The Power of Potassium

Jerry Coleby-Williams

JERRY COLEBY-WILLIAMS: I’m adding potash to my strawberries. By adding potash, I can strengthen my plants, increase their ability to fight disease and my fruit tastes better. Potash is good stuff.


In gardening, the words potash and potassium are interchangeable. It is a standard ingredient in most fertilisers and you can also get it on its own in a crystalline form which you can dissolve in water and in a granular, slow release form which you sprinkle directly around plants and you’ll sometimes see it in liquid form.

Now along with nitrogen and phosphorous, potassium is what’s known as a macro-nutrient – plants need a lot of them. If you buy fertilisers, you’ll see them displayed on the back of the pack – ‘N’ for nitrogen, ‘P’ for phosphorous and and ‘K’ – that’s Latin – kalium which stands for potassium – and this is how it works.

Firstly, potassium helps plants to move water and sugar inside themselves, so it makes fruit juicier and sweeter and it also improves the quality of flowers.

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Secondly, potassium helps strengthen plants – it thickens their cell walls. Look at these spring onions. If I applied a general purpose fertiliser to them, containing nitrogen, it will produce a surge of growth, but the growth is soft and sappy and prone to rot. Applied on its own, potassium produces the same surge of growth, only that growth is strong and less susceptible to rot.

Thirdly, potassium helps defend plants against disease. By creating thicker cell walls, it makes it difficult for germinating fungal spores to punch a hole through the side of the cell wall and cause disease.

Fruit trees like mango, avocado and this custard apple, suffer from a fungal disease known as anthracnose and usually gardeners discover this when it’s too late – when the fruit has spoiled. Disease prevention starts in winter – using potassium – and you apply it once a month during winter and spring and that prevents the flowers becoming invaded by this disease.

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Now, how much potassium to use? It varies according to the brand, so check the back of the pack and remember that a little bit goes a long way. You can overdo potassium and you can kill plants. That said, just for a few bucks, I can buy a kilo and that’ll last the whole of my garden for an entire year – fruit, vegetables, turf – everything, so this is a wonderful thing to use. It puts you in charge as a gardener. You can see the result. Look forward to using potassium and you’ll get fresh flowers, strong plants and delicious fruit – and it doesn’t get better than that.

COSTA GEORGIADIS: Well that’s all we have time for. I hope you’ve enjoyed the show and that we’ve inspired you to get out into your own garden. I look forward to seeing you next week. Here’s what’s on offer then.

Jerry will be helping to relocate a huge collection of bromeliads to a new home.

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Angus will be showing how the power of deep planting has encouraged impressive growth in his trees.

ANGUS STEWART: We get extra roots called adventitious roots, growing all along the buried stem, which means that the plant has got access to more water and nutrients, all through the soil profile.

COSTA GEORGIADIS: And Tino’s busy with some last minute winter jobs so his borders will be bursting with colour in spring.

I’ll see you then.