Crop rotation, in its many aspects, is about growing your own organic fertilizer and building great soil at the same time. A complete garden plan will include crop rotation for more than one growing season (or more than one year) in each area of the garden. Crop rotation could be considered to be a type of companion planting over time, based on which plants are related – or not – according to their plant family (see GFT, July 2010). Plants of different families should follow each other season-to-season in the same location. For example, instead of following a crop of spinach with more spinach or chard, plant something like zucchini or cucumbers. In this way, plants of different families grown in succession support nutrient balance in the soil and help to eliminate disease and weed problems as well.
A cycle of a green manure crop or cover crop, one that will not be harvested, but turned in or left until the following season to decompose, is an aspect of crop rotation that is tempting to overlook when there is limited garden space. But planning in a crop rotation is well worth the effort and space. Green manures (plowed in, mowed or left on the soil) and cover crops (planted to prevent erosion and left on the soil) allow for buildup of organic matter, as well as the return of nutrients. They add sugar and protein to the soil, while compost is adding humus, lignins and other compounds that take longer for soil microorganisms to process.
Crop rotation is a very effective way to prevent recurring problems in the garden that are disease-, pathogen- and insect-related. Planting green manures and cover crops breaks the problematic cycles that build up in the garden beds after 2, 3 or 4 years of repeated growing of the same plant or family-related plants. These crops will also disrupt the cycle of weeds. An ideal rest of any given area under green manure or cover crop would be 2 – 4 years to most effectively break the pathogen and disease cycles that reduce the ability of crops of the same family to grow optimally in a given site.
The selection of green manures or cover crops may depend on the season in which you want to plant and whether you wish to plant an annual or a perennial crop. Alfalfa and white clover are perennial legumes, red clover is biennial, and all are good at giving back nutrients, especially nitrogen, to the soil. Hairy vetch, another popular green manure is an annual. Hairy vetch is planted in fall and comes back in the spring, when it is mowed and tilled in. One growing season, a dormant season and another growing season are required for a legume to give the maximum benefit of nitrogen return to the soil, corresponding to the cycle of the nitrogen-fixing microorganisms associated with the roots. Other winter annuals that are planted in the fall are winter rye, barley and wheat. Cool season annuals include oats, rye and subterranean clover. Warm season annuals include buckwheat, cow peas, fava beans, sweet clover and berseem clover. Buckwheat will flower vigorously and produce seeds profusely, so you may want to cut it when no more than 25% of the plants are flowering. One unfriendly companion cover crop relationship I am aware of is that sweet potatoes should not be followed with a legume; pick something else for your green manure.
Besides family relationships, the level of nutrients a particular crop requires is an important consideration in crop rotation. Veggies are categorized as being low-, medium- or high-demand. Steve Soloman, in his book Gardening When it Counts, offers a good discussion on crop demands. Some popular low-demand crops include arugula, beans, beets, carrots, herbs, kale, parsnip, sorrel and chard. Crops that are medium-demand include artichokes, basil, fall cabbage, eggplant, garlic, parley, potatoes (these do use lots of nitrogen), radish, fall spinach, turnips and watermelons. Asparagus, corn, lettuce, melons, cauliflower, leeks, onions, peppers, spring crops of spinach, squash, and tomatoes are high-demand. Ideally, plan to follow a high-demand crop with a low-demand crop or a green manure that gives back nitrogen. Turnips, sweet potatoes and green peppers are all light on nitrogen. You can mix heavy feeders and light feeders in the same space. And, of course low- and medium-demand crops will love having a soil more fertile than the bare minimum for their basic requirements.
Clearly, the benefits of crop rotation are ultimately derived from a combination of factors that work together best in a given soil condition – particularly if the soil has earned a reputation for being troublesome! In many areas of New Mexico, where soils are hard and nutrient-poor, you still may want to consider an organic fertilizer to truly balance the nutrients in your garden soil. In any case, dedicating a couple of patches in your garden for crop rotation not only is a great insurance towards building balanced soil for your harvest; it also helps to ensure that the crops you harvest and eat have balanced nutrients for you, as well!
Susan Waterman has a Ph.D. in botany and over 25 years of experience in sustainable agriculture. For more info, visit www.harvestbyhand.com. Questions? E-mail email@example.com