Tuesday, 1 March 2011

Do potatoes need a concentrated NPK fertiliser?

I have to keep reminding myself of this:
N - Nitrogen: promotes the growth of leaves and vegetation
P - Phosphorous: promotes root growth
K - Potassium: promotes flower and fruit growth

I know that the biochemistry is much more complicated than this but this is good enough for me.

The Marshalls's potato fertiliser is N:P:K 15:21:24.5 which is massive and I don't know why potatoes would need so much potassium.

Whatever, I use comfrey for potatoes and this has a higher ratio of potassium, but nothing like the Marshalls's fertiliser.
Comfrey's percentage NPK is 0.74:0.24:1.19 nowhere near the Marshalls's fertiliser.

Relative to the amount of carbon dioxide and water that the plants need to produce sugars, the amounts of N:P:K that they require is tiny. I would question the necessity for such high ratios.

It is remarkable how the amounts of these elements in plants mimic the amounts in the soil. There is some evidence that excessive nitrogen in food is not a good thing.

I like to put quite a lot of animal manure on the potatoes and this year it was mixed in well with tree leaves.

Apart from this, I am sticking to home made compost, comfrey tea and nettle tea to fertilise my veg.

Have a look at what my potatoes are like now in May with only comfrey, horse manure and leaves added to the soil. 
Kestrel potatoes at the end of May

Kestrel potatoes in June


  1. The general rule in commercial growing is to keep the NPK at a ratio of 1-1-1.5. We use 14-14-21 NPK on most of our potatoes but as long as you keep that ratio you should get a good consistent growth.

  2. Thanks for that JBA. I am surprised that it is such a big ratio but you are the experts. As an organic gardener, I doubt if I could get a ratio as high as commercial growers use. Comfrey seems to nearly have the right ratio although it might need some bone meal to keep the phosphorus up.