How to practise crop rotation

Crop rotation has two main aims: to provide nutrients to your plants and protect them from garden pests.

All plants have different nutrient requirements. When you grow the same crops in the same parcel of land over several consecutive years, you run the risk of depriving the soil of certain nutrients. This is known as soil depletion.

Garden pests tend to set in and reproduce more easily when the crops remain unchanged. Varying your crop from one year to the next helps to limit the spread of parasites in the vegetable garden by depriving them of their preferred food source and habitat.

What nutrients do vegetable crops need?

The types of minerals plants take from the soil depends on the crop type and their needs. Each type of plant requires a different set of minerals and trace elements.Alternating different vegetable crops on the same plot of land helps to avoid depleting the soil as

the soil has time to replenish its reserves. At the same time, some vegetables require a lot of nutrients and therefore a lot of manure; other crops are less demanding and are able to grow without the use of fertiliser.

Vegetables are often split into three different categories depending on their needs in terms of nutrients:

• high nutrient requirements: squashes, courgettes, tomatoes, cabbages, aubergines and peppers;

• average nutrient requirements: carrots, lettuce and beetroot;

• modest nutrient requirements: onions, garlic and radishes.

Crop rotation also makes the most of the ability of certain plants to enrich the soil with minerals. The most common example of this phenomenon is legumes and pulses (peas, beans, etc). These crops use the bacteria found on their roots to fix atmospheric nitrogen in the soil. After harvesting, the plant roots decompose in the soil and nitrogen is released to the benefit of any subsequent crops.

Depth of the roots and mineral depletion

Plants deplete minerals from the soil at different depths depending on the root system of the plant. Some crops, such as lettuce or beans, have shallow roots.At the other end of the scale are plants like asparagus, squashes or root vegetables (carrots, turnips, potatoes, etc.), which seek their nutrients deep down in the soil.

Changing your crops from year to year will interrupt the life cycle of parasites or disease, whether you're dealing with fungi or garden pests. This is because many plant parasites are able to survive in the soil only to crop up again the following year. For example, at the end of the summer, surviving adult potato beetles can bury over 30 cm down into the soil to re-emerge the following spring.

Vegetables from the same botanical family often have the same enemies. The flea beetles that riddle your radish leaves with holes are the same ones found on cabbage leaves. That's why it's a good idea to wait a while before seeding or planting vegetables from the same family on the same parcel of land. In order to practise crop rotation effectively, you will therefore have to know which family your vegetables belong to.

A question that comes up a lot is how long you should wait before growing the same crop again in a given spot.  The response is simple: as long as possible.This is because plant parasites can survive for up to several years in the soil in many resistant forms; for example fungi spores, eggs, larvae, etc. That said, it is generally thought that a cycle of crop rotation should last about 3 or 4 years.

Remember that some crops won't be affected by rotation as they stay in place for a number of years, such as:

• strawberries;

• artichokes;

• asparagus;

• rhubarb;

• 'perpetual' vegetables such as some types of cabbage and onions.

These crops are often given their own isolated spot in the vegetable garden.

You'll quickly realise that attempting to practise effective crop rotation within all the correct parameters (plant family, crop type and nutrient requirements) can prove a tricky task. The following table should help you find your way.

It is fairly straightforward for those growing around three or four crops over a large area to get started with crop rotation and the practice is well worthwhile in this case. However, crop rotation can become a real challenge if you grow a wide variety of crops in a smaller space, such is often the case in vegetable gardens, greenhouses and home gardens.

Crop rotation requires organisation and forward-planning. You'll need to think about your crops several years in advance and break down your garden into parcels.  If you also want to factor in companion planting, the challenge can become almost impossible.

However, to keep things simple, bear in mind that the most important crops to rotate are potatoes and cabbages. If you can, you should also rotate the position of your carrots and any plants from the onion family (garlic, shallots, leeks, etc.).

Divide your vegetable garden into as many zones as you plan your rotating cycle to last. For example, for a traditional four-year rotation plan, break your garden into four large parcels.

In this case, each parcel should contain vegetable crops with the same basic requirements in terms of fertilisation during any given year. Start with vegetables that need a lot of nutrients (e.g. aubergines, squashes and cauliflowers); for this, you'll need a strong fertiliser such as a mature compost.

The following year, the parcel should be used to grow less demanding crops such as carrots, beetroot, peas and lettuce. The third year, you'll need to grow vegetables with low nutrient requirements, such as garlic, onions, lamb's lettuce; you do not need to add compost at this stage.

It is then recommended to add green manure to the soil and leave the earth to replenish itself for the fourth year.

In very small gardens or raised vegetable beds containing a lot of different crops in a tight space, it generally won't be possible to practise crop rotation as any vacant space will quickly be occupied by new seedlings or plants. However, small gardens offer crop diversity and the quick switchover of plants means that they tend not to fall victim to diseases or pests.