From Gardeners' Question Time - Factsheet for 12 August 2001

Question from Alan Llewellyn: "We are told to use crop rotation to assist against pests and diseases, but what do the panel consider is the correct order for that crop rotation?"

Bob: "The important thing is to make sure the same plant isn't in the same place two years running.  Then the pests and diseases that over winter in the soil aren't there for the next crop.   It isn't so important what you rotate with what, the first and most important thing is that you rotate.

"The next thing is, some plants are more susceptible to pests and diseases than others.  It's important to move carrots and parsnips so they don't get root fly the next year.   And brassicas, so they don't get club root the following year.  And onions, for onion fly.

"Brassicas like lime but potatoes don't like lime, so it makes a lot of sense to have them well apart in the rotation.  Peas and beans also like lime, so it's common to add the lime when you have peas and beans.   That leaves the soil rich, so then you plant your brassicas.

"After the brassicas the soil is quite firm, which is good for the onions.  You don't even have to dig the soil, you can just put the onions where the brassicas were.

"After the onions, carrots come rather well.  Because carrots are a root vegetable you break the soil up getting them out, which leaves the soil in lovely condition for planting potatoes!

"The potatoes take a lot of goodness out of the soil, so I earth them up with grass clippings, which increases the humus content.  At that point it's good to add lime so it's time for the peas and beans to come back again!"

Eric: "Is that one year's rotation or three?"

Bob: "I try and work a minimum of a three year rotation before anything ever comes back, but the aim is for a five or six year rotation.   But it really doesn't matter, just make sure you never have the same plants in the same place two years running."