Crop rotation is the method of planting different kinds of crops in the same area over several growing seasons. This method helps crops use different nutrients, lowers the chance of pests and weeds becoming too common, and reduces the risk of pests and weeds becoming resistant to control methods.

Planting the same crop in the same place for many years, called monocropping, can cause the soil to lose certain nutrients and allow pests and weeds that specifically target that crop to grow more easily. Without using a variety of crops to balance nutrients and reduce pest and weed problems, monocropping depends heavily on outside resources, such as fertilizers and chemicals, which can harm soil health. On the other hand, a carefully planned crop rotation can decrease the need for chemical fertilizers and weedkillers by using the natural benefits of growing different crops. Crop rotation also helps improve soil structure and increase the amount of organic material in the soil, which reduces erosion and makes farming systems stronger.

History

Farmers have always known that using good crop rotation, like planting spring crops for animals instead of grains for people, helps keep soil healthy. Farmers in the Ancient Near East used crop rotation around 6000 BC by alternating legumes and cereals.

In the two-field system, half the land was used for planting each year, while the other half remained unused (fallow). The next year, the fields were switched. In China, both two-field and three-field systems were used during the Eastern Zhou period.

Between the 9th and 11th centuries, European farmers changed from a two-field system to a three-field system. This method lasted until the 20th century. Land was divided into three parts. One part grew autumn crops like rye and wheat. Another part grew spring crops such as oats, barley, and legumes (peas, lentils, or beans). The third part stayed fallow. Every three years, each field rested once. In the two-field system, only half the land was planted each year. With the three-field system, two-thirds of the land was planted, possibly leading to bigger harvests. The extra crops had more benefits than just more food. Spring crops, mainly legumes, add nitrogen to the soil, which helps plants make proteins, improving the nutrition of Europeans.

In the 18th century, British agriculturist Charles Townshend promoted the three-field system

Crop choice

A basic look at how crops affect each other can be seen in the following ways:

• Helps add organic matter to the soil.
• Helps control pests.
• Helps manage soil nutrients that are too much or too little.
• Helps prevent or reduce soil erosion.
• Mixes with other crops to create hybrid plants.
• Affects nearby plants and animals in the environment.

Farmers choose crops based on their goals for the rotation, such as controlling weeds, adding nitrogen to the soil, reducing erosion, or improving soil structure and plant growth. When discussing crop rotations, crops are grouped in different ways depending on what is being studied: by plant family, by their need for or benefit to nutrients, and by their value (such as cash crops or cover crops). For example, paying attention to plant family helps reduce pests and diseases. However, many farmers manage rotations by planning the order of crops and using cover crops around their main cash crops. The following is a simple way to group crops based on their purpose and quality.

Some crops that are important for selling, like vegetables, are row crops (grown in straight rows). These crops are often the most profitable for farmers, but they can harm the soil more. Row crops usually have little plant material and shallow roots, meaning they leave little organic matter in the soil and have limited effects on soil structure. Because much of the soil around these plants is exposed to rain and farming activities, fields with row crops lose organic matter quickly, leaving fewer nutrients for future plants.

In short, while these crops may be profitable, they take away nutrients from the soil. Crop rotation helps balance the need for short-term profit with long-term soil health.

A major benefit of crop rotation comes from pairing crops that add nitrogen with those that need nitrogen. Plants like legumes (alfalfa and clover) take nitrogen from the air and store it in their roots. When these plants are harvested, the leftover roots break down, releasing nitrogen for future crops.

Cereals and grasses are often used as cover crops because they improve soil quality and structure. Their thick, widespread roots help hold the soil together and add a lot of organic matter.

Grasses and cereals also help control weeds by competing with them for space and nutrients.

Green manure is a type of crop that is mixed into the soil. Both legumes that add nitrogen and plants that take up nutrients, like grasses, can be used as green manure. Legumes are a good source of nitrogen, especially for organic farming, but their plant material does not add as much long-lasting organic matter to the soil as grasses do.

Planning a rotation

When planning a crop rotation, many factors must be considered. These include things that do not change, like farm size and soil type, and things that can change, like market prices, labor availability, climate, and farming methods. A successful crop rotation also depends on how one crop affects the soil for the next crop and how crops are planted together. For example, a crop that adds nitrogen to the soil, like legumes, should be planted before a crop that uses up nitrogen. Similarly, a crop that leaves little plant material should be followed by a cover crop that grows a lot of plant material, such as a mix of grasses and legumes.

There is no set number of crops that can be included in a rotation, nor is there a set time needed to complete it. Farmers may plan rotations years or seasons in advance, or they may make changes quickly if an opportunity arises to improve profits or soil health.

Implementation

Crop rotation systems can be improved by adding practices such as raising livestock and using manure, and by growing more than one crop in the same field at the same time. A monoculture is when only one type of crop is grown in a field. A polyculture is when two or more crops are grown together in the same area at the same time. Crop rotation can be used with both monocultures and polycultures, which helps increase the variety of plants in farming (table).

Adding livestock helps use grasses and cover crops more efficiently. Livestock, through manure, spread the nutrients from these plants across the soil instead of taking them away when hay is sold.

Mixed farming, which includes growing crops and raising livestock together, helps manage crop rotation and move nutrients through the soil. Crop leftovers can be used as animal food, and the animals provide manure to help crops grow and offer power for farming tasks. These methods support natural nutrient movement and reduce the need for chemical fertilizers and large machines. Also, animals like cattle, sheep, or goats can provide milk and serve as a source of income during tough times.

Polyculture systems, such as intercropping or companion planting, add more variety and complexity to farming in the same season or rotation. For example, the "Three Sisters" method grows corn, pole beans, and vining squash or pumpkins together. In this system, beans add nitrogen to the soil, corn supports the beans and protects them from pests, and squash covers the ground to stop weeds and keep raccoons away from corn.

Double-cropping is common in areas where two different crops are grown one after the other in the same growing season, or where a main crop, like vegetables, is grown alongside a cover crop, like wheat. This is helpful for small farms that cannot afford to let cover crops grow for long periods, unlike larger farms. Using multiple crops on small farms helps make the most of limited land through crop rotation.

In the United States, crop rotation is required for farms that want to be certified as organic. The "Crop Rotation Practice Standard" in the National Organic Program, under the U.S. Code of Federal Regulations, section §205.205, states that

Crop rotation helps reduce the need for outside resources by controlling pests and weeds and increasing soil nutrients. It also helps organic farmers increase the variety of plants on their farms. Biodiversity is required for organic certification, but there are no specific rules to enforce this standard. Increasing plant diversity can improve the surrounding environment and support more wildlife, insects, and helpful soil microbes, as shown in studies by McDaniel et al. (2014) and Lori et al. (2017). Some research shows that crop rotation in organic farming can make more nutrients available than in traditional farming because organic methods are better for keeping helpful soil microbes alive.

While multiple cropping and intercropping share some benefits with crop rotation, they do not meet the requirements set by the National Organic Program.

Benefits

Agronomists call the increase in crop yield from using crop rotation "The Rotation Effect." This happens because rotating crops helps reduce problems caused by growing the same crop repeatedly. Benefits include better soil nutrition, less harm from pests and weeds, and improved soil structure. These improvements have been linked to better crop growth in some studies.

Other advantages of crop rotation include lower farming costs. When farmers grow a variety of crops or raise livestock, they spread financial risks more evenly. They also need fewer purchased supplies over time, and crops can still meet production goals with less input. Combined with higher short- and long-term yields, crop rotation is a strong method for improving farming systems.

Using different plant types in rotation increases soil organic matter (SOM), which improves soil structure and the chemical and biological environment for crops. More SOM helps soil absorb and hold water better, making crops more drought-resistant and reducing erosion.

Soil organic matter is made from decaying plant and animal material mixed with active microorganisms. Crop rotation increases the amount of plant material added to the soil, such as from sod, green manure, and plant debris. Less need for deep tilling in crop rotation allows plant material to build up, helping the soil hold nutrients better and reducing the need for added fertilizers. Tilling harms microorganisms that help plants get nutrients. Soils with low microbial activity provide fewer nutrients to plants, even if the same amount of plant material is present.

Soil microorganisms also reduce pests by competing with them. Plants release chemicals from their roots that affect both their soil environment and nearby weeds. This means crop rotation helps crops grow better by increasing nutrient availability and reducing competition from weeds.

Crop rotations increase soil organic carbon (SOC), the main part of soil organic matter. Carbon, along with hydrogen and oxygen, is a key nutrient for plants. Rotating many different crops over long periods increases SOC more than other methods. However, soil disturbances like tilling cause SOC levels to drop quickly. In Brazil, using no-till methods with crop rotations has increased SOC by 0.41 tonnes per hectare each year.

Crop rotation also helps remove carbon dioxide from the air, which can slow climate change.

Rotations add nutrients to the soil. Legumes, such as beans and peas, have root nodules with bacteria called rhizobia. These bacteria convert air nitrogen into ammonia, which plants can use. Rotating legumes with crops like corn or wheat, which need nitrogen, helps improve soil fertility. How much nitrogen is available depends on the type of legume, the bacteria’s effectiveness, soil conditions, and other factors.

Crop rotation helps control pests and diseases. Changing crops breaks pest life cycles and limits their habitat. Plants in the same family often share pests and diseases. Rotating crops and using cover crops instead of leaving soil bare reduces pest problems, especially those that survive in crop residue. This method is used in organic farming, where pesticides are not allowed.

Including cover crops in rotations helps control weeds. These crops compete with weeds for space and resources. Cover crops also slow weed growth, giving main crops an advantage. This reduces weeds for future crops, like shallow-rooted plants, which are more vulnerable to weeds. Cover crops are called conservation crops because they protect land from becoming overgrown with weeds.

Crop rotation is better than tilling for weed control. Tilling turns over soil to stop weeds, but it can expose buried weed seeds and bury crop seeds. Rotating crops reduces the number of weed seeds in the soil.

Weeds lower crop quality and yield. They also slow harvesting because weeds like bindweed and knotgrass can tangle with farm equipment, causing delays.

Crop rotation reduces soil loss from water erosion. In areas prone to erosion, combining crop rotation with no-till or reduced-till methods helps. These methods reduce rain impact, soil movement, and water runoff.

Soil erosion is best controlled with rotations that leave the most crop residue on the soil. This residue reduces water speed and power, preventing soil from being washed away. Soil erosion and sealing can block water flow, reduce infiltration, and increase runoff. This makes soil more resilient during erosion.

When forage crops break down, they create sticky substances that help soil particles stick together, forming aggregates. These aggregates resist water erosion and reduce wind erosion because they are larger and harder to break apart.

Crop rotation’s effect on erosion depends on the climate. In areas with stable weather, strict crop rotations can provide enough plant cover. In areas with unpredictable weather, more flexible rotations are better. An opportunity cropping system plants crops when soil moisture is good and planting is possible. This method likely provides better soil cover than strict rotations because crops are planted only when conditions are right.

Challenges

Crop rotation needs careful planning, but the choice of crops must consider many factors. These include soil type, land shape, climate, and water availability, which do not change. Other factors, like weather, market prices, and labor supply, can change from year to year. Because of this, it is not a good idea to plan crops many years ahead. If a crop rotation plan is not done correctly, it can cause problems in the soil, such as an imbalance in nutrients or the growth of harmful diseases that harm important crops. These problems may take many years to notice, even for experts, and may take just as long to fix.

There are many challenges in using crop rotation. For example, using plants like legumes to improve soil health can sometimes lead to an increase in snails or slugs. Also, the breakdown of these plants can sometimes stop other crops from growing well.