Dear Readers, I am a great fan of botanist James Wong, who writes a regular column in New Scientist. He is a great one for debunking garden myths, and this week he was taking on the idea of having different plant fertilisers for different kinds of plants. If you’ve been to a garden centre lately you’re bound to have noticed that there are liquid feeds for almost every kind of plant, from chrysanthemums to fuchsias, not to mention for different fruit and vegetable crops. But do we really need to buy all of them?
Wong explains that although plants need 16 micronutrients in order to thrive (and there’s a very useful article explaining this here), most fertilisers are based on three macronutrients: nitrogen (N) for leaf growth, phosphorous (P) for root development, and potassium(K) for fruit and flower formation. Fertilisers often display an NPK ratio, showing the make-up of the formula. So, lawn fertilisers often feature nitrogen, while tomato feeds might be heavier on potassium. However, as Wong points out, it’s not so simple – he looked at three ‘individual’ fertilisers, for roses, tomatoes and strawberries, and all three had an identical NPK ratio. So, if you were growing all three ‘crops’ you wouldn’t need three bottles of fertiliser – just one would do.
Furthermore, as Wong points out, plants only take what they need from the soil, in the proportions that they need it. The way to check out what’s going on is to buy a cheap, simple soil test, to make sure that your soil has what plants need. There will always be some exceptions, such as ericaceous plants, which need more acidic soil conditions, but even here, the plants will sometimes surprise you: I’ve seen camellias growing to 20 feet tall in a London front garden, on clay soil. And anything in a container will probably need top dressing occasionally, to make sure that all the nutrients aren’t used up. But we don’t need a whole shelf full of different bottles of fertiliser, pretty as it might look.
And also, let’s not forget that over-fertilised soils produce run-off, which pollutes our rivers and streams. Sometimes, less is definitely more.
This bears out what I have thought for years. Good compost does plants a lot of good – as does mulch.