Crop rotation involves planning where to grow a crop in a given production area when transitioning from one harvest cycle to the next. There are three key reasons that growers rotate crops in their fields. First, it balances soil fertility by maintaining or improving the soil organic matter content. Second, it helps to reduce diseases and pests. Lastly, it controls soil erosion.
Growers usually know that continuous planting of crops from the same families on the same spot results in a buildup of plant pathogens (disease-causing agents). To reverse this situation and avoid pathogen buildup, wait at least three to five years before planting crops of the same family in the same location. When a non-host crop is planted, the pathogen germinates; however, it will not be able to penetrate, infect and reproduce. When denied its susceptible host, such an inoculum (fungal or bacterial spore) gradually dies in the soil. Over time, the inoculum levels are greatly reduced.
Crop rotation can be an effective disease management tool, especially if the pathogen overwinters in crop residue or soil. It works less well for airborne foliar (leaf) diseases, such as powdery mildew or rusts. Also, this method works well when the pathogen causing the disease has a narrow host range, affecting only one or few plant families. The rotation plan will work even better if combined with very good sanitation. This includes proper removal of diseased plant residue and any alternate hosts of a disease (e.g., weeds). When planning alternating crops on a piece of land, remove any plant residue from the previous crop. Use of cover crops, including growing some brassica (of the family including broccoli, cabbage, radish, etc.), will also minimize soil borne diseases for the next season.
An effective crop rotation plan is often based on one of two options: botanical family or the plant parts that are eaten. Plants in the same family usually are prone to similar diseases and pests. As a general rule, crops in the same family should not follow one another in rotation. Rather than using botanical family, crops can be grouped based in their edible parts, such as fruit and seed, leaf and stem or root and bulb. This plan is a simple and easy way to avoid any overlap of consecutive plants from the same family.
Here are some important aspects to consider for an effective rotation plan:
1. Available space: The area that can be allotted to each crop depends on the available space and on the nature of the crop.
2. Growing season: Cool-season crops need cool soil and air temperatures; they are grown in the spring or fall. They also tend to be shallow-rooted and susceptible to drought. They are usually cultivated for their leaves or roots. Warm-season crops need warm soil and air temperatures to germinate, grow and mature. They are deep-rooted and resistant to drought. These crops are usually raised for their seed or fruit. Alternating cool- and warm-season crops allows a producer to include a cover crop or grow multiple crops during the same growing season. For example, a cool-season pea could be followed by a buckwheat cover crop in summer. This could be followed by a fall planting of onions or radishes.
3. Nutrient demands: Having two nutrient-depleting (heavy feeding) crops follow one another robs the soil. It can result in poor yields. Heavy feeding crops should be sequenced with light feeders or a soil-building cover crop such as hairy vetch. Legumes are good to rotate after heavy feeding crops, such as sweet corn or tomatoes. Another example is growing the grass family (sweet corn), followed by the pea family (bean or pea), then the mustard family (cabbage, broccoli, mustard, radish) and finally the nightshade family (tomato, pepper, potato).
4. Rotation length: Flexibility can be built into longer rotations. For example, rotation periods of several years might be needed to suppress soil borne pathogens. Longer rotations also allow for perennial crops, such as grass or legume hay, to be added. This results in healthier soil; it builds organic matter and improves soil aggregation (allows soil particles to bind together to form granular structures).
5. Rooting depth: Take advantage of the variable rooting depth of plants in a crop rotation. Follow deep-rooted plants with shallow-rooted plants to allow for more complete use of the nutrients within the entire soil profile. For example, follow spinach (shallow-rooted) with potatoes (deep-rooted) or a rye cover crop. The table below gives the rooting depth of a number of vegetables.
(By Dr. Zelalem Mersha, Lincoln University, State Extension Specialist - Plant Pathology)