What is the best soil for growing tomatoes?

Garden Experts
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Dear Mr Kötter, I want to grow some tomato plants in ten-litre tubs. What is the best type of soil to use for this? Can I use a peat-based potting compost or a mixture of garden soil and peat-based compost with a little sand? Or are there other effective options? I also want to put lettuces in window boxes on my balcony. What kind of soil should I use for this? I look forward to hearing from you and thank you for your time.

The GARDENA gardening expert

That is a really interesting and topical question. There have been some very interesting discoveries made in France recently, showing that the composition of soil for growing tomato plants has a real impact on the tomatoes' flavour, which otherwise essentially depends on the genetics involved, i.e. the variety.

To put it simply: tomatoes get their flavour from the soil in which they grow, just like wine. Flavoursome tomatoes need loamy soil that are rich in humus. Good quality peat-based potting compost is just the ticket. Tomato cultivation tests have shown that sandy soil and peat soil do not provide a fully developed flavour, so it is best to keep away from cheap composts that are too rich in peat and not loamy enough. What's more, they also contain sludge!
It is also best to avoid the mixture of garden soil and sand you mentioned, unless you have very loamy soil. Then it can make up 20% of the soil volume, so in your case that would be about two litres for every ten-litre plant tub.

Bear in mind the following: if you make your own mix, this will change the composition of the peat-based compost as specified by the manufacturer's label. The amount and pH value of the nutrients it contains will be different and probably reduced. The pH value influences the nutrients available to the plant.
Tomatoes already lower the soil's pH value due to the secretion from their roots, so it is worth putting garden compost on your tomato plants. Garden compost is generally slightly alkaline and therefore counters this natural reduction of the pH level. Alternatively, in this context, it might be a good idea to add some lime. You would add between 50 grams (loamy soil) and 100 grams (sandy soil) per square metre in the ground, so a heaped teaspoonful should be sufficient for one of your ten-litre plant tubs.

To feed your tomatoes while they are growing, choose a fertiliser with high potassium and phosphorus content (K, P) and lots of micronutrients. In your garden centre, you are likely find one amongst the tomato feeds or flower fertilisers. Avoid herbaceous plant fertilisers as they contain too much nitrogen.
Just a quick word of advice about the quantity to grow: Unless you are a big tomato eater, six tomato plants are usually sufficient for a two-person household, twelve if there are four of you, and sixteen for six of you.

My extra tip: to make sure the tomatoes have the right amount of feed and water when they are ripening, I would advise putting them in a plant tub with a water reservoir. Alternatively, you could also use an automatic watering system.
To finish, here is my answer to your question about lettuces:

you can use a good-quality peat-based compost for these too. The best solution for a balcony window box is leaf lettuce. With proper fertilising, you can sow seeds more densely, the lettuce grows fast and you can reseed within a short time. If you have four or five window boxes set aside to grow lettuce and sow seeds in a different box each week, once the first heads are ready, you will then have a fresh harvest in one of the boxes every week.

To be able to reuse the soil rather than having the cost of changing it before replanting, you can then sow Asian salad greens afterwards. After harvesting, sow leaf lettuce again and then lamb's lettuce in the autumn. Before reseeding, loosen the soil in the window boxes each time.

It is best to feed your salad using 3 grams of slow-release fertiliser per litre of soil. This is a six-month fertiliser which should be mixed in with the soil when first preparing the window boxes. Depending on the manufacturer's instructions, it can be supplemented on a weekly basis with a liquid fertiliser for herbaceous plants. This advice on feeding depends on the weather, so you will need to develop the knack for avoiding over-feeding or under-feeding and to adapt appropriately.