A guide to crop rotation and why it’s important
You grew tomatoes successfully in that sunny corner of your garden last year, so why shouldn’t you plant this year’s seedlings in the same spot? It’s tempting to follow the identical garden plan that worked well last growing season, but the advice from gardening experts is to rotate locations for annual edibles. There are research-based reasons for following their suggestions.
Crop rotation is an ancient practice, dating back to pre-Roman times, to preserve soil fertility. The main strategy in crop rotation is to alternate crops with different characteristics. Plants differ in nutrient needs. Because vegetable crops from the same botanical family tend to use the same types and amounts of soil nutrients, repeated planting in the same location depletes the soil of elements essential for growth. Some plants, for example, are heavy users of nitrogen. Alternating these plants with plants that fix nitrogen in the soil can help to even out soil nitrogen levels.
Rotating annual crops can also reduce issues with diseases and pests. Plants in the same botanical family are susceptible to similar diseases and insects. Because many disease organisms can remain in the soil for several years, avoiding successive planting of the same crop in a garden area decreases the opportunity for soil-borne diseases to persist. Likewise, rotating crops deters insect pests from returning to their favorite host plants, thus disrupting their life cycle. Any debris or volunteers left behind by the previously grown crop can also harbor disease, so good cleanup before planting is important.
Major benefits of crop rotation include reducing the need for pesticides and fertilizer. Studies have also demonstrated an increase in crop yields with diversified rotational planting. Crop rotation can also enable the soil to retain more organic matter and store or “sequester” carbon when combined with low-till or no-till practices.
A crop rotation plan requires dividing crops into groups for planting and rotating together. There are different ways to do this. One way is to split crops into categories based on the part harvested. Leafy crops, for example, would be planted together and moved to a new location each season, followed by fruit crops, root crops and legumes. Another method is to group crops as heavy or light feeders corresponding to their nutrient needs. Moving heavy feeders, such as tomatoes, corn and squash, to beds previously occupied by light feeders like carrots, spinach and beets can make better use of soil nutrients. Legumes (beans and peas) help retain nitrogen in the soil and fit into this rotation scheme as nutrient contributors.
The most common grouping for crop rotation is according to the botanical plant family. Tomatoes, eggplant, peppers and potatoes are members of the Solanaceae family. Solanaceous crops share the same pests and are susceptible to some serious soil-borne viruses. They should not be planted in the same place more than once every three to four years. Multiple plant families can be planted and moved together in the same bed. For example, plants in the cabbage family (Brassicaceae) can share space with onions, leeks and garlic (Amaryllidaceae) and carrot family plants (Apiaceae) can rotate with beets, spinach and other members of the Amaranthaceae family.
Crop rotation requires advanced planning. Start with a list of annual edibles you want to grow and divide them according to one of the groups previously described. Create a chart or map, sectioning your garden space into quadrants or separate rows. Indicate where each group will be planted in year one and where they will move in years two, three and four. The ideal rotation plan would involve a four-year cycle. This can work well in a garden large enough to be divided into quadrants or separate rows. When space is restricted, the number of plant groups may need to be reduced, or the cycle decreased. Good soil preparation to replenish nutrients is particularly important when cycles are shorter. Containers can be added to the available planting space and included in the rotation. At the very least, home gardeners with limited space should avoid planting the same crop or crops from the same family year after year in the same part of the garden. Those tomatoes need to move around!
Sponsored by UC Cooperative Extension, the University of California Marin Master Gardeners provides science- and research-based information for Marin home gardeners. Email questions to helpdesk@marinmg.org. Attach photos for inquiries about plant pests or diseases. Please call 415-473-4910 to see when a master gardener will be at the office or drop off samples 24/7 in the sample box outside the office. To attend a gardening workshop or subscribe to Leaflet, a free quarterly e-newsletter, go to marinmg.ucanr.edu.