The Importance of Crop Rotation

Why Rotate your Crops?

The primary reason gardeners and farmers rotate their crops is to balance nutrient depletion in the soil. Different crops have unique nutrient requirements, so growing the same crops in the same place year after year will eventually deplete the nutrients unevenly and require more and more expensive fertilizer inputs. Another important benefit of rotating crops is disrupting pests and diseases. Many pests and plant diseases overwinter in the soil and in plant matter just beneath their typical host plants. When the growing season comes back around, they emerge and begin seeking out a suitable host. If you have an unsuitable crop growing where they emerged it reduces the chances of them finding their target crop in your garden.

Rotating by Plant Family

As mentioned above, most crops use a unique proportion of different nutrients in the soil which means that planting them in the same place for many years can deplete the nutrients that they use most heavily. Often, plants in the same plant family have similar nutritional habits. The same is true for pests and diseases which can target multiple species in the same plant family. For both of these reasons, most gardeners and farmers base their crop rotation around families of plants. Here are some of the most common plant families used in North American gardens and the crops they contain:

Solanaceae: Tomatoes, Peppers, Potatoes, Tomatillos, Eggplant Fabaceae: Beans, Peas, Peanuts Cucurbitaceae: Cucumbers, Squash, Pumpkin, Melons Brassicaceae: Cabbage, Kale, Broccoli, Collards, Radishes, Turnips, Mustard Greens, Arugula Poaceae: Corn, Wheat, Rice, Oats Apiaceae: Carrots, Parsnips, Parsley Amaranthaceae: Beets, Swiss Chard, Spinach, Quinoa Amaryllidaceae: Onions, Garlic, Leeks

To illustrate how a crop rotation can reduce pest pressure on your garden, I’ll use cabbage pests as an example. Let’s say you grew cabbage in a particular garden bed one year and then grew kale in that same bed the next year. The cabbage pests and diseases that overwintered in the soil would emerge in the spring to find kale, which is in the same family as cabbage, making it a suitable host for their larvae. Instead, if you planted peppers—which are in a different plant family—the lifecycle of those pests and diseases would be interrupted, and they would either die off or leave your garden.

As for crop rotation balancing nutrient depletion, let’s use the example of corn and beans. Corn is known as a heavy nitrogen feeder. It uses tons of available nitrogen in the soil to fuel its leafy growth and produce nice big ears. If you grow corn in the same plot year after year without amending the soil, it will eventually deplete the nitrogen leaving you with anemic, starving plants and tiny ears of corn.

On the other hand, if you rotate your corn patch with a legume like beans and peas, you can reduce the depletion of nitrogen. Legumes actually produce nitrogen in their roots through a relationship with nitrogen-fixing bacteria, so growing them and leaving the spent plants to compost in your garden beds actually replaces some of the available nitrogen. This is why large-scale commercial corn farms will often rotate soybeans every few years where they typically grow corn.

When to Rotate?

Some growers, especially small farmers, recommend rotating crops annually. For instance, a field in which carrots were grown one year will not contain carrots again for 3 or 4 years until the rotation comes back around to that field.

In other cases, crop rotation is only used when pest and disease problems build up in one field in order to interrupt their lifecycle. To keep track of what was planted where in previous years, it is helpful to draw a map of your garden and note which crops were grown in what bed each year. How often you rotate your crops is your personal preference, but if you find that your garden is plagued by pests and disease year after year, you likely need to implement a more frequent and strict crop rotation.

How Does this Jive with Companion Planting?

With all this talk of rotating by plant family and avoiding growing the same crops year after year, you might be wondering how exactly this works with companion plantings like a Three Sisters Garden, which includes plants from three different families. Because the purpose of many companion plantings is to reduce the impact on the soil and confuse pests, it might feel like overkill to also rotate them, but the compounding effect of growing a polyculture and rotating your crops only further confuses pests and preserves your soil that much more.

Don’t get me wrong—crop rotation is not a magic bullet. You’ll still need to fertilize your garden and manage pests and diseases, but as illustrated above, crop rotation reduces the severity of the pests in your garden and decreases the amount of fertilizer needed for healthy, productive crops.