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Tomatoes grown in cultivation need fertilizers, or at the very least, vastly improved soil. In this article, I’ll break down NPK, share some tomato growing hacks, and explore how to choose the right fertilizer for the right stage, and the best NPK ratio for tomatoes.
What is NPK?
NPK is a common fertilizer nutrient guide given all over the world, and while it has uses in the vegetable garden, it can play an important part in ornamental gardens and houseplant collections too. Different mixes of NPK give different results and help duplicate very specific soil requirements.
The letters in NPK stand for:
- (N) Nitrogen
- (P) Phosphorous
- (K) Potassium
What do the numbers mean in NPK?
Each nutrient has its own role in the lifecycle of plants, and while it’s simple to calculate the ratio of each nutrient to the other, I’m often asked what the difference between 5-5-5 and 10-10-10 NPK is.
Simply put, they are the percentage ratios where water or base materials are removed. So for a liquid feed, it is actually 10-10-10-70, where water is 70% of the content. For a compost that 70% is pure filler.
What other nutrients do tomatoes need?
Nitrogen, potassium and phosphorous are macronutrients. This means they’re the most available nutrients in the soil, and therefore are the easiest for commercial fertilizers to provide, but there are several micronutrients that your tomatoes need just as much, and in many ways are more important during fruiting.
Zinc
Zinc supports the development of chloroplast in tomatoes. Chloroplast is the part of the cal structure of plant leaves where photosynthesis happens. Without Zinc, tomatoes produce paler, weaker leaves that don’t turn sunlight into sugars.
So adding zinc to a fertiliser mix via liquid seaweed, or even diluting zinc sulphate tablets, can actively improve the flavour and health of tomato plants.
There’s a great study by Penn State’s College of Agricultural Sciences into the benefits of Zinc to tomatoes in commercial settings here.
Magnesium
Magnesium is present naturally in most soils, and in most commercial composts, but tomatoes can develop magnesium deficiencies if the soil pH is too low. Tomatoes need slightly acidic soil to take up nutrients. If the soil is not acidic enough, the roots fail to process nutrients in the soil.
To increase magnesium uptake from tomatoes, add coffee grounds to your soil, or add old over-brewed tea to a watering can when you notice yellow patches on leaves.
Calcium
The best way to add calcium to tomato growing mediums is to mulch with crushed eggshells. Not only does it deter slugs and snails, but it is a great slow-release source of calcium. However, when tomatoes show calcium deficiency (usually as fruits are ripening, it is possible to give them a quick boost).
If you see signs of calcium deficiency, then dissolve tums (the heartburn pills) into the water and use them to treat the soil. It’s a great way to quickly tackle early signs of blossom end rot in tomato fruits.
What NPK do tomatoes need at different growth stages?
What NPK do tomato seedlings need?
None. If you do fertilize them, only use a spreadable fertilizer that is slow release, ad targetted as general growth. Ideally around 5-5-5 (Ecoscraps works well as a slow-release early-stage fertilizer for tomatoes).
We grow hundreds of tomato plants every year here, partly for food, and partly because we’re addicted to variety and can never make our minds up, and my honest advice is that there is no need to fertilize tomatoes before they get their flowers with anything other than good compost.
What NPK do young tomato plants need before flowering?
Young tomato plants that have started to develop fruiting trusses that have yet to flower do benefit from a weak dilution of tomato fertilizer, or liquid seaweed. Both are very general fertilizers with balanced nutrients at slightly higher concentrations than Ecoscraps. When diluted (always buy concentrated plant foods – it saves you money and space.
What NPK do flowering tomatoes need?
Flowering tomatoes need a high phosphorous plant food, so either give them a rich balanced feed at around 10-10-10 or something with a slightly higher weighting towards flowering and fruit, with more potassium, like 7-6-9 (a great fertilizer at this ratio is Miracle Gro’s Performance Organics)
What NPK do fruiting tomatoes need?
Fruiting tomatoes should continue with the same fertilization as they had at the flowering stage, but if you can increase their calcium, either by making eggshell teas or using a fertilizer like Cal-Mag which is derived from calcium, it helps to strengthen their skins and decreases the chance of blossom end rot (but do not rely on this entirely as it is just 2-0-0 NPK and will do nothing to support growth).
The Journal Of Agricultural Science And Agriculture published an extensive study of the effects of fertilization at different tomato growth stages for further reading on the subject here.
What NPK do tomatoes different types of tomatoes need?
What NPK do cordon tomatoes need?
For cordon tomatoes (indeterminate tomatoes), try to limit their nitrogen. High nitrogen feeds encourage root and leaf growth, and your tomato will put all its energy into growing bigger, longer stems (which in the wild would essentially just crawl along the ground and root), and only start fruiting later in the season.
What NPK do bush tomatoes need?
Bush tomatoes (determinate tomatoes) are much more self-sufficient than cordon varieties. They need more fertilizer at the fruiting stage to promote general plant health and ensure nutrients reach the leaves to prevent any wilt, which increases the chances of mid-season blight in particular.
A balanced fertilizer, once a week, throughout the growing season should give good results on outdoor bush tomatoes, ensuring regular new growth, and regular repeat flowering. Some varieties of cherry bush tomatoes like Romello will continue flowering and fruiting on new trusses for up to a month.
(Note: The best NPK for trailing tomatoes is the same as for bush tomatoes)
Should I stop feeding tomatoes when they ripen?
Tomatoes still need feeding when they ripen, but depending on the variety and the location, you can forgo any specific NPK and give them more general fertilizer like liquid seaweed, which has an average NPK of around 1-0-4 NPK, which helps to sustain general health, while leaving the rest up to the sunshine, the rain, and you.
By proving them with more general-purpose fertilizers at the end of the season you’re boosting soil health with the nutrients they don’t absorb and giving tire plants a final flourish at the end of the season when they begin to struggle taking water to their fruits.
Conclusion
In an ideal world, tomatoes would get all their nutrients from the slow release and uptake of nutrients in balanced rich soil. Tomatoes grow naturally in equatorial South America, where nutrients flow generously through rich soils on the hillside in exceptionally heavy and regular rainfall.
In our gardens, we rely entirely on compost where nutrients are quickly used up by tomatoes to try to replicate the nutrients tomatoes need. While you can reduce your fertiliser use by mulching tomato beds with manure in winter, and adding copious amounts of enriched compost in spring, you will still need to use the best NPK fertilizer for tomatoes at the right time.