Fertiliser
Organic Tomato Fertiliser | Slow Release 3-4-6
A slow-release 3-4-6 feed for tomatoes.
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Dr Forest
Organic Potato Fertiliser | Slow Release Feed
Organic Potato Fertiliser | Slow Release Feed
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£11.50 GBP
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£11.50 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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Potato Fertiliser 3-5-8 — designed for starch, flavour and skin quality
3-5-8 NPK 20 Ingredients Starch & Flavour Led Slow Release No Slaughterhouse Waste Handmade in the UK
The potato is one of the most nutritionally demanding crops a home grower can grow — and one of the most rewarding when grown well. The 3-5-8 NPK ratio is built around what the peer-reviewed literature consistently identifies as decisive for tuber quality: potassium dominant to drive starch accumulation, dry matter and flavour; elevated phosphorus for tuber initiation and root development; nitrogen deliberately moderate to prevent the excess vegetative growth that diverts energy away from the crop underground.
Twenty ingredients including Yorkshire Polyhalite, Scottish Seaweed, Phosphorous Meal, Gypsum, Mealworm Frass, EM-1 Microorganisms and Fermented Biochar deliver a complete slow-release feed with 7% calcium from four sources for skin quality and disease suppression, and a K:N ratio exceeding 2.5:1. Handcrafted in Stockport — no slaughterhouse waste.
3-5-8NPK Ratio
7%Calcium (4 sources)
2.4%Magnesium (3 sources)
2.7:1K:N Ratio
What this formula does for your potatoes
- Higher starch, deeper flavour — chloride-free potassium at 8% activates starch synthase and drives sugar-to-starch conversion in bulking tubers
- More tubers, better set — elevated phosphorus from Phosphorous Meal and Micronised Rock Phosphate drives stolon elongation and tuber initiation in the critical 4–8 week window
- Clean skin, less scab — sulphur from Yorkshire Polyhalite and Gypsum suppresses Streptomyces scabies; Silica Meal strengthens the tuber periderm
- Lower nitrate in tubers — moderate organic nitrogen produces 30–50% lower nitrate than mineral-fed tubers with cleaner taste and longer storage life
- Living soil biology — EM-1 Microorganisms and chitin from Mealworm Frass activate systemic acquired resistance and suppress soil-borne pathogens
- Trace mineral depth — Scottish Seaweed, Volcanic Rock Dust and Clay Minerals supply the enzyme cofactors for volatile aroma compounds
Dr Forest Potato vs liquid potato feed
Dr Forest Potato Fertiliser 3-5-8
- 20 ingredients — 7% Ca, 2.4% Mg and broad trace minerals
- Chloride-free potassium only — chloride suppresses starch synthesis
- Slow-release organic fractions feed for weeks per application
- Every application permanently improves soil structure and biology
- Sulphur supply suppresses common scab
- Apply at planting and earthing up — not weekly
Typical Liquid Potato Feed
- 3 nutrients — NPK and nothing else
- Often contains muriate of potash — actively suppresses starch
- No calcium, no sulphur, no trace minerals
- Feast-and-famine cycle — leaches before tubers utilise it
- Adds nothing to soil structure or biology
- Weekly dosing required throughout the season
Handcrafted in Stockport
Dr Forest fertilisers are blended in small batches from traceable British ingredients. Named after Joe's grandfather — an NHS GP who believed in doing things properly. No slaughterhouse waste. No shortcuts.
All 20 ingredients — what they do and why they are in the formula
Every ingredient is here for a specific, research-backed reason. Nothing is filler. The potassium mineral is mined in North Yorkshire. The seaweed is hand-harvested from Scottish waters. The biochar is British-sourced and fermented before blending.
01
Nitrogen Plant Extract — Plant-derived, controlled release
Primary nitrogen carrier at 12% N. Mineralises over 6–8 weeks without nitrate spikes. A nitrogen spike during bulking redirects energy into haulm at the expense of starch accumulation.
02
Phosphorous Meal — Heat-treated plant meal, fast-moderate release
Heat treatment dramatically increases P availability. At 15% P and 9% Ca, this is the primary fast-acting P source for stolon elongation and tuber initiation.
03
Sulphate of Potash (SOP) — Mineral, immediate release
Fast-acting potassium at 50% K₂O — chloride-free. Activates starch synthase and drives sugar-to-starch conversion immediately. No muriate forms anywhere in this formula.
04
Gypsum (Calcium Sulphate) — Mineral, immediate release
23.3% calcium and 18.6% sulphur in immediately plant-available sulphate form. Strengthens cell walls and tuber skin. Sulphur acidifies the rhizosphere to suppress common scab.
05
Micronised Magnesium Mineral — Mineral, sustained release
20.9% Mg at 5-micron particle size. The central atom of every chlorophyll molecule — without it, photosynthesis fails and carbohydrate transport to tubers stalls.
06
Yorkshire Polyhalite — North Yorkshire, slow release 50–60 days
K, Ca, Mg and S from a single crystal. Mined 1,200m below the North Sea. Extends K supply by 50–60 days after SOP's immediate release is exhausted. Sulphur increases dry matter and suppresses scab.
07
Alfalfa Meal — Plant-based, slow release, biostimulant
Contains triacontanol — increases chlorophyll content by 15–20%. More photosynthate means more sucrose available for starch conversion in developing tubers.
08
Micronised Rock Phosphate — Mineral, slow reserve
31% P and 49% Ca. Dissolves slowly as long-term reserve. Handles the final stretch when tubers are still bulking in August.
09
Magnesium Carbonate — Mineral, moderate release
The fastest-acting of the three Mg sources at 20% Mg. Dissolves within weeks in moist soil, providing early-season correction while Micronised Magnesium Mineral and Yorkshire Polyhalite build through the season.
10
Scottish Seaweed Meal — Hand-harvested
Over 60 trace elements. Natural cytokinins delay haulm senescence. Alginates improve soil structure and moisture retention in the tuber development zone.
11
Rapeseed Meal — British, slow release
Steady nitrogen over 6–8 weeks through microbial breakdown. Acts as a prebiotic carbon source. Gradual mineralisation avoids nitrate spikes that suppress starch.
12
Mealworm Frass — Sustainably reared, SAR activator
Contains chitin. Plants detect it as a pest signal and upregulate Systemic Acquired Resistance, priming defences against Pythium, Rhizoctonia and other potato pathogens.
13
Humic Acid & Fulvic Acid — Mineral organic, chelation
Chelates micronutrients, increases soil bacterial biomass by 30–60%, stimulates mycorrhizal colonisation by 25–40%. Fulvic acid increases nutrient uptake during rapid bulking.
14
Fermented Biochar — British, activated
Permanent porous carbon scaffold. Increases K retention by 18–35% under leaching conditions — particularly valuable in potato beds where earthing up and watering leach soluble nutrients.
15
Clay Minerals — British, permanent CEC reservoir
Montmorillonite and illite clays. Ionic reservoirs that bind and slowly release K, Ca and Mg. Valuable in the light or sandy soils where potatoes are commonly grown.
16
Seaweed Extracts — British coastal, biostimulant
Concentrated cytokinins, betaines for drought tolerance, and mannitol for beneficial rhizobacteria. Enhances nutrient uptake and tuber set.
17
Volcanic Rock Dust (Basalt) — Mineral, trace elements
Zinc, iron, copper, manganese — the enzyme cofactors for volatile aroma compounds that give well-grown potatoes their characteristic earthy depth.
18
EM-1 Microorganisms — Living culture
Bacteria, yeasts, actinomycetes and lactic acid bacteria. Suppresses pathogens, accelerates organic matter decomposition, builds the diverse rhizosphere that keeps scab in check.
19
Silica Meal — Mineral, structural
Silicon strengthens the tuber periderm — improving skin finish, reducing scab damage and increasing storage life. Also a physical barrier against pest damage and fungal penetration.
20
Herbal Mixture — Plant-derived, biostimulant
Comfrey (K-rich), nettle (iron, silica), yarrow (phosphorus-solubilising bacteria), chamomile (rhizobacteria support). Broad-spectrum biological stimulus.
How to use: rates, timing & method
Before you start
This product is a milled powder with a bulk density of 1 g/ml — grams and millilitres are interchangeable. Never apply in direct contact with seed tubers — always mix into surrounding soil with a 10 cm gap. Water thoroughly after application. For best results, mix 1:1 with compost before applying.
Step-by-step: containers and grow bags
- Prepare your compost. 5–7.5g per litre. Mix thoroughly. 5g/L for enriched compost; 7.5g/L for plain or peat-free mixes.
- Fill and plant. Half depth of prepared compost. Seed tuber eyes upward. Cover with 10–15cm.
- Earth up as shoots emerge. When 10–15cm tall, add compost leaving 5cm exposed. Add 1.5–2.5g per litre of added compost.
- Continue until full. Include top-dress dose each time. Once full, switch to surface top-dressing.
- Surface top-dress every 3–4 weeks. 1.5–2.5g per litre of container volume. 10cm from stem. Scratch in and water.
- Monitor plant signals. Very dark haulm, few flowers: stretch to 5–6 weeks. Pale lower leaves from midsummer: normal remobilisation.
Application rates
Containers, grow bags & pots
| Situation | Rate (g = ml) | Frequency | Notes |
|---|---|---|---|
| Potting mix preparation | 5–7.5g per litre | Once at planting | 5g/L enriched mixes. 7.5g/L plain or peat-free. |
| Container top-dressing | 1.5–2.5g per litre of pot volume | Every 3–4 weeks | 1.5g/L earlies. 2–2.5g/L maincrops. |
Outdoor beds, raised beds & allotments
| Situation | Rate (g = ml) | Frequency | Notes |
|---|---|---|---|
| Initial soil preparation | 125–175g per m² | Once before planting | Fork into top 15–20cm. 175g/m² for depleted beds. |
| Outdoor top-dressing | 100–150g per m² | Every 3–4 weeks | 100g/m² earlies. 150g/m² maincrops through bulking. |
| Single plant at planting | 30–45g per plant | Once at planting | Mix into planting hole. 10cm gap from tuber. |
| Single plant top-dressing | 20–35g per plant | Every 3–4 weeks | Ring 10–15cm from stem. Time with earthing up. |
Seasonal feeding guide
| Stage | Timing | Rate & Frequency | Goal |
|---|---|---|---|
| Bed / container prep | 2–4 weeks before planting | Beds: 125–175g/m². Pots: 5–7.5g/L | Build nutrient-rich root zone |
| At planting | At time of planting | 30–45g per planting position | Localised nutrition for emerging stolons |
| First earthing up | 3–4 weeks after emergence | Beds: 100–150g/m². Pots: 1.5–2.5g/L | K and P into tuber initiation zone |
| Active bulking | 4–6 weeks after tuber initiation | Same, every 3–4 weeks | Sustained K for starch accumulation |
| Ripening & curing | Haulm yellowing | Stop or single low-rate application | Allow skin set; excess N reduces storage life |
Works well combined with…
Use Dr Forest Seaweed Powder as a fortnightly foliar. Apply Dr Forest Liquid Gypsum as a root drench for additional calcium during tuber initiation. Use the Dr Forest All-Purpose 6-6-6 for companion plantings.
Storage
Cool, dry place out of direct sunlight. Keep sealed. Effective for at least 18 months.
Potato growing guide — varieties, chitting, earthing up & feeding by type
Not all potatoes grow the same way. This guide covers the practical differences between variety types grown in the UK — and how to adjust your fertiliser programme to get the best from each one.
Feeding adjustments by variety type
First earlies — Rocket, Swift, Casablanca, Lady Christl, Pentland Javelin
Rate: Lower end of range | Frequency: Every 4 weeks | Top-dresses: 1–2 max
Short season (10–12 weeks). Full base dressing, then 1 top-dress at first earthing up. A second only if the season is long. Earlies are harvested before full starch accumulation — they are valued for thin skins and fresh, waxy texture. Lower K demand than maincrops.
Second earlies & salad — Charlotte, Anya, Nicola, Kestrel, International Kidney (Jersey Royal)
Rate: Lower to mid-range | Frequency: Every 3–4 weeks | Top-dresses: 1–2
Waxy varieties bred for firm texture and buttery flavour. The 3-5-8 formula's K emphasis improves sugar content and skin quality without pushing starch so high it changes the waxy character. Moderate feeding. Harvest when haulm just begins to yellow.
Floury maincrops — King Edward, Maris Piper, Rooster, Kerr's Pink, Golden Wonder
Rate: Upper end of range | Frequency: Every 3 weeks at peak | Top-dresses: 2–3
The varieties where the 3-5-8 formula makes the most dramatic difference. Floury varieties are genetically programmed for high starch — but that potential is only realised with sustained potassium throughout the extended bulking period. Full programme. Upper rates. This is what the formula was designed for.
All-rounders — Desiree, Cara, Sarpo Mira, Wilja, Estima
Rate: Mid-range | Frequency: Every 3–4 weeks | Top-dresses: 2–3
Versatile varieties that respond well to standard rates. Sarpo Mira has exceptional blight resistance and benefits from the full maincrop programme. Desiree's red skin colour intensifies with adequate potassium and the anthocyanin support from the trace mineral profile.
Heritage & specialist — Pink Fir Apple, Shetland Black, Highland Burgundy Red, Ratte, Salad Blue
Rate: Mid-range | Frequency: Every 3–4 weeks | Top-dresses: 2–3
Heritage varieties retain genetic capacity for complex flavour and respond more dramatically to organic feeding than modern commercial types. The volatile aroma compounds and anthocyanin pigments that make these varieties special are produced more abundantly under K-rich organic nutrition. Worth the extra attention.
Containers vs beds
Containers, grow bags & patio pots
- Best for first and second earlies — shorter varieties, faster harvest
- Limited compost volume means nutrients leach faster — biochar and clay minerals in the formula help
- Earth up by adding compost layers as shoots grow
- Water daily in warm weather — consistent moisture prevents scab and hollow heart
- Feed at every earthing up, then surface top-dress every 3–4 weeks
- Drain freely — waterlogging rots tubers
Outdoor beds, raised beds & allotments
- Full range of varieties — earlies through to late maincrops
- Larger soil volume buffers nutrients and moisture better
- Earth up by drawing soil from between rows
- Water deeply 2–3 times per week rather than little and often
- Mulch after final earthing up to conserve moisture and suppress weeds
- Rotate — never grow potatoes in the same bed two years running
Chitting — do you need to?
Chitting is optional but valuable for earlies
Chitting (pre-sprouting) gives first and second earlies a 2–3 week head start. It is less important for maincrops — some growers skip it entirely for late varieties. If you chit: stand tubers upright in egg boxes in a cool (8–10°C), bright room from late January. Plant when sprouts are 1.5–2.5cm long. Do not let sprouts exceed 3cm — they become fragile and snap during planting.
Earthing up — why it matters
- Triggers stolon formation. Burying the base of the stem stimulates the plant to produce the underground lateral stems from which tubers develop. More earthing up = more potential tuber sites.
- Prevents greening. Tubers exposed to light produce solanine — toxic and bitter. Earthing up keeps developing tubers in darkness.
- Delivers fertiliser to the stolon zone. Apply your top-dress dose just before or during earthing up. The fresh soil drawn over the fertiliser creates the ideal microbial breakdown environment.
- When to earth up. First time: when shoots reach 15–20cm. Second time: 2–3 weeks later when shoots re-emerge. Container growers: add compost layers instead of drawing soil.
UK seasonal timeline
| Month | What to Do |
|---|---|
| Jan–Feb | Chit early varieties in egg boxes. 8–10°C, bright but not direct sun. Order seed tubers early — popular varieties sell out. |
| March | Prepare beds: fork in 125–175g/m² base dressing 2–4 weeks before planting. Plant first earlies mid-March in mild areas; late March elsewhere. |
| April | Plant second earlies and maincrops. 30–45g per planting hole. Prepare containers with 5–7.5g/L charged compost. |
| May | First earthing up when shoots reach 15–20cm. Apply first top-dress (100–150g/m²) during earthing up. Keep soil moist. |
| June | Second earthing up. Continue top-dressing every 3–4 weeks. Earlies: harvest when flowers open or haulm begins to yellow. Consistent watering critical for scab prevention. |
| July | Harvest first and second earlies. Maincrops in active bulking — feed at upper end every 3 weeks. Monitor for blight in warm, humid weather. |
| August | Final top-dress for maincrops. Reduce watering slightly in the last 2 weeks to concentrate starch and dry matter. Cut haulm if blight appears. |
| Sep–Oct | Harvest maincrops on a dry day. Cure on soil surface for 1–2 hours. Store in cool, dark, ventilated conditions. Pull spent haulm and compost (unless blighted). |
Common problems
Common scab
Cause: Streptomyces scabies in alkaline, dry, biologically poor soil
The sulphur, silica and EM-1 in this formula directly address all three risk factors. Consistent watering during weeks 2–6 after tuber initiation is the single most important non-fertiliser intervention. Do not lime before potatoes. Choose scab-resistant varieties (Cara, Sarpo Mira, Kestrel) if your soil is prone.
Blight (Phytophthora infestans)
Cause: Fungal spores spread in warm, humid, wet conditions — typically July–August
No fertiliser prevents blight — it is an airborne fungal disease. Choose blight-resistant varieties for outdoor growing (Sarpo Mira, Sarpo Axona, Carolus). Monitor the Hutton Criteria blight forecast. Cut haulm at ground level immediately if blight appears. Do not compost blighted foliage. Tubers below ground are usually safe if haulm is removed promptly.
Hollow heart
Cause: Rapid, uneven growth — typically from erratic watering or excess nitrogen
The moderate 3% N in the formula prevents the growth surges that cause hollow heart. Consistent watering and even fertiliser distribution are the prevention. Most common in large-tubered varieties (King Edward, Maris Piper) during rapid bulking.
Greening (solanine formation)
Cause: Tubers exposed to light during growth or after harvest
Thorough earthing up is the prevention. The stronger periderm from Silica Meal and calcium slows light penetration, but physical coverage is essential. After harvest: store immediately in darkness. Green potatoes should not be eaten.
Slug damage
Cause: Slugs tunnelling into tubers — especially in wet conditions and heavy soils
Harvest earlies promptly. For maincrops, harvest before autumn rains intensify. Choose slug-resistant varieties (Sarpo Mira, Cara, Kestrel). Organic slug pellets around the base of the earthed-up ridge are effective. The strong skin structure from calcium and silica provides some resistance.
Blackleg
Cause: Bacterial infection (Pectobacterium) from infected seed tubers or waterlogged soil
Buy certified seed tubers. Ensure good drainage. Do not overwater. Remove and destroy affected plants immediately — do not compost. The competitive microbial community from EM-1, biochar and humic acid provides some suppression but cannot overcome infected seed stock.
The science of potato flavour, starch and why nutrition is decisive
Potato flavour combines four systems: starch (texture and body), sugars (sweetness and Maillard browning), amino acids (Strecker degradation volatiles), and VOCs (earthy, nutty aroma). All four are profoundly influenced by nutrition.
N 3%Prevents dilution
P 5%Tuber set
K 8%Starch & dry matter
Ca 7%Skin & scab defence
Why 3-5-8
Potato NPK uptake approximates 1 : 0.25 : 1.4–1.5. The 3-5-8 ratio delivers K:N of 2.7:1 — firmly in the range where potassium dominates starch synthase activation, sugar-to-starch conversion and carbohydrate transport into developing tubers.
Why sulphate, not muriate
The chloride ion in muriate of potash inhibits starch synthase. SOP and polyhalite deliver potassium as sulphate — promoting maximum starch accumulation. No chloride anywhere in the formula.
Mechanisms of action
01
Starch accumulation via potassium
K activates starch synthase and drives phloem loading of sucrose into developing tubers. Chloride-free K from SOP (immediate) and Polyhalite (50–60 day sustained).
02
Low nitrogen preserves flavour
Excess N produces watery tubers with low starch, elevated nitrate and bitter taste. 3% N through slow organic mineralisation prevents this.
03
Dual-speed phosphorus for tuber set
Phosphorous Meal for early demand; Micronised Rock Phosphate for months-long reserve. Undersupplied P is one of the most common causes of poor set.
04
Disease suppression and skin quality
Sulphur acidifies the rhizosphere and suppresses Streptomyces — reducing scab 30–60%. Silica strengthens the periderm. EM-1 and chitin build competitive microbial communities.
05
Microbial metabolites and flavour
Decomposing organic ingredients generate secondary metabolites that directly influence volatile aroma biosynthesis. Biologically active soil produces higher methional and hexanal.
06
7% calcium from four sources
Gypsum (immediate), Phosphorous Meal (9% Ca), Micronised Rock Phosphate (49% Ca, slow), Yorkshire Polyhalite (17% Ca, sustained). Continuous availability.
Study data
| Study | Finding |
|---|---|
| Field trials incl. Impala (2020–2024) | Organic: dry matter +10–15%, starch +15–30%. Nitrate below permissible norms. |
| Biofertiliser trials (2023–2024) | Organic NPK + biologicals: yield +104%, starch +79%. Low-N, high P/K outperformed high-N. |
| Global meta-analysis (2023) | Combined NPK + organic: yield +31%, nutritional quality +12–30%. |
| K and starch synthesis | Optimal K:N 1.5–2.5:1. SOP better than muriate for starch and cooking quality. |
| Sulphur and scab | Sulphur (incl. polyhalite) reduced scab 30–60%. |
| Organic matter and flavour VOCs | Higher methional, 2-ethyl-1-hexanol, hexanal across all cooking methods. |
References
- Hopkins, B.G. et al. (2010). Potato nutrition and quality. American Journal of Potato Research.
- Lal, M.K. et al. (2022). Starch biosynthesis in potato. Carbohydrate Polymers.
- Klikocka, H. et al. (2016). Sulphur and nitrogen effects on potato tuber amino acids. Journal of Elementology.
- Wang, Y. et al. (2023). Combined organic–mineral meta-analysis. Science of the Total Environment.
- Leake, A.R. (2003). Organic and conventionally grown potatoes. Aspects of Applied Biology.
Frequently asked questions
Yes — sulphur from Yorkshire Polyhalite and Gypsum acidifies the rhizosphere to suppress Streptomyces. Silica Meal strengthens the tuber skin. EM-1 and chitin from Mealworm Frass build competitive biology. Combine with consistent watering during weeks 2–6 after tuber initiation.
Tuber set is phosphorus-driven. If P is limiting during the 4–8 week window after emergence, tuber number is permanently reduced. Phosphorous Meal and Micronised Rock Phosphate at different speeds ensure unbroken P supply.
Excess nitrogen relative to potassium. Skip the next top-dress, reduce rates and stretch to 5–6 weeks. Common in recently manured soil.
Yes — use 5g/L rather than 7.5g/L for the initial mix. Begin top-dressing at 6–8 weeks rather than immediately at first earthing up.
Yes — see the Growing Guide tab. Earlies: lower rates, 1–2 top-dresses. Maincrops: upper rates, 2–3 top-dresses through bulking. Same base dressing for both.
The chloride ion in muriate inhibits starch synthase. SOP and polyhalite deliver potassium as sulphate, promoting maximum starch. No chloride anywhere in this formula.
The high K and moderate N suits sweet potato nutrition broadly. Use lower rates. The formula was specifically designed for Solanum tuberosum.
Yes. Organic and mineral ingredients, no synthetic chemicals, no slaughterhouse by-products, no persistent toxins. No withholding period.
Made from organic and natural ingredients. Not certified organic. No synthetic chemicals, no slaughterhouse waste, no GMO inputs.
Cure 1–2 hours on harvest day. Store at 4–8°C in darkness with ventilation. Hessian sacks or wooden boxes. Never plastic. Maincrops store for months; earlies eat within weeks.
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