Tomato feeding

Best fertiliser for tomatoes: an honest guide from someone who sells one

By Joe, Founder of Dr Forest · May 2026

Two declarations up front. One: I make and sell a tomato fertiliser. Two: this is not the post where I tell you to buy it whatever the question.

The best fertiliser for tomatoes depends on three choices you make before you ever pick up a bag. Whether to use organic or mineral, whether to feed dry or liquid, and whether to use a tomato-specific feed or an all-purpose one. The three choices matter more than the brand on the bag. Pick well across them and almost any decent product will give you a good crop.

This guide walks through each choice in order. 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.

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. Microbial diversity drops because the salts shift conditions in favour of a narrow group of organisms. 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 dramatically more of the aromatic volatile compounds that make a tomato smell and taste like a tomato (+72.1%). 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%. The honest hedge on the other side: yield dropped 5.50% and fruit firmness 2.99% under organic-only treatment. Mineral wins on weight per plant; organic wins on every quality metric the same meta-analysis tracked.

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 pooled across 9,752 organic and 5,915 organic-inorganic observations in 313 published trials (Yang et al. 2023). Bottom two rows from a single-site Brazilian field trial (Oliveira et al. 2013): strong direction, large effect, not pooled.

The single most-cited tomato quality paper, Oliveira and colleagues in PLOS ONE, gives you a sense of the upper end of the effect in a controlled field trial: vitamin C up 55%, soluble solids up 57%, total phenolics up 139% in organic compared with conventional [3]. These are big numbers and they come from one site, so treat them as illustrative rather than expected. The Yang meta is the place to anchor your expectations; Oliveira shows what is possible.

One more useful study: Mitchell and colleagues ran a ten-year comparison of organic and conventional tomatoes on the same farm and found that flavonoid content in the organic crop rose steadily as soil organic matter built up, while the conventional crop stayed flat [4]. That is the time-dependent piece of the picture. The quality gain compounds as the soil's organic matter accumulates, season after season.

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. A tomato fed nitrogen slowly grows at a gentler pace than one given a quick mineral hit, and that pace changes what it does with its energy:

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 usually closes by year three.

If you are in either edge case, mineral is fine, and saying otherwise would be daft.

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.

Granular feeds release slowly. The granules sit in the soil, soil microbes break them down at the rate biology allows, and the nutrients enter solution gradually over two to eight weeks depending on granule size. The curve is flat, not spiky. The plant has access to roughly the same amount of nitrogen, phosphorus and potassium on day three as it does on day thirty.

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.

Nitrogen spikes show up in the fruit. Tomatoes are efficient nitrogen uptakers. When soil-solution N spikes after a liquid feed, the plant grabs nitrogen faster than it can metabolise it, and the surplus accumulates as nitrate in the tissue and in the fruit. The Yang 2023 meta-analysis I cited earlier found organic-grown tomatoes carried 16.9% less nitrate in the fruit than mineral-fed equivalents. The same logic applies to any high-nitrogen liquid, organic or mineral: a feed that gives the plant a sudden N surplus produces fruit with more nitrate per bite, and nitrate in the fruit is the single factor most strongly associated with flat, watery flavour in independent taste panels. A slow-release granular avoids the spike, so the plant takes up N at the rate it can use, and the surplus that drives nitrate accumulation never builds.

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. As a result, virtually no liquid mineral fertiliser on the UK retail shelf contains calcium. Read the back of a bottle of Tomorite, the best-known liquid tomato feed on the UK shelf: nitrogen, phosphorus, potassium and a few trace elements, but no calcium listed. 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.

That matters because calcium does the work that prevents blossom end rot and toughens cell walls against pest and disease pressure. Blossom end rot, the dark sunken patch at the base of the fruit, is a calcium-transport problem inside the plant, and the immediate trigger is usually that the plant cannot get calcium to the developing fruit fast enough. The chronic background cause is that nothing in the feeding programme is putting calcium into the soil in the first place. 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.

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 (Growmore-style 7-7-7, equal N, P and K). 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 sets the flowers. 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.

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. The rate at which a tomato fills with sugar is, more than any other single factor, set by the rate at which potassium is moving through the plant. The University of Florida IFAS commercial target sits at 200-150-225 lb/acre N-P₂O₅-K₂O for exactly this reason [5]. K is the dominant nutrient for fruit. An all-purpose feed at 5–7% K cannot do what a tomato feed at 6–10% K can.

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. Tomato-specific feeds usually carry Mg as Epsom salt or polyhalite. All-purpose feeds rarely include it at a meaningful rate.

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, with elevated phosphorus through flowering and fruit set.

Our Tomato 3-4-6 lands in that family.

3 nitrogen, 4 phosphate, 6 potash. Moderate N so the plant does not run away on leaf at the expense of fruit. Elevated P through the critical flower-to-fruit window. K leading the mix because potassium drives sugar transport into the fruit. The ratio sits inside the band the research supports without trying to be tighter than the evidence justifies.

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 2:1 K:N treatment produced the highest total soluble solids (the sugar content in the fruit) and the heaviest ripe fruit weight [6]. Dr Forest 3-4-6 sits in that 2:1 K:N band on the bag label. 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.

If you have read this far and you are still on the fence, the honest answer is that any well-made organic tomato fertiliser 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.