container vegetable growing

I’m experimenting with container growing. I am a complete beginner, so please be patient with my newbie questions.

I’ve got some young tomato and potato plants, and some onions and cabbages growing in small pots indoors. Getting ready to plant them in larger pots outdoors. I also sowed some leeks and carrots, but they aren’t looking good so far.

What kind of soil should I use for planting?
I’ve got some bags of “potting compost” (contains peat) which is described as being good for container growing, and some multi-purpose compost" which is described as being good for growing vegetables. Should I just use one or other of these? A mixture of the two?

Here’s the particular brand I’ve got.

Anything I should add to the mix? I understand that plants need traces of minerals from rock. Does the compost have all the needed minerals? Should I add a little gravel or sand to the mix?

I understand too that plants need certain bacteria in the soil for growth. Does the compost have the correct ones already? Maybe I should add a few scoops of soil from the garden?

What about perlite? It’s supposed to increase water retention and aeration. Is this needed?
Should I add fertilizer? How about plant food? I’ve seen potato food sold in pellets, and tomato food in liquid. Should I add some of these?
How about water-retaining crystals? Are these useful? I heard they are good for preventing over-watering. Is that correct?

Multipurpose compost probably already has most of the plant food that your plants will need to get started - once they’re established, they may need additional feeding, but there’s no rule of thumb for that - you need to look up the feeding regime for the types of plants you have.

For example, tomatoes will need feeding with a diluted tomato food once they begin flowering - this provides the potassium they need to form fruit.

Cabbages need to be a bit starved of potassium, or they may run to flower - but they need nitrogen-rich fertiliser to form healthy leaves.

Most of the things you listed can be planted out into larger containers once they’ve grown on a bit, but potatoes aren’t normally transplanted - you would usually plant seed potatoes in a large container and leave them there until they are ready for harvesting.

I wouldn’t transplant carrots either- if you disturb the taproot, it tends to split and wiggle, and you get good contenders for the comedy vegetable section in the local show, but you don’t get usable carrots. I’d direct sow carrots in a deep pot outside. They do like very loose soil (it helps them grow bigger, straighter roots), which perlite or sand can help with, so for them I’d probably add some.

Your spuds should be OK if you plant them out, I don’t think transplanting will hurt the plants, but plant them really deep in the pots- bury the stems- as this encourages the plant to produce more tubers. Tomatoes (which are closely related) also respond very well to being planted deep, as they produce more roots from the stem.

Generally, most half decent multipurpose composts are fine for most stuff for a while, but the nutrients do get depleted over time, so adding some fertiliser later is a good plan. Not too much though, after a point in positively hinders growth. You can mix a bit of soil in if you want, but they should grow fine in the straight compost. Don’t make it more complicated than it needs to be when you’re starting out.

May I ask why you’re keeping them inside? Personally, I would not for anything but the tomatoes (assuming, as your link would imply, that you’re in the UK), as the others are all quite happy with fairly low temperatures, and prefer more sun. Plants grown too ‘soft’ are more prone to pests and disease. They need to be introduced to outdoor life gradually, even the tough plants, keeping them in a very sheltered spot and bringing them back in if it gets frosty or windy for the first week or so (soft growth can be damaged by frost even if the species is normally totally hardy).

The advantage to adding perlite or vermiculite to containers used for vegetable growing relates to aeration, allowing the growing medium to drain more quickly (important in a rainy growing season, to lessen the chance of soggy soil/root rot) and permits the pots/tubs to be moved more easily, saving your back (tomatoes and potatoes will need large containers). The downside of lightweight mostly soilless mixes is that they dry out faster and need more watering, plus are more prone to blow over in high winds. I can’t tell from the OP’s first link exactly what’s in that potting mix, other than peat (“compost” is a very broad term).

I just started 7 large pots/tubs to grow tomatoes and eggplant, mixing in a slow-release balanced fertilizer. That will be supplemented as needed with liquid feedings. Something like a 20-20-20 fertilizer is often recommended for tomatoes early on (the numbers refer to N-P-K, nitrogen, phosphorus and potassium). Then you can switch to a fertilizer higher in the last number (potassium) once flowering begins. I wouldn’t worry too much about trace nutrients, which are generally supplied in liquid fertilizers.

Agreed, although if very great care is taken, it’s possible to transplant them and achieve better results than with direct sowing - I’ve seen cases where gardeners with slightly stony soil germinated carrots in paper tubes of sandy compost, then planted them into the ground by pushing a large dibber into the soil, partially filling with more sandy mix, then planting the paper tube with the carrot seedling in the top - this resulted in the most uniform and nice-looking carrots I have seen, but is a lot of effort and risk, if you don’t need to do it.

What I thought of doing with the carrots and leeks is this: start them off in seed trays with individual square cells like this. I put a few seeds in each cell. I was intending to keep only the strongest shoot in each cell and remove the weaker ones. Then when they are established I’d move the entire square of soil into larger containers. Would that work, without splitting the carrots and making amusing shapes?

For the moment it’s a moot point, since very few of the seeds germinated at all, and those that did are very weak.

Maybe, but for the carrots, once you get to the point where the roots are out of the bottom of the cell, you’re going to bend or disrupt them when you move them, and at the very least, your carrots will be kinky.
And if you try to transplant them before that point, there won’t be enough roots to hold the compost in the cell together in a coherent block - so it will crumble, and you will be transplanting a bare seedling, which has a high risk of damaging the roots, or bending them.

Leeks, on the other hand, are OK with transplantation - it’s quite common to grow them in a seed tray, then ‘prick them out’ to a dense planting in a seed bed where they grow to a little smaller than the size of spring onions, then dig them up and plant them to their final position at proper spacing (where the normal method of planting is to use a dibber to make a hole, drop in the plant and just water it in, rather than pressing the soil down)

It may be that you should choose your vegetables to fit the growing conditions that you have available - if you’re growing in containers, these sorts of things will work:

Bunching onions (sort of like chive-zilla - but you can dig them up at pretty much any time, split off as much as you need for the kitchen, then divide and replant the remainder, and let them grow again)

‘Tumbler’ varieties of tomato - they’ll grow in a hanging basket, and they form compact plants that form a sort of small, overhanging bush, with cherry tomatoes on it.

‘Cut and come again’ mixed salads - these are seed mixtures containing various assortments of lettuce and other salad greens which you can sow directly in your container - when you thin out the seedlings, you can eat the thinnings as baby salad leaves - and as the plants mature, you can either pick a few leaves at a time, or cut them off and they regrow a bit.

Radishes - plant direct in containers and they crop in a few weeks