releases over weeks and months. If a plant is already showing scorched leaf margins in mid-season, use a soluble potash for the quick correction and use Micro-K to stop it happening next year.
Application rates
Soil drench, liquid application
Suspend in water and apply to the root zone. Particularly useful during peak fruiting and flowering when potassium demand spikes. Use at the higher rate for heavy-feeding crops like tomatoes and peppers.
Foliar spray, rapid potassium delivery
Spray both leaf surfaces in early morning or late evening. Stir the solution regularly during spraying to maintain suspension. Foliar potassium is absorbed rapidly and can supplement root uptake during peak demand.
Soil broadcast, dry application for beds and borders
Scatter evenly and work into the top layer if possible. Water in. The micronised particles release potassium gradually as they dissolve in soil moisture. Use the higher rate for known-deficient soils or heavy-feeding crops.
Trees and shrubs
Measure the canopy from the trunk out to the drip line, apply at the bed rate across that area, and work it into the soil. Water in thoroughly. Particularly important for fruit trees, vines and ornamental trees where potassium supports fruit quality and winter hardiness.
Lawns and turf
Broadcast evenly and water in. The autumn application is particularly important for winter hardening. Potassium improves frost tolerance, wear resistance and spring recovery.
Step-by-step preparation for liquid application
- Measure the powder. For a 10-litre watering can, measure 5–10 g (1–2 level teaspoons).
- Add to water and stir vigorously. The micronised powder suspends in water but will settle over time. Stir or shake regularly during application.
- Apply to foliage or root zone. For foliar sprays, use a sprayer with good agitation. For root drenches, apply evenly around the base of the plant.
- Time to demand. Potassium demand peaks during flowering and fruit development. Begin supplementing when the first flowers appear and continue through to harvest.
- Store dry powder sealed. Keep in a cool, dry place. Shelf life is indefinite when kept dry.
Works well combined with…
For higher-concentration soluble potassium during peak fruiting, combine with Sulphate of Potash (50% K₂O). Use alongside Yorkshire Polyhalite for balanced K, Ca, Mg and S. Pair with Liquid Magnesium to hold the K:Mg balance, since heavy potassium can induce magnesium deficiency if magnesium is not also supplied. Add Seaweed Powder for biostimulant activity. For a top-dressing method that works on any bed, see How to top dress like a pro.
Storage
Keep the bag sealed, cool and dry; shelf life is indefinite when kept dry. Mixed suspensions are for same-day use, as the particles settle and cake in the can. Keep out of reach of children and pets.
Questions & Answers
Frequently asked questions about potassium rich fertiliser
A potassium rich fertiliser supplies mainly potassium (potash, K₂O) with little or no nitrogen or phosphorus, for plants that are flowering, fruiting or going into winter. Micro-K is a single quarried rock micronised to a fine powder with 11% K₂O and no chloride. Use it as a drench, a foliar spray, or a dry slow-release dressing for beds, trees and lawns.
Because it is a ground rock. The potassium is locked in the mineral and comes out as particles dissolve, so it feeds slowly and cannot burn roots. Sulphate of Potash has more potassium per gram and acts within days, but it leaches quickly and has a higher salt index. They do different jobs: Micro-K for background potassium and building a reserve, sulphate of potash for a sharp correction at peak demand. Many growers use both.
Four potassium sources, four jobs. Sulphate of Potash (50% K₂O, soluble) is the strongest and quickest, for fixing a deficiency or feeding tomatoes at full fruiting. Liquid Potassium Feed (15% K₂O, chloride-free) is the easiest to dose in a can every week or two. Yorkshire Polyhalite (14% K₂O with calcium, magnesium and sulphur) is the slow granular option when you want all four in one. Micro-K is the slow option for potassium alone, and the only one you can also spray on the leaf.
Brown, dry, scorched edges on the older, lower leaves, working inwards. Potassium is mobile inside the plant, so a short plant robs its old leaves to feed new growth and the damage shows low down first. Poor fruit set, small or bland fruit and weak stems follow. In a lawn it shows as poor winter survival and a slow spring. If the yellowing is between the veins rather than at the margin, suspect magnesium; our magnesium deficiency guide shows the difference.
Yes. Suspend 0.5–1 g per litre, keep the sprayer agitated so the particles stay in suspension, and spray both leaf surfaces early morning or late evening every 7–14 days during fruiting. Damp air and open stomata help foliar uptake, which is why the timing matters. Foliar potassium tops up root uptake at peak demand; it does not replace a soil supply.
No. Like every rock dust it stays as fine particles in the water. The micronised grind keeps it suspended long enough to spray or drench, but it settles on standing, so stir or shake during use. That is normal and does not reduce what the plant gets.
Yes. Micro-K is a rock dust containing no chloride, so it suits chloride-sensitive crops: tomatoes, strawberries, potatoes, grapes and salad leaves. Muriate of potash (potassium chloride) is the one to avoid on those.
It can. Potassium and magnesium compete at the root, and potassium wins more often than it loses, so a heavy potassium programme in a confined pot can push magnesium short. If you are feeding potassium hard through fruiting, add Liquid Magnesium or use Yorkshire Polyhalite, which carries both.
Yes. Broadcast 50–150 g per m² in spring and again in autumn and water in. The autumn dressing is the one that matters most: it raises tissue potassium before frost, which is linked to better cold tolerance, wear resistance and spring recovery. Being a low-salt mineral it will not scorch the sward.
It is a quarried rock with nothing added, no synthetic processing and no animal by-products, so it is vegan and suits organic growing systems; we do not claim organic certification. The 1.5 kg, 3 kg and 9 kg sizes come in recyclable packaging. The 500 g size is not currently in recyclable packaging.
No. Epsom salt is magnesium sulphate, so it supplies magnesium and sulphur but no potash. It's useful alongside a potassium feed, though, because heavy potassium feeding can push magnesium short, especially in pots. If you need magnesium, use a magnesium product. For potassium, use a potash source such as this one, sulphate of potash or polyhalite, which carries both.
Quickly with a soluble source, then long term with a slow one. For brown, scorched edges on older leaves this season, use sulphate of potash (50% K₂O, soluble) or a liquid potassium feed, and spray this powder at 0.5–1 g per litre on both leaf surfaces every 7–14 days during fruiting. To build the soil's reserve, broadcast it at 150–350 g per m² once or twice a year on beds, or 50–150 g per m² on lawns in spring and autumn. Keep an eye on magnesium while you do it.
Here. This micronised rock potash (11% K₂O, no chloride) comes in 500 g, 1.5 kg, 3 kg and 9 kg sizes, packed in Stockport and posted to any UK address. If you need something stronger or faster, we also make sulphate of potash at 50% K₂O and a liquid potassium feed at 15% K₂O.
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