Best Fertilizer for Tomatoes — 4 Picks, Potassium Late
The best fertilizer for tomatoes changes once fruit sets. 4 picks compared across the season, plus the soil test first. See which your plants need.
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The commonest tomato problem is not a pest or a disease. It is a magnificent six-foot plant with four tomatoes on it, and it is caused by feeding the same fertiliser all season.
Tomatoes need two different things at two different stages, and the switch happens when the first fruit sets.
Reading the numbers
Fertiliser labels carry three numbers — N-P-K — the percentages by weight of nitrogen, phosphorus and potassium.
- N, nitrogen — leaves and stems. Green growth.
- P, phosphorus — roots, flowering, early establishment.
- K, potassium — fruit development, size, flavour and ripening.
For tomatoes, the ratio matters more than the strength. A weak feed with the right balance beats a strong one with the wrong one.
| Stage | Want | Typical NPK | Frequency |
|---|---|---|---|
| Seedlings | Very little | Weak balanced, quarter strength | Rarely |
| Potting on to first flowers | Balanced, some N | 10-10-10 or similar | Every 2 weeks |
| First truss sets | Switch now | High K, e.g. 4-5-8 | Every 7–14 days |
| Full fruiting | High K | High K tomato feed | Every 7 days |
| Late season, tip pinched | Reduce | High K, half strength | Every 2 weeks |
Bagged “tomato feed” is formulated for the fruiting stage. That is why it is the wrong thing to use on a young plant and the right thing from flowering onward.
The nitrogen trap
Nitrogen produces exactly what it looks like it should — big, dark, lush foliage. That is why people keep applying it: the plant looks superb. Meanwhile flowering is suppressed and fruit set is poor. If your tomato plants are enormous, dark green and barely fruiting, stop the nitrogen and switch to potassium. The plant will not shrink, but the next trusses will set.
Soil preparation matters more than feeding
Work compost or well-rotted manure into the bed before planting. A tomato in decently prepared ground needs far less intervention later than one planted into thin soil and rescued with liquid feed.
In containers, nutrients wash out with every watering, so container tomatoes genuinely do need regular feeding — see growing in pots for how quickly a pot runs out.
Organic options
| Feed | Supplies | Notes |
|---|---|---|
| Comfrey liquid feed | High potassium | Homemade, free, excellent, smells appalling |
| Seaweed extract | K plus trace elements | Good all-rounder; mild |
| Wood ash | Potassium, raises pH | Use sparingly; not near potatoes |
| Fish, blood and bone | Balanced, slow | Good pre-planting soil feed |
| Well-rotted manure | Balanced, slow | Dig in before planting |
| Worm castings | Balanced, mild | Excellent side dressing |
| Chicken manure pellets | High nitrogen | Early season only |
Comfrey is the classic homemade tomato feed and it earns the reputation. Pack leaves into a container, weigh them down, and collect the dark liquid that drains off after a few weeks. Dilute it heavily. It is genuinely high in potassium and costs nothing. It also smells extraordinarily bad, so site the bucket well away from the house.
More options in organic fertilizers.
Blossom end rot: not a fertiliser problem
The dark, sunken, leathery patch on the base of a fruit is blossom end rot, and it is the most commonly misdiagnosed tomato issue.
It is a calcium transport failure, not usually a calcium shortage. Calcium moves through the plant in the water stream, so when watering swings between dry and soaked, the plant cannot deliver calcium to the rapidly expanding fruit — even with plenty of calcium in the soil.
The fix is consistent watering and mulch, not calcium supplements. Adding calcium to soil that is not deficient achieves nothing. It is the same underlying mechanism that causes splitting fruit.
The folk remedies, honestly
- Eggshells. Take years to break down. They will not supply calcium this season, and blossom end rot is not a calcium supply problem anyway.
- Epsom salts. Magnesium sulphate. Useful only if you actually have a magnesium deficiency — yellowing between the veins on older leaves. Applied routinely, excess magnesium can interfere with calcium uptake and make things worse.
- Banana peels. Some potassium, released very slowly. Compost them first.
- Aspirin sprays, milk sprays. Little good evidence.
- Coffee grounds. See what they actually do — not a tomato feed.
A simple schedule
- Before planting: compost or well-rotted manure dug into the bed.
- Planting to first flowers: balanced feed every two weeks, or nothing at all if the soil is good.
- First fruit sets: switch to high-potassium tomato feed, every 7 to 14 days.
- Full fruiting, in containers: weekly.
- After pinching out the tip: reduce to half strength.
Follow the dilution on the bottle. Over-concentrated feed scorches roots, and the damage is not quickly reversible.
Our picks
| # | Pick | Best for | Where to buy |
|---|---|---|---|
| 1 | High-potassium tomato feed | Best once fruit has set | Check price for High-potassium tomato feed → |
| 2 | Balanced granular feed | Best early in the season | Check price for Balanced granular feed → |
| 3 | Soil test kit | Buy before any fertiliser | Check price for Soil test kit → |
| 4 | Fish emulsion or seaweed feed | Best organic option | Check price for Fish emulsion or seaweed feed → |
Best once fruit has set
High-potassium tomato feed
The feed to be using from fruit set onward. Read the third NPK number and ignore marketing on the front of the bottle.
- NPK
- Low N, high K — the third number is the one that matters
- Form
- Liquid concentrate
- Frequency
- Every 7–14 days once fruiting, more in containers
- Switch at
- First truss setting fruit
What we liked
- Potassium is what fruit development actually needs
- Liquid feed is available to the plant immediately
- Easy to apply at the right interval through the season
- Containers lose nutrients to every watering and need it most
Worth knowing
- Easy to over-apply; follow the dilution
- Does nothing for a plant that is short of water
Best early in the season
Balanced granular feed
Right for the first half of the season and wrong for the second. The switch to high-potassium is the whole technique.
- NPK
- Balanced, e.g. 10-10-10
- Form
- Granular, worked into soil
- When
- At planting and through early growth
- Switch off
- Once fruit sets
What we liked
- Nitrogen early builds the frame that carries the fruit
- Granular releases slowly, so less frequent application
- Cheap per plant
Worth knowing
- Keep using it after fruit set and you get leaves, not tomatoes
- Slower to act than liquid feed
Buy before any fertiliser
Soil test kit
The step people skip. Feeding soil you have not tested is how gardens end up high in phosphorus and still disappointing.
- Measures
- pH and N-P-K
- Why
- Soil preparation matters more than feeding
- When
- Before the season starts
- Cost
- Less than a season of feed
What we liked
- Tells you what the soil lacks instead of guessing
- pH decides whether nutrients present are available at all
- Stops you adding what is already there
- Cheap relative to a year of unnecessary feeding
Worth knowing
- Home kits are approximate; a lab test is more accurate
- Needs doing before planting to be useful
Best organic option
Fish emulsion or seaweed feed
A good choice if you prefer organic inputs. Pair it with compost rather than relying on it alone for potassium.
- Type
- Fish emulsion, seaweed, or a blend
- NPK
- Modest numbers, fast availability
- Frequency
- Every 1–2 weeks in season
- Note
- Smells, briefly
What we liked
- Feeds soil biology as well as the plant
- Hard to burn roots with, unlike concentrated synthetics
- Seaweed supplies trace elements a basic NPK does not
Worth knowing
- Genuinely smells for a day after application
- Lower analysis, so more volume for the same feed
Common questions
What is the best fertilizer for tomatoes?
What do the NPK numbers mean on fertilizer?
Does adding calcium fix blossom end rot?
Are eggshells, epsom salts or banana peels good for tomatoes?
How often should you feed tomatoes?
Can you use general purpose fertilizer on tomatoes?
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