Fertilising tomatoes: when, how often & tips

When should you start feeding tomato plants? How often should you feed them? And what fertiliser should you use? Here is everything you need to know about fertilising tomatoes.

Tomato plants (Solanum lycopersicum) are a fantastic addition to any garden. However, they can be demanding. Tomatoes require plenty of nutrients and regular fertilisation. But how often should you feed tomato plants? When should you start feeding tomatoes? And which fertiliser should you use? For the answers to all of these questions and more, read on!

Why do tomatoes need to be fertilised?

Compared to many vegetable plants, tomatoes require a lot of nutrition to grow healthy and produce fruit. In fact, they are such hungry plants that nutrient-rich potting soil is not usually enough to sustain them.

Regular fertilisation ensures that your tomato plants remain healthy and produce a high yield. If the plants remain underfed, they will begin to show signs of nutrient-deficiency – at the latest, when the plant’s tomato fruits emerge.

When and how often should tomatoes be fed?

Depending on their developmental stage, tomatoes have different nutrient requirements:

• From sowing to pricking: tomato plants do not need any fertiliser
• Before planting: apply two small doses of fertiliser
• Once planted outdoors: first proper fertilisation
• Throughout the growing season: regular fertilisation

Initially, tomato seedlings feed on nutrient reserves stored inside their seed. Irregular and excessive fertilisation at this stage will likely harm your seedling’s root system. So much so in fact, that it is best to give your tomato seedlings a low-nutrient seeding soil, such as our Plantura Organic Herb & Seedling Compost. The low salt content of such soils facilitates tomato seed germination.

When you prick out and repot your tomato seedlings, it is a good idea to use a specialty tomato soil, such as our Plantura Organic Tomato & Vegetable Compost. The young tomato plants will feed on the nutrients already contained in such soil for some time. Nonetheless, it is also worth fertilising these young plants a couple of times using a gentle, liquid fertiliser, such as our Plantura Liquid Tomato Food. Liquid fertilisers are less likely to stress plants because they are diluted with water before they are applied.

Once you plant your tomato seedlings outside, you will need to fertilise them properly. As you move your tomato plant into its new location, it is a good idea to add a slow-release fertiliser, like our Plantura Tomato Food, into the hole with it.

Tip: You can plant tomatoes in a greenhouse in mid-April, or outside in mid-May – just be sure that the threat of frost has passed. In cooler climates, harden off your tomatoes before moving them outside.

When should tomato plants in pots be fed?

Potted tomato plants demand just as many nutrients as those in a garden bed. Slow-release fertilisers are a great option for container plants, because they gradually and consistently supply plants with all the nutrients they need.

Do remember, however, that nutrients do not wash out of pots as quickly as they do in garden beds. As such, potted tomato plants do not need to be fertilised as often as those outside. In fact, excessive fertilisation can damage potted tomato plants because the nutrients do not distribute well. In general, it is best to fertilise potted tomato plants little and often and not overload them with too many nutrients at once. To correctly fertilise potted tomato plants, mix a slow-release fertiliser into the soil as you pot the plants. This will provide the tomatoes with sufficient nutrients in their first few weeks. After that, liquid fertilisers, like our Plantura Liquid Tomato Food, are best for potted tomatoes, as they are easier to apply than granular fertiliser.

Liquid Tomato Food, 800ml

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From June, once they begin to flower and fruit, fertilise your tomato plants weekly. As a rule of thumb, add 15 to 25ml of liquid fertiliser to every 5 litres of irrigation water, mix, and apply.

Remember: It is important to consistently water tomato plants. The microorganisms that convert fertiliser to nutrients thrive in moist soil; they need water to function properly. Plants also absorb nutrients better if the soil is wet. As such, dehydration not only stresses tomato plants, but inhibits their access to nutrients.

The right time to feed potted tomato plants:

• While planting tomatoes, mix slow-release fertiliser into the potting soil.
• From June, fertilise weekly with liquid fertiliser and irrigation water.

When should tomato plants in garden beds be fed?

For young tomato plants outside, use a mature compost or speciality tomato fertiliser, such as our Plantura Tomato Food. Just 120 to 230g of our potassium-rich fertiliser is enough to feed a tomato plant for an entire year.

Tomato Food, 1.5kg

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However, this fertiliser should be applied in two sessions. First, as you plant out your tomato, work 70 to 150g of fertiliser into the soil about as deep as your spade. Then, after about two months, apply a further 50 to 80g of fertiliser. This time, rake the fertiliser into the soil, being careful not to damage any roots. Our 1.5kg box of tomato fertiliser is enough to feed up to 20 tomato plants.

Slow-release fertilisers are far better than mineral fertilisers. This is because mineral-based fertilisers, like blue fertiliser, provide a short, sharp nutrient boost that is insufficient for a tomato plants’ long-term development. They stimulate vegetative growth, which can increase the plant’s susceptibility to disease. In fact, if it rains too much, these water-soluble minerals will wash out and end up in groundwater instead of in the plant.

What is more, on top of gently feeding tomato plants, slow-release fertilisers enrich soil life – they are actually environmentally friendly!

When to fertilise garden bed tomato plants:

• Mix one round of fertiliser (70-150g) into your planting hole, as you plant out your tomato.
• After two months, apply a second round of fertiliser (50-80g).

Tip: Nettle liquid manure can support and strengthen tomato plants.

Recognising nutrient deficiencies in tomato plants

When nutrient-deficient, tomato plants tend to display leaf discolouration. Here is what to look out for.

Nitrogen (N): If tomato plants do not have enough nitrogen, the older leaves will begin to turn yellow, brown and fall off. The younger upper leaves will remain green for a long time until the deficiency takes over the entire plant. In general, the plant will appear pale green, grow poorly and form few shoots.

Potassium (K): Potassium deficiency damages leaf tissue. Here, the edges of tomato leaves dry and wither. This withering slowly spreads throughout the leaves causing them to dry out, even though their veins are green because water is being transported to them.

Phosphorus (P): If your tomato plant lacks phosphorus, its older leaves will begin to turn purple, first on the underside and then all over.

Calcium (Ca): Another important nutrient is calcium (Ca). It is partly responsible for the stability of the plant’s cell walls. Typically, calcium deficiency produces blossom end rot. To avoid blossom end rot and prevent tomato plant problems, apply calcium once when you plant out your tomatoes, and again, two months later. 

Over-fertilised tomato plants: what should I do?

If you apply too much fertiliser to your tomato plants, they may become over-fertilised. This is particularly problematic for fast-acting mineral fertilisers. Depending on the contents of the fertiliser, over-fertilised tomatoes can display burnt roots, excessive leaf and shoot growth, growth disorders, and stunted growth, all of which increase your plant’s susceptibility to disease. If that wasn’t bad enough, tomato fruits also suffer from over-fertilisation!

In acute cases, you will need to transplant your over-fertilised tomatoes into fresh soil. Another option is to apply mulch to the ground. Certain mulch materials, like wood chips or straw, absorb excess nitrogen, so that the mineral is not be absorbed by the plants.

To prevent over-fertilisation, it is worth carrying out a soil analysis on-site before planting out your tomatoes. The pH value of your soil, amongst other things, determines what fertiliser to use and how to apply it. When fertilising tomatoes in containers and outdoors, never exceed the fertiliser’s recommendations.

In addition to fertilisation, there are a number of other measures you can take to ensure healthy, plentiful tomatoes in your garden. Read our article on watering and mulching tomatoes to find out more!