Best fertiliser for tomatoes: what to use, and when
For what tomato feed usually is, and whether it is high in nitrogen, see is tomato feed good for all plants — that page owns those questions.
For most home growers, the best fertiliser for tomatoes is a tomato feed with more potash (the K on the label) than nitrogen (N), started when the first fruits begin to swell. The RHS suggests feeding tomatoes in pots every 10–14 days from then on. Whatever you use, steady watering matters more than the brand.
For what is on the Tomorite label, see Tomorite vs Dr Forest tomato feed.
This guide walks through each choice in order — organic vs mineral, dry vs liquid, tomato-specific vs all-purpose — with the chemistry that sits under supermarket liquids like Tomorite and under dry organic meals. I sell an organic tomato fertiliser because I care about flavour, fruit quality and the health of the soil over the long run. Those are what organic feeds do better, and they're the reason I started Dr Forest in the first place.
Looking for stage-based organic feeding instead? Our page on the best organic fertiliser for tomatoes takes you through what to feed at each stage, from planting to the first truss to peak fruiting, in a bed, growbag or pot.
In short · the three choices
Organic or mineral?
Mineral (synthetic) wins on raw yield and on cost per gram of nutrient. Organic returns organic matter to the soil over the years. On flavour, the research isn't conclusive, but a lot of it points towards organic, and it's widely accepted among cooks that organic tomatoes taste better.
Dry or liquid?
A dry feed releases over a few weeks, so you feed less often. Liquid acts faster and gives precision for foliar feeding and containers.
Tomato-specific or all-purpose?
All-purpose feeds tend to carry similar nitrogen and potassium, or more nitrogen, because they're built for lawns and leafy plants. Tomatoes through fruit set want the reverse. A tomato-specific feed gets the ratio right.
The trade-off that nobody on a fertiliser bottle is going to tell you
Mineral fertilisers are dissolved salts. Nitrogen as ammonium or nitrate, phosphorus as superphosphate, potassium as potassium chloride or potassium sulphate. They were engineered for one thing: to put nutrients into a plant fast and at the lowest cost per gram of nutrient on the shelf. They do that very well. That is why commercial tomato growers and most allotment veterans who chase a competition weight have used them for decades.
The problem shows up over time, not in a single season. When you feed soil with nothing but soluble salts, three things happen. Organic matter declines because there is no carbon coming in. A 2020 study of long-term field trials found less carbon, more acidity, and fewer kinds of bacteria on chemical-only plots. Salinity rises. A 2020 study in mSystems from long-running field trials showed that decades of chemical-only fertilisation pushed bacterial diversity down and selected hard for a few dominant species [1]. That matters because microbial diversity is what underwrites nutrient cycling in healthy soil.
Organic fertilisers come at the problem from the other side. The nitrogen, phosphorus and potassium they carry are bound up in plant or mineral matrices. Microbes have to break them down before the plant can take them up. The feed also returns organic matter and carbon to the soil along with the nutrients. The carrier is part of the benefit, not just the package. Which is why, in the long run, the soil gets better rather than worse.
What the peer-reviewed evidence says about flavour
Yield is the easy thing to measure. Flavour is harder. The largest synthesis we have on this is a 2023 meta-analysis by Yang and colleagues in the European Journal of Agronomy, pulling together 313 published studies covering 9,752 organic and 5,915 organic-inorganic combination treatments across 353 sites and 268 tomato varieties [2]. That is a serious sample.
The findings are clear and consistent. Compared with mineral fertilisers, organic produced tomatoes with significantly higher total sugars (+9.58%, p<0.001), higher soluble sugars (+12.0%, p<0.001), and more of the aromatic volatile compounds that make a tomato smell and taste like a tomato (+72.1% across the pooled studies; treat that as a direction of travel for a UK grower, not a promise for one crop). Organic acids dropped by 6.76%, which is what you want for sweet-tasting fruit. Disease incidence dropped by 45 to 73%, and nitrate levels in the fruit dropped by 16.9%. On the other side, yield dropped 5.50% and fruit firmness 2.99% under organic-only treatment. So mineral feeding gives slightly more fruit, and organic gives slightly sweeter, more aromatic fruit that is a little softer. The same review found that mixing organic and mineral feeds gave 12.4% more fruit than mineral alone. The longer comparison is organic vs chemical tomato feed.
Figure 1 · Organic vs mineral on tomato quality
Where organic beats mineral on the things tomatoes are eaten for
Percentage change vs mineral baseline. Top four rows: organic vs mineral feeds, from 313 studies (Yang et al. 2023). (Yang et al. 2023). Bottom two rows from a single-site Brazilian field trial (Oliveira et al. 2013): strong direction, large effect, not pooled.
A Brazilian study compared one tomato variety from an organic farm and a conventional farm 1.5 km apart [3]. The organic fruit had 55% more vitamin C, 57% more soluble solids and 139% more phenolics. It was also about 40% smaller. The authors put the difference down to the organic plants being more stressed. It is one pair of farms, so treat the numbers as illustrative rather than expected. The Yang review is the place to anchor your expectations.
One more useful study: Mitchell and colleagues ran a ten-year comparison of organic and conventional tomatoes on the same farm and found organic tomatoes averaged 79% more quercetin and 97% more kaempferol, two flavonoids, than conventional ones over ten years. The levels rose most after the organic plots were given less manure, so less nitrogen may matter as much as richer soil. Yields were about the same in both systems. [4].
Figure 2 · A ten-year curve
Organic feeding builds flavour, year on year.
Indexed flavonoid content (quercetin + kaempferol aglycones) in tomatoes from a side-by-side organic and conventional trial run on the same farm for ten years. The organic line tracks soil organic-matter build-up; the conventional line tracks nothing in particular.
Alyson Mitchell, who led that ten-year study, explained the mechanism in an interview with NPR. With less nitrogen to hand, she said, the organic plants may grow more slowly, and that changes what they do with their energy:
A slower-growing plant has more time to put its resources into making secondary plant metabolites.
So which should you actually pick
For most home growers most of the time, the answer is organic. The reasoning is straightforward. You are not chasing a per-tonne commercial yield. You are growing food you will eat, in a patch you will use again next year, ideally with a child or a dog wandering through it. Flavour matters. Soil matters. The mild yield trade-off in year one often narrows later, though that depends on the soil and how you grow.
Pick organic if
- You grow tomatoes for flavour and fruit quality
- You want a feed that returns organic matter to the soil alongside the nutrients
- You're gardening on a plot you keep year on year and want it to improve over time
- You care about soil biology, no-dig, or allotment health
- You have children, pets or pollinators in the garden
Pick mineral if
- You're tight on space and want maximum yield per plant
- You're growing for a single competition weight
- You want the cheapest cost per gram of nutrient
- You need a precise, fast nutrient correction mid-season
If you are in either edge case, mineral is fine, and saying otherwise would be daft.
More guides like this, sent as the season turns — and 10% off your first order.
Dry or liquid? The spike and the steady curve
Once you have settled the organic question, the next practical choice is the form. Each form does things the other cannot, and a good feeding programme uses both. The point of this section is not to pick one but to set out what each is good for.
The simplest way to see the difference is to look at what nutrient availability does over time.
Liquid feeds are fast-acting. Dilute the concentrate, water it in, and the plant takes up most of what it needs within hours. That sounds like an advantage, and in narrow situations it is. The cost is the shape of the curve. Each application is a spike: nutrient availability in the root zone goes from low to very high in the space of one watering, the plant grabs what it can, microbes and the next rainfall move the rest below the root zone within a few days, and you are back where you started until the next application. Liquid feeding is a series of pulses.
Dry organic feeds release slowly. Soil microbes break them down and the nutrients come through over two to four weeks, faster in warm, moist compost. The curve is flat, not spiky.
Figure 3 · Nutrient release over time
Liquid spikes. Granular holds steady.
Conceptual time-course of nutrient availability in the root zone over a 12-week period. Liquid feeds applied every two weeks produce a sharp peak followed by rapid decay back to near-zero. A slow-release granular delivers a sustained, even supply across the whole application window.
Organic-fed tomatoes in the Yang review had about 17% less nitrate in the fruit than mineral-fed ones. The review compared organic with mineral feeds, not dry with liquid.
It is chemically very difficult to put calcium in a liquid mineral fertiliser. Almost no garden centre will mention this, but the chemistry is straightforward. In aqueous solution, calcium ions (Ca²⁺) and phosphate ions (PO₄³⁻) react spontaneously to form insoluble tricalcium phosphate, which precipitates out as a chalky sludge. The two cannot coexist in liquid at meaningful concentrations. That is why calcium feeds are usually sold on their own. Read the back of a bottle of Tomorite, the best-known liquid tomato feed on the UK shelf: NPK 4-3-8, enriched with seaweed. The standard label does not declare magnesium, calcium or trace elements. The product has to assume your soil already has enough. In dry granular form, with no aqueous medium driving the reaction, calcium and phosphate sit side by side in the bag, and the plant gets both at once.
Figure 4 · The chemistry of the missing nutrient
Why almost no liquid mineral fertiliser on the UK shelf contains calcium
In water, calcium ions and phosphate ions react spontaneously to form insoluble tricalcium phosphate and drop out of solution. In a dry granule there is no water, so the same ingredients sit side by side until they reach the soil.
Calcium matters because it is part of every plant cell wall. Calcium only travels in the water the plant draws up, so the usual cause is uneven watering, not a lack of calcium in the soil (blossom end rot in tomatoes). Cell walls reinforced with calcium are mechanically tougher and chemically less appealing to piercing-sucking insects; weakened cell walls invite the aphids, whitefly, thrips and fungal pathogens that come along behind. A granular feed carrying gypsum, polyhalite or calcified seaweed delivers calcium alongside the other nutrients. A liquid mineral usually doesn't.
| Form | Release | What it does well |
|---|---|---|
| Granular (dry) | 2 to 8 weeks (varies with granule size) | Steady supply, no nutrient spikes, carries calcium alongside phosphate. |
| Liquid concentrate | Hours, once watered in | Fast correction, foliar feeding, dose precision, responsive in containers. |
| Soluble powder | Hours, once mixed | Same as liquid concentrate. Dry to store, liquid to apply. |
The three forms compared. Each does something the others don't.
What dry granular gives you
- A steady release curve that keeps nitrogen from spiking in the fruit
- Calcium, magnesium and sulphur in the same bag
- Fewer applications. Ours goes on every three weeks.
- Soil biology activity, because microbes do the work of breaking the granule down
What liquid gives you
- Fast correction when something is acutely missing
- Foliar feeding for trace elements that bypass slow soil mineralisation
- Responsiveness in pots and grow bags where soil volume can't buffer supply (i.e. small pots for the plant size)
- Precise dose control week to week
- Easy mixing with biostimulants and other inputs in the same watering can
Why tomato-specific beats all-purpose, organic or not
Picking organic is only half the choice. The other half is whether to use a generic plant food or one formulated specifically for tomatoes. A generic organic feed will keep a tomato plant alive and growing. It will not give you the fruit it could have done, and the reason comes down to nitrogen and potassium in the wrong proportion.
All-purpose fertilisers are formulated for the average UK garden, where the typical job is feeding lawn and leafy ornamentals. The NPK ratios vary. Some are balanced, such as Growmore at 7-7-7. Others lead with nitrogen (10-5-5 lawn feeds, or 24-8-16 in the soluble mineral world). Tomato-specific feeds are built for a different job: feeding a fruiting tomato through the flower-to-fruit window, where the plant wants potassium higher than nitrogen, calcium for cell walls and fruit, and magnesium for chlorophyll. The NPK ratio, calcium source, and magnesium content on the bag all reflect that. Apply an N-leading feed to a fruiting tomato and the plant does what the nutrition tells it to: it makes leaves at the expense of fruit.
Figure 5 · Where each nutrient does its work
A tomato plant is five different jobs in one organism
N grows the leaves. P is for roots and energy. K moves sugars into fruit. Ca builds the cell walls. Mg powers photosynthesis. An all-purpose feed lands the same ratio on every part. A tomato feed lands the right ratio for the part doing the work right now.
| Type of feed | Typical NPK | Where K sits |
|---|---|---|
| Soluble mineral all-purpose | 24-8-16 | K is two-thirds of N |
| Balanced organic all-purpose | 7-7-7 | K equals N |
| Leafy veg or lawn feed | 10-5-5 | K is half of N |
| Tomato-specific liquid feed (mineral) | 4-3-8 | K is twice N |
| Tomato-specific liquid feed (organic, e.g. Tomorite Organic) | 3-3-3 | K equals N |
| Dr Forest Tomato 3-4-6 | 3-4-6 | K is twice N |
Typical UK retail NPK ratios. Most tomato-specific feeds put K above N; the organic liquid category is an exception.
One exception worth flagging on that table. Tomorite Organic is a 3-3-3 balanced feed. Despite the tomato branding, the ratio is closer to an all-purpose feed dressed as a tomato product. That probably says more about product limitations than about what tomatoes want. Organic liquids are difficult to fine-tune: plant-based inputs are themselves roughly balanced, and the chemistry needed to concentrate one element without precipitating the others is expensive. The principle still applies. Tomorite Organic just doesn't deliver on it, which means the all-purpose argument lands on it the same way it lands on a generic 7-7-7.
Potassium is the other half of the equation. It is the nutrient that drives sugar transport from leaves into fruit, moving through the phloem alongside sucrose. Potassium helps move sugar into the fruit. Sunlight, variety and steady watering matter at least as much. cut the sentence and source [5]. What matters is the ratio. A tomato feed has about twice as much potash as nitrogen, while an all-purpose feed has about the same of each.
Two other things tomato-specific feeds tend to address that all-purpose feeds skip:
- Calcium. Discussed above in the dry-vs-liquid section: tomato-specific granular feeds usually include a calcium source where all-purpose feeds rarely do.
- Magnesium. The classic yellowing between the veins of lower tomato leaves is magnesium deficiency. Tomatoes are heavier Mg feeders than most garden plants. Some tomato feeds add magnesium. Many don't: Tomorite's label lists none. All-purpose feeds rarely include it at a meaningful rate. Yellow leaves have other causes too, so check why tomato leaves turn yellow before you reach for magnesium.
So the short answer is that a generic organic plant food gives a tomato the wrong ratio of the right nutrients. Crop-specific formulation is what closes that gap.
An NPK ratio that actually matches what tomatoes want
If you read three random tomato-fertiliser recommendations, you will get three different NPK ratios. There is no single magic number. What every credible source agrees on is the principle: potassium higher than nitrogen. Phosphate matters less, because most garden soil and compost already hold enough.
Our Tomato 3-4-6 lands in that family.
N
3%
Nitrogen
P
4%
Phosphate
K
6%
Potash
3 nitrogen, 4 phosphate, 6 potash. Moderate N so the plant does not run away on leaf at the expense of fruit. Moderate phosphate. K leading the mix because potassium drives sugar transport into the fruit.
More specific support: Fallah and colleagues in Horticulture, Environment, and Biotechnology compared K:N ratios of 2:1 and 4:1 in greenhouse cherry tomato nutrient solutions and found the sweetest, heaviest ripe fruit came from plants fed at 2:1 with each truss pruned to four fruit. Total yield was highest on unpruned plants at either ratio [6]. On the bag, Dr Forest 3-4-6 has twice as much potash as nitrogen. Counted as potassium, that's about 1.7 to 1, close to the trial's 2:1. It is a single greenhouse paper on cherry tomatoes rather than a field meta-analysis, so treat it as one data point that lines up with the wider principle rather than the definitive word.
Why this product looks like it does
I made Tomato 3-4-6 to feed my own tomatoes, so it started with the ingredient list I would buy if it existed rather than a price point. Most of those ingredients you can buy separately and blend in your own shed; we bundle and mill them to an even coarse powder because the proportions matter and most growers will not weigh out a recipe every time they feed. We do the boring bit so you do not have to.
Plant-based nitrogen extract
Concentrated plant protein at 12% N. Replaces dried blood with no slaughterhouse-derived ingredients.
Yorkshire polyhalite
Mined at Boulby, between Staithes and Loftus, about ten miles north-west of Whitby. One crystal carrying K, Ca, Mg and S, slow-release, about 3% chloride, from ICL, suited to tomatoes.
Sulphate of potash
50% K₂O, chloride-free, for a faster potassium hit alongside the polyhalite. Never muriate of potash.
Biochar
Carbonised wood whose pore structure gives the microbes that release the other ingredients somewhere stable to live.
And more besides
The full list is on the back of the bag, every line of it.
Related: Tomato 3-4-6 · multi-input organic plant food · Brix+ · seaweed biostimulant for better yield and fruit quality · Liquid amino acids · biostimulant that supports yield and quality · Read the full ingredient list
If you have read this far and you are still on the fence, If you're still unsure, any well-made organic tomato feed will do most of the work. The difference between ours and a supermarket organic feed is in the bits the supermarket cannot afford to put in.
The organic options compared in this guide all live together if you want to browse the organic fertilisers side by side.
Frequently asked questions
Is organic tomato fertiliser better than mineral?
It depends on what you are optimising for. Mineral wins on raw single-season yield because the salts are pre-dissolved and the plant takes them up fast. Organic tends to edge ahead on flavour and fruit quality, and it feeds the soil over time (the difference is usually small): a 2023 meta-analysis of 313 studies found organic-grown tomatoes had 9.58% more sugars, 12% more soluble sugars and 72.1% more aromatic volatiles than mineral-fed equivalents.
What NPK ratio is best for tomatoes?
There is no single ratio that every credible source agrees on. What they all agree on is the principle: potassium higher than nitrogen. Phosphate matters less, because most garden soil and compost already hold enough. Tomorite is 4-3-8 and ours is 3-4-6. Both have about twice as much potash as nitrogen. Our Tomato 3-4-6 sits at the moderate-nitrogen, elevated-phosphate, potassium-leading end of that band.
Should I use granular or liquid fertiliser on my tomatoes?
A good feeding programme uses both. Each form does something the other cannot. Granular gives a steady release of nutrients over weeks rather than the spike a liquid produces, and can carry calcium and phosphate in the same bag. Liquid gives fast correction when something is acutely missing, foliar feeding for trace elements, dose precision week to week, and responsiveness in containers and grow bags where soil volume can't buffer supply (i.e. small pots for the plant size). Used together, they cover what no single form can.
Can I use an all-purpose fertiliser on my tomatoes?
You can, and the plants will grow, but you will get a leafier plant with lower-quality fruit. All-purpose feeds are formulated for the average UK garden, where the typical job is lawn and leafy ornamentals. The NPK ratios vary, and so do calcium and magnesium content. A tomato-specific feed is built for a different job: feeding a fruiting tomato through the flower-to-fruit window, where the plant wants potassium higher than nitrogen, calcium for cell walls, and magnesium for chlorophyll. A tomato feed at 6 to 10% K does what an all-purpose feed at 5 to 7% K cannot.
Why does Dr Forest use sulphate of potash instead of muriate of potash?
Tomatoes cope with chloride better than most crops. The ones that really mind it are strawberries and other soft fruit, beans and potatoes. We use sulphate of potash anyway, because it adds sulphur and keeps salt down in a pot. Muriate of potash (potassium chloride) is around 47% chloride by weight; sulphate of potash is under 1.5%. Direct peer-reviewed tomato trials are thin, but a 2022 potato study published in Frontiers in Plant Science showed that potassium chloride reduced starch and ascorbic acid compared with potassium sulphate at the same potassium rate.
What is polyhalite and why is it in Dr Forest Tomato 3-4-6?
Polyhalite is a naturally occurring single-crystal mineral (K₂Ca₂Mg(SO₄)₄·2H₂O) mined from Boulby in North Yorkshire, the only operating polyhalite mine in the world. It carries roughly 14% potassium oxide, 17% calcium oxide, 6% magnesium oxide and 48% SO₃, all as sulphates, with about 3% chloride, from ICL, in a slow-release form. We include it because it gives a long, steady release of four nutrients tomatoes need throughout the season, complementing the faster-acting sulphate of potash.
How often should I feed tomatoes with Tomato 3-4-6?
For pots and containers: mix 5–10g of coarse milled powder per litre of compost at planting (5g per litre for nutrient-rich mixes, 10g for plain or peat-free), then top-dress every three weeks at 1–3g per litre. For beds: 150–200g per square metre. A 1.5kg bag covers a typical season for six to ten plants. The feed is slow-release, so there is no need to feed weekly. Adjust by what the plant shows you: feed a little more often if the lower leaves pale, and less often if growth turns dark and leafy with few flowers.
Is Dr Forest Tomato 3-4-6 certified organic?
Our tomato fertiliser is made with organic ingredients, but the finished product is not certified organic. Certification is a separate process from choosing good ingredients. The full ingredient list is on the back of the bag so you can verify every line.
Does Dr Forest Tomato 3-4-6 contain blood, fish or bone?
No. This Tomato 3-4-6 feed has no blood, fish or bone. Its nitrogen comes from a concentrated plant protein source delivering 12% N, alongside Scottish seaweed meal and humic acid. Our All Purpose and Bloom feeds do contain bone meal.
Sources cited
- Xu, Q., Ling, N., Chen, H., Duan, Y., Wang, S., Shen, Q., Vandenkoornhuyse, P. & Kent, A.D. (2020). Long-term chemical-only fertilization induces a diversity decline and deep selection on the soil bacteria. mSystems 5(4): e00337-20. DOI: 10.1128/mSystems.00337-20.
- Yang, J., Mattoo, A.K., Liu, Y., Zvomuya, F. & He, H. (2023). Trade-offs of organic and organic-inorganic fertilizer combinations in tomato quality and yield: A global meta-analysis (1992–2021). European Journal of Agronomy 152: 126985. DOI: 10.1016/j.eja.2023.126985.
- Oliveira, A.B., Moura, C.F.H., Gomes-Filho, E., Marco, C.A., Urban, L. & Miranda, M.R.A. (2013). The Impact of Organic Farming on Quality of Tomatoes Is Associated to Increased Oxidative Stress during Fruit Development. PLOS ONE 8(2): e56354. DOI: 10.1371/journal.pone.0056354.
- Mitchell, A.E., Hong, Y.-J., Koh, E., Barrett, D.M., Bryant, D.E., Denison, R.F. & Kaffka, S. (2007). Ten-year comparison of the influence of organic and conventional crop management practices on the content of flavonoids in tomatoes. Journal of Agricultural and Food Chemistry 55(15): 6154–6159.
- Olson, S.M., Simonne, E.H., Stall, W.M., Smith, S.A., Hochmuth, G.J., Mossler, M.A., Webb, S.E., Taylor, T.G., Datnoff, L.E. & Vallad, G.E. (2010, updated). Vegetable Production Handbook for Florida: Tomato. SL355/CV236, UF/IFAS Extension. Research-based fertilisation target: 200–150–225 lb/acre N–P₂O₅–K₂O.
- Fallah, M., Delshad, M. & Sheikhi, H. (2021). The effects of cluster pruning and the K:N ratio on greenhouse tomato yield and quality. Horticulture, Environment, and Biotechnology 62: 691–700. DOI: 10.1007/s13580-021-00358-7.
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