Crop rotation is a food gardening plan that minimises pests and diseases by growing different crops in succession
Crop rotation sounds stranger and more complicated than it really is. It refers to the system of growing vegetables in different parts of the garden in successive years to reduce pest and disease problems. In particular, it reduces the chance of soil-borne diseases and crop-specific pests building up and making it hard to grow crops. Rotation also allows each crop to benefit from the soil conditions left behind by the previous crop. As crop rotation is a non-chemical approach to pest and disease prevention and control, it’s an important concept for organic gardeners to implement. It can be done on a large scale using four to six separate beds, or in a small way by varying crops from year to year.
Understanding plant families
For crop rotation to work effectively, good plant knowledge and careful planning is required. The first thing to understand is that plant diseases and some pests affect groups of plants that are related to each other. Related plants are grouped together in plant families. For example, potatoes and tomatoes are in the same plant family of Solanaceae, so they shouldn’t be grown one after the other in the same bed.
As well as being classified into family groups, they can also be grouped according to how they grow and which part of them is harvested, giving rise to root crops (such as parsnip, carrot and potato), leaf crops (such as spinach) and fruiting crops (including tomato, cucumber, beans and peas). Depending on which part is harvested, plants use different amounts of nutrients.
One major plant family, the legume or Fabaceae (the pea family), is the wild card in any plant rotation system. While most vegetables use up soil nutrients, legumes have the ability to add nitrogen to soils. To make the most of the added soil nitrogen from legumes, they are followed by plantings of leafy crops that make good use of that increased soil nitrogen.
Some approaches to crop rotation
To make the most of soil nutrients and to reduce disease problems, crops are rotated in a set pattern across three to six garden beds, depending on the space available and the crops grown.
In a six-bed system, one bed can be left fallow (that is, not planted with a crop) to rest for a season to further reduce diseases and allow nutrients to be restored. Fallow beds may be left heavily mulched or else sown with a green manure crop that’s then dug in to replenish soil nutrients.
In agriculture, particularly when growing certain disease-prone crops such as potatoes and brassicas, farmers leave five years before the same crop is grown again in the same field.In very small spaces, such crop rotation is difficult to manage without restricting crops, but it’s possible to include a raised bed or containers in a crop rotation scheme in a small area to lengthen the time between replanting a crop in the same soil.
Not every crop is rotated. Vegetables that are perennial, which means they grow year after year in the same space, such as asparagus, artichokes, rhubarb, herbs and some fruiting plants, are grown in permanent beds. Rotation is also not critical for plants that have few soil-borne disease problems, such as lettuce and spinach, or that take up lots of space, such as pumpkins, squash and zucchini.
One of the simplest forms of crop rotation is just growing summer and winter crops, as most actually come from different plant families.
Crop rotation is complicated and it’s easy to forget what was planted where and when. For the system to work, note each year’s planting scheme in a diary or notebook with a planting plan, draw it on a chalkboard in the garden shed or garage, or take photographs of your plantings and include the date.
Speeding up rotations
Current research is showing that growing a biofumigant crop in the rotation system can speed up rotation times by eliminating pest and disease problems in soils. Mustards and other brassicas containing high levels of glucosinolates can be grown as a winter crop and dug into the soil to release mustard gas. Tasmanian farmer Darren Long claims that growing biofumigants on his farm has allowed him to replant potatoes into fields within three years rather than waiting five years.
Five-year crop rotation system
Rotate crops from bed to bed in a systematic way to reduce the likelihood of pest and disease problems and to make the most of soil nutrients. The pattern outlined below shows the planting pattern per bed in year 1. In the next year, plant new crops in the adjacent bed (for example, the brassicas grow where the beans were growing, and root and stem vegetables (in bed 5) move to bed 1. The same pattern is repeated in following years, with whatever is in bed 5 moving to 1. Where space permits, include a sixth bed in the system for a green manure crop, which is dug into the soil to increase soil nutrients. Under this system, brassicas won’t return to bed 1 for five years.
Bed 1: Brassicas (Brassicaceae), corn (Poaceae)
Bed 2: Legumes (Fabaceae)
Bed 3: Potato, tomato, capsicum (Solanaceae)
Bed 4: Onions (Alliaceae)
Bed 5: Root and stem vegetables (Amaranthaceae and Apiaceae)
Bed 1: Root and stem vegetables
Bed 2: Brassicas
Bed 3: Legumes
Bed 4: Potato, tomato, capsicum
Bed 5: Onions
Written by Jennifer Stackhouse
Originally from Good Organic Gardening Volume 7 Issue 6