Practising crop rotation when growing annual vegetables not only increases long-term yield, it preserves the fertility of your soil while deterring pests and diseases.
We can’t treat annual vegetable plants like perennials because they grow in a completely different way. Perennials are longer-lived and send their roots down deeper to access more water and nutrients. Generally slow feeders, perennials use only moderate amounts of nutrients to drive equally moderate rates of growth.
Annual plants, which have less than a year to complete their life cycle from seed to seed-bearing plant, are much shorter lived. Most have 80 percent of their roots in the first 30 centimetres of soil, so they can’t reach very deep for nutrients or water. They compensate by having extremely rapid growth rates and have high nutrient demands to fuel that growth.
Monocultures of annual plants don’t typically exist in nature, because most can’t form stable ecosystems. Collapse is inevitable when large numbers of shallow-rooted heavy feeders are planted together. The soil ecology cannot support them for extended periods and pests and diseases proliferate more quickly when plants are concentrated in a single area.
However there are many things we can do to maintain a healthy and productive annual vegetable garden, and one proven strategy is crop rotation.
What is Crop Rotation?
Crop rotation is the practice of planting each type of crop in a separate location, in a certain sequence, with each new season. Every year, each type of crop is moved to the next planting location in the sequence. It is a systematic approach to ensure a crop type isn’t grown in the same garden bed for longer than one season.
For example, we plant tomatoes in the first garden bed, carrots in the second and lettuce in the third. So when you come to the second year, your planting sequence in the three beds will move to lettuce in bed one, tomatoes in bed two and carrots in bed three; the third year will be carrots, lettuce and tomatoes.
In this example, tomatoes move to the next bed each year, and the first bed does not grow any tomatoes for two years afterwards. This may sound like extra garden planning and record keeping, but crop rotation is important for many reasons.
Certain vegetables use more of some nutrients and less of others, so crop rotation prevents depletion of the soil. If the same vegetable is grown in the same location season after season, it will quickly deplete the soil of the most used nutrients it requires.
Certain plants used in crop rotation, such as legumes, add nutrients back into the soil. By rotating your crops, you’re increasing soil fertility by maintaining nutrient levels and soil health to support plant growth.
Pests have preferences for particular plants so if certain vegetables are mass-planted in the same location season after season, pests will naturally gather there and their populations will rapidly increase. Crop rotation means pest breeding cycles will be disrupted and their numbers will reduce.
It’s a similar story with diseases. Soil-borne pathogens such as parasitic nematodes, fungi and bacteria all have preferences for particular plant families. If favourable conditions are created by growing plants from the same family in the same location year after year, their numbers will build in the soil, reinfecting the plants they favour.
Givers and takers
COMMON EXAMPLES OF WHICH VEGETABLE FALLS UNDER WHICH FEEDING OR GIVING CATEGORY.
Asparagus, beetroot, broccoli, Brussels sprouts, cabbage, capsicum, cauliflower, celery, sweet corn, eggplant, endive, kale, kohlrabi, lettuce, okra, parsley, potato, pumpkin, radish, rhubarb, rockmelon, spinach, squash, strawberry, sunflower, tomato and watermelon.
Carrot, garlic, leek, mustard greens, onion, parsnip, swede, shallot, sweet potato and Swiss chard.
Alfalfa, beans, clover, peas, peanut and soybeans.
ROTATION BY PLANT FAMILY
All plants belonging to the same family have common traits and properties. Take tomatoes and potatoes, for example. Both are in the Solanaceae family, both are the same Solanum genus, but a tomato’s species is Solanum lycopersicum, while a potato’s is Solanum tuberosum. From this example, we can see that tomatoes and potatoes are closely related and will have many things in common, including susceptibility to similar diseases.
By rotating whole plant families from one garden bed to another each year or growing season, you can maximise your yields.
There are two major systems of crop rotation, crop rotation based on the plant family and crop rotation based on nutrient demands. Crop rotation by family is a fairly complicated system which some find discouraging for that reason. By comparison, crop rotation by plant nutrient demands is a relatively simple system which is often easier to implement.
Rotation By Nutrient Demands
In John Jeavons’ book How to Grow More Vegetables, he introduces us to a system which categorises plants by three separate categories; heavy feeders, heavy givers and light feeders. Most vegetables fall into the heavyfeeder category, extracting lots of nutrients from the soil – especially nitrogen for producing leafy growth.
Heavy givers are nitrogen-fixing plants which take nitrogen from the air and return it to the soil, converting it into a form which is usable by plants. Light feeders, on the other hand, have relatively low nutrient demands and include all root crops.
With this system, plant groups are rotated in the order of heavy feeders followed by heavy givers followed by light feeders. A fourth green-manure category can be grown in a bed for half a year, warm or cool season, to further build soil much like the heavy-giver stage.
The other soil-building option is to add compost and manure to the soil, before covering it with mulch and allowing it to rest for six months. It’s important to respect the soil, and sometimes we just need to stop taking for a moment and give something back.
Family Rotation System
In a garden with six beds, each group (see sidebar) can be planted in separate beds (Group A in the first bed, Group B in the second bed, Group C in the third, etc.) and then rotated to the next bed the following year (Group A in bed two, Group B in bed three, etc.).
But since some of the plant groups are comprised only of warm- or cool-season vegetables, it’s not possible to plant all six groups out at once. Two rotation schemes may be required each year, as seasonal plants only live for half a year.
To use all available garden beds each half-year season, two beds can be planted with members of the same family. For example, (Group C) tomatoes and chillies can be planted in beds one and three, then the following year in beds two and four, followed by beds three and six, creating a three-year crop rotation cycle for that family. In the cooler seasons, the same beds can be planted with the brassica family (Group B) for example, and rotated through in the same way.
Planting calendars are useful for planning when to sow seeds from each group. If working with seedlings, be aware that they’re usually transplanted into the garden about four weeks later if they’re established enough to transplant and the risk of frost has passed.
EACH PLANT FAMILY IS ROTATED AS A SINGLE GROUP, AND THERE ARE UP TO SIX GROUPS WHICH CAN BE ROTATED WITH THIS SYSTEM.
Group A (cucurbits)
Cucumber, pumpkin, rockmelon, squash, watermelon.
Group B (brassicas)
Brussels sprouts, cabbage, cauliflower, kale, lettuce*, mustard, radish, spinach*, swede, Swiss chard*, turnip.
Group C (solanums)
Eggplant, potato, okra, pepper, tomato.
Group D (root crops)
Beetroot, carrot, garlic, onion, shallot, sweet potato.
Group E (corn)
Group F (legumes)
All beans, peas.
*Lettuce, spinach and Swiss chard aren’t brassicas, but are rotated within that group for convenience.
This article represents the permaculture principle OBTAIN A YIELD.