Thursday 12 March 2020

Crop rotations: the basics for urban growers

My kitchen garden and forest garden border now. The raised beds currently have garlic,
broad beans and corn salad. Now it's warming up, I can start more seed sowing
and really get the season going.
When I published my post on my crop rotation plans for this year in January, I got a lot of interest, particularly from people on Mastodon, who asked me a number of questions. So I thought I'd write a post covering the basics as my contribution to Solarpunk Action Week. The focus is on urban growers, as that's from where my experience comes.

Why rotate your crops?
Or, do you have to rotate your crops, even in a small garden? Yes, yes, you do.

The key principle of crop rotation is that you rotate plants of the same botanical family, which are prone to the same soil-living pests and diseases, and should not be grown in the same spot every year. The aim is to prevent the build-up of soil borne problems, such as club root, white rot, and eelworms, which often have no means of control (1).

If you continue to grow the same few crops in your garden or allotment, you risk a build up of diseases in the soil that might take years to get back under control. Particular diseases include potato cyst eelworms which affects potatoes and tomatoes, and club root which affects brassicas. Of course, in smaller gardens, disease can spread to other soil areas. Best practice is to do what you can to reduce the chance of this happening.

The other key reasons to rotation your crops is for nutrient availability/soil fertility. Vegetables have different nutrient requirements, so moving them around the growing area helps to avoid nutrient depletion in the soil (and remember, you'll be adding compost and manure/green manure each year to also improve soil fertility). Furthermore, rotating crops with alternating deep- and shallow-rooting vegetables, improves soil structure.(2)
A wooden raised bed sitting on top of acid soil.
The raised bed is filled with compost, giving it an alkaline-neutral pH. 

Does soil type (pH) matter?
The pH in soil matters because crop rotations are needed for annual vegetables, which won't grow in acid soil. A pH of 7.0 is considered neutral. An acid soil has a pH value below 7.0 and above 7.0 the soil is alkaline.

When it comes to perennial vegetables, few will have much of a problem with a slightly acid soil (pH 6-7) and some will be ok in pH 5-6. Below pH 5, you can only grow acid loving fruit and vegetables, such as most fruit, including blueberries and strawberries, or acid tolerating perennials such as sorrel.

If you have heavy acid soil (pH 5 and below), as I do, then building raised beds is the way to get around this to grow annual vegetables. My raised beds are 40cms high or more, allowing me enough depth to grow any type of annual vegetable.

Basic crop rotations
Crop rotations are usually practiced on a 3- or 4-year plan. If you have the space, plant your crops on a 4-year plan. For smaller spaces, go for a 3-year plan as a minimum.

For a simple 3-year crop rotation, divide your space into 3 sections or beds, then plant:
  1. Root crops (including potatoes, carrots, beetroot, garlic, onions and parsnips). You could include tomatoes in this group to keep the solanum family (potatoes and tomatoes), together.
  2. Brassicas (including cauliflower, broccoli, kale, radish, kohl rabi and cabbage).
  3. Legumes (peas, broad beans and beans). You could add the cucubits (pumpkin and squash) and vegetables that don't really have a group, such as corn and lettuce, here.
For a simple 4 year rotation, you would use the above, but separate out potatoes and tomatoes into their own group.

The garlic grew in this raised bed last year. This year I have created sections within this, and it
will contain a mix of broad beans, climbing French beans, squash and courgette, parsnip and beetroot.

However, even a basic crop rotation doesn't stay quite this simple, because plants have different seasons in which they grow. You plant garlic and broad beans in autumn and they'll be in their spaces from November to the following July-August. For example, one bed might have the following 4 year rotation:

Year 1: Broad beans Spring and Summer, then winter salads and green manures in Autumn and Winter.
Year 2: Peas, courgette and squash (the peas grow up, the courgette and squash underneath) for Spring, Summer and into mid-Autumn. Then plant garlic from mid-October, which will be  that will be there from October to the following July.
Year 3: Garlic up until mid-Summer, then salads and green manures for Autumn and Winter.
Year 4: Potatoes from Spring until Autumn.

After year 4, you return to year 1 and start again.

This photo shows an overview of a previous garden in mid-summer, 2015, which ran on a 4-year schedule. You can see how much you can fit into a small urban garden. The raised beds have been divided into roughly a metre squared sections, with different annual vegetables growing in each. The same view looked different next year as the crops are rotated.

Below are the plans I have, using a 3-year crop rotation cycle. Visit my post on crop rotations for my garden to see more detail.

Detailed crop rotations
Here's an example of crop rotations for a larger amount of beds, from a previous garden. This also includes a larger amount of crops, including brassicas and solanums. The beds were a mix of 3 smaller beds (1-3), and two long beds (4-8 and 9-13). The long beds were broken up into sections in order to make it easier to plan for crop rotations. This meant I could go with a 4 year rotation plan.

This is another photo from 2015, more clearly showing the different vegetables growing in different sections. In this space is: garlic, potatoes, climbing French beans, broad beans, parsnips, and carrots under white netting.

Final thoughts
You don't even need to start of with making big crop rotation plans as I do (I'm a bit of a nerd in that area). As long as you keep a record of what you plant, and where, each year, you'll be fine. Crop rotations can be flexible, just keep botanical families together. You can mix botanical families, but move them around the beds/rotations, together.

* * * * *
My thanks to Alison from Backyard Larder for information regarding growing perennials in acid soil.

1. Growing fruit and vegetables on a bed system the organic way, Pauline Pears, 2004, p25.
2.  Encyclopedia of Organic Gardening, editor Pauline Pears, 2001, p301.