The basics of Crop Rotation.

A Guide To Vegetable Crop Rotation

The practice of crop rotation in the vegetable garden or the allotment is a sensible, plant health conscious approach to avoiding some of the most common problems caused by pests and diseases. To continue to grow the same crops in exactly the same plot year after year can be detrimental to plant health and can contribute to the depletion of a number of essential nutrients from the soil. However, by rotating the crops over a 3 or 4-year cycle, your vegetable plants are more likely to grow heathy, be relatively pest free and the soil can be improved or at least maintained with every crop. So, why does crop rotation work so effectively?

Understanding what different vegetable plants need

To understand this aspect you need to know something about what each type of vegetable plant needs from the soil, in terms of nutrients and in some cases, trace elements. Also, what types of pests and diseases can adversely affect different vegetable plants. It basically comes down to good soil management and knowing how much Nitrogen, Phosphorous and Potassium (NPK) fertiliser should be applied or organic matter added for each type of crop.

For example, almost all vegetable plants that are grown for their leaves, such as Cabbages, Brussels Sprouts and Spinach will require a high level of Nitrogen. Plants such as Onions, Swede and Carrots need much less nitrogen but require more phosphorous and plants that are grown for their flowers and fruits will need more potassium. In summary then, Nitrogen for leaves, Phosphorous for roots and Potassium for flowers.        

Incorporating organic matter and fertiliser to the soil

Replenishing the soil after a crop has been harvested is best done by incorporating plenty of well-rotted organic matter such as farm yard manure or horse manure. Well-rotted garden compost from a compost heap can also be quite effective. Leaf mould, spent mushroom compost or similar light composts are useful but will not contain all the nutrients required and are best used as a surface mulch. Whatever crops are to be grown, all types of vegetable plants will benefit from the addition of organic matter before planting. This is usually added to the soil during the late autumn or early winter months, allowing at least 6-8 weeks before planting out.

At planting time, a sprinkling of a general NPK fertiliser can also be beneficial or the addition of blood and bonemeal, lightly hoed into the soil surface, will provide a source of NPK nutrients together with some other trace elements. Alternatively, a liquid feed can be applied to plants as they grow but take care not to overfeed plants. Keeping the soil in a good fertile condition will pay dividends when it comes to harvesting your healthy vegetable crops.

Why crop rotation is best practice

Unless you are able to test your soil on a regular basis to know what level of nutrients it contains and what pH level it is, the practice of crop rotation will ensure the soil is never totally depleted of all nutrients. Planting leguminous plants, such as peas and beans, will actually add nitrogen to the soil through a bacterial process within nodes found on their roots. So by planting leafy plants, such as cabbage for example, in the same soil that peas or beans were grown in during the previous year, the young cabbage plants will benefit from the free nitrogen that has been left. There are several examples of plants, which leave a beneficial nutrient legacy for others that follow.

Other important reasons why you should practice crop rotation includes reducing the risk of passing on plant diseases within the same group of plants. For example, all brassicas are susceptible to club root, so if you plant brassicas in the same infected plot year on year then you can expect all will succumb to the disease. Rotating crops will significantly reduce this risk. The same can be said for many diseases that mainly affect specific groups of plants. Similarly, pests that overwinter in the ground, such as chafer grubs and wire worms, will attack the next available crop they are presented with especially root crops. By planting something else in the plot you will deprive them of their favourite food and their numbers will eventually decline.

The chart below illustrates an example of a typical 4-year crop rotation plan:

Plot A - Peas, Beans- plus Lettuce, Celery, Spinach and Tomatoes.

Plot B - Brassicas- Cabbage, Cauliflower, Sprouts, Broccoli etc.

Plot C - Root crops - Swedes, Beetroot, Carrots and Parsnips plus Onions & Leeks

Plot D - Potatoes

Each year each plot moves round one so no crop is repeated in the same place for another three or four years.

There are many variations. For example, Potatoes may not be grown at all and this plot could be used for other crops like Courgettes or Sweet Corn. Climbing Beans are often grown on the same plot each year, and in their particular case this may be an advantage unless disease shows up. There are also permanent crops like Asparagus and Rhubarb which of course do not come into this rotation at all.

Summary

Soil that has been well prepared in advance with the incorporation of plenty of organic matter will drain easier, retain moisture longer and is more likely to produce bigger and healthier plants. A healthy plant is better able to fend off attacks by pests and disease. By rotating crops over a 3 or 4 year cycle, the likelihood of total nutrient depletion and the spread of disease will be significantly reduced.

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This article was previously published on SimplySeed in 2010. Revised and updated in 2020.

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