When I first started growing my own fruit and veg in containers, I found
information about how, when and what to feed plants in containers surprisingly
hard to come by. Perhaps this isn’t surprising. Crop nutrition is never going
to be a sexy subject. At its best it’s a complicated and slightly smelly one. Still,
if you want your plants to thrive in containers, you need to put a bit of
thought into feeding them. You can dedicate a lifetime to learning about crop
nutrition. But I’ve found a little knowledge can go a long way.
Plants need a mix of nutrients, just as humans need to
consume a mix of protein, fat, carbohydrate, minerals and vitamins. They
particularly need nitrogen (N) for leaf growth, phosphorus (P) for root
growth, and potassium (K) for fruit growth. All plants need all three, but leafy
crops particularly need nitrogen, and fruit crops won’t develop well without
enough potassium. They also need a
wide range of other nutrients, often in tiny quantities. I think of these as
the equivalent of the vitamins and minerals we need to keep us healthy.
Finally, plants need bacteria and fungi in the soil to break it down and
release food to their roots, just as humans need bacteria to digest food in our
Most compost or growing mixes you can buy in garden centres
only contain enough nutrients for six weeks’ growth. So for optimum results,
you need the right fertiliser (rich in either nitrogen or potassium) and you
need to make sure the plant has enough vitamins and minerals and that there is
plenty of soil life in your pots.
Here are some of the easiest ways to add these critical
Your salads and other leafy crops won’t flourish unless they
have enough nitrogen.
Chicken manure pellets are cheap, easy to source, and
contain most of the essential nutrients for plants. They are particularly high
in nitrogen. I use them all the time for rejuvenating old compost to grow
salads. Simply mix a handful into your compost before you plant your salads.
If you have a supply of nettles nearby, you can make nettle
tea by soaking nettles in a bucket of water for two weeks. The resulting brew is high in nitrogen
and other goodies (if a bit whiffy!).
Take care not to add too much nitrogen to fruiting crops –
you may get lots of leaves and not many fruits. Try experimenting – a little bit at a time is the safest strategy.
To get good yields of tomatoes, runner beans, squash,
chillies, strawberries and other fruiting crops from containers, you need to
regularly add additional potassium (K).
The easiest way to do this is to buy a bottle of tomato
feed. Although called tomato feed, this will do the job for all fruiting crops.
You can also make an excellent potassium feed by soaking comfrey
leaves in water for a week to make comfrey tea, which is a more organic
solution. You can find comfrey alongside canals and in town marshes where it
can grow wild in abundance – or you can buy liquid comfrey online. This is
wonderful stuff, though it is even smellier than nettle tea.
Vitamins and minerals
Liquid seaweed is an excellent source of vitamins and minerals.
You can add it to your watering can once a week to keep your plants healthy. Or,
to make a precious bottle go further, you can spray it on to the leaves of your
crops using a simple hand spray (an empty spray bottle, cleaned well, works
Rock dust (available in some garden shops) is another source
of minerals – mix a few handfuls into the soil in each container.
Healthy soil contains an invisible and amazing array of bacteria
and fungi which digest the nutrients you are adding, releasing it to the roots
of your hungry plants.
You can buy various products such as mycorrhizal fungi to add
life to your pots. But one of the best sources is worm compost or worm tea. You
can make this at home in a wormery.
Worm compost is fantastic for containers: as well as soil
life it’s rich in the vitamins and minerals your plants need, as well as some
of the major nutrients. Mix 10 – 25% into old compost to rejuvenate it, or add
5 – 10% to new compost to add soil life. Add a layer of an inch or two to
hungry crops like courgettes and tomatoes to give them a boost half way through
the growing season.
How much to feed?
Feeding crops is as an art as well as a science. How much you
feed depends on lots of variables: the size of pot, what compost you’re using, how
big your plant is, and how fast it’s growing. The secret is to give it a go,
observe the difference, and learn from the results. As a general rule, little
and often is the safest strategy (too much feeding is as bad as too little). And
feed more when your crops are fruiting.
I’m only scratching the surface of a big subject here – but following
these simple rules can, in my experience, transform small harvests into more
Live Better Challenge is funded by Unilever; its focus is sustainable
living. All content is editorially independent except for pieces
labelled advertisement feature. Find out more here.