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Crop rotation is the practice of growing a series of dissimilar/different types of crops in the same area in sequential seasons.
Crop rotation gives various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals and other crops. Crop rotation also mitigates the build-up of pathogens and pests that often occurs when one species is continuously cropped, and can also improve soil structure and fertility by alternating deep-rooted and shallow-rooted plants.
Crop rotation is one component of polyculture.
Historic crop rotation methods are mentioned in Roman literature, and referred to by several civilizations in Asia and on three major elements: sophisticated systems of crop rotation, highly developed irrigation techniques and the introduction of a large variety of crops which were studied and catalogued according to the season, type of land and amount of water they require. Numerous farming encyclopedias were produced.
In Europe, since the times of Charlemagne, there was a transition from a two-field crop rotation to a three-field crop rotation. Under a two-field rotation, half the land was planted in a year while the other half lay fallow. Then, in the next year, the two fields were reversed. Under three-field rotation, the land was divided into three parts. One section was planted in the autumn with winter wheat or rye. The next spring, the second field was planted with other crops such as peas, lentils, or beans and the third field was left fallow. The three fields were rotated in this manner so that every three years, a field would rest and be fallow. Under the two field system, if one has a total of 600 acres (2.4 km2) of fertile land, one would only plant 300 acres. Under the new three-field rotation system, one would plant (and therefore harvest) 400 acres. But, the additional crops had a more significant effect than mere productivity. Since the spring crops were mostly legumes, they increased the overall nutrition of the people of Northern Europe.
From the end of the Middle Ages until the 20th century, the three-year rotation was practised by farmers in Europe with a rotation of rye or winter wheat, followed by spring oats or barley, then letting the soil rest (leaving it fallow) during the third stage. That suitable rotations make it possible to restore or to maintain a productive soil has long been recognized by planting spring crops for livestock in place of grains for human consumption.
A four-field rotation was pioneered by farmers, namely in the region Waasland in the early 16th century and popularised by the British agriculturist Charles Townshend in the 18th century. The system (wheat, turnips, barley and clover), opened up a fodder crop and grazing crop allowing livestock to be bred year-round. The four-field crop rotation was a key development in the British Agricultural Revolution.
In the Green Revolution, the traditional practice of crop rotation gave way in some parts of the world to the practice of supplementing the chemical inputs to the soil through top dressing with fertilizers, e.g. adding ammonium nitrate or urea and restoring soil pH with lime in the search for increased yields, preparing soil for specialist crops, and seeking to reduce waste and inefficiency by simplifying planting and harvesting.
Growing the same crop in the same place for many years in a row disproportionately depletes the soil of certain nutrients. With rotation, a crop that leaches the soil of one kind of nutrient is followed during the next growing season by a dissimilar crop that returns that nutrient to the soil or draws a different ratio of nutrients: for example, rice followed by cotton.
Choice of crops 
The choice and sequence of rotation crops depends on the nature of the soil, the climate, and precipitation which together determine the type of plants that may be cultivated. Other important aspects of farming such as crop marketing and economic variables must also be considered when deciding crop rotations.
Crop rotations may include two to six or more crop rotations over numerous seasons. A two crop rotation such as corn and soybean in cash grains or corn and alfalfa in forage systems use legumes to help fix nitrogen in the soil for utilization over the long term. Multiple cropping systems, such as intercropping or companion planting, offer more diversity and complexity within the same season or rotation. Carrots can be shaded by tomatoes and loosen soil below them. Double cropping is common where two crops, typically of different species, are grown sequentially in the same growing season. Winter rye and barley can be sown after oats or rice and harvested before the next crop goes in of oats or rice. These systems can maximize benefits of the rotation as well as available land resources.
More complex rotations commonly utilize people for greater use of on-farm nutrient management and additional farm products. A soil-feeding crop of clover could be replaced or aided by an application of manure to set up a field for a double crop of winter grains after potatoes. Soil building and pest population management benefits can be further utilized with different complexities of crop rotation. In general the complexity of a field’s rotation is limited by what soil, climate, and other environmental conditions permit. This also includes the current or desired management tools and goals of the farmer.
Incorporation of animals 
In Sub-Saharan Africa, as animal husbandry becomes less of a nomadic practice many herders have begun integrating crop production into their practice. This is known as mixed farming, or the practice of crop cultivation with the incorporation of raising cattle, sheep and/or goats by the same economic entity, is increasingly common. This interaction between the animal, the land and the crops are being done on a small scale all across this region. Crop residues provide animal feed, while the animals provide manure for replenishing crop nutrients and draft power. Both processes are extremely important in this region of the world as it is expensive and logistically unfeasible to transport in synthetic fertilizers and large-scale machinery. As an additional benefit, the cattle, sheep and/or goat provide milk and can act as a cash crop in the times of economic hardship.
Using some forms of crop rotation farmers can keep their fields under continuous production, instead of letting them lie fallow, as well as reducing the need for artificial fertilizers, both of which can be expensive.
A general effect of crop rotation is that there is a geographic mixing of crops, which can slow the spread of pests and diseases during the growing season. The different crops can also reduce the effects of adverse weather for the individual farmer and, by requiring planting and harvest at different times, allow more land to be farmed with the same amount of machinery and labour.
Agronomists describe the benefits to yield in rotated crops as "The Rotation Effect". There are many found benefits of rotation systems: however, there is no specific scientific basis for the sometimes 10-25% yield increase in a crop grown in rotation versus monoculture. The factors related to the increase are simply described as alleviation of the negative factors of monoculture cropping systems. Explanations due to improved nutrition; pest, pathogen, and weed stress reduction; and improved soil structure have been found in some cases to be correlated, but causation has not been determined for the majority of cropping systems.
Other benefits of rotation cropping systems include production costs advantages. Overall financial risks are more widely distributed over more diverse production of crops and/or livestock. Less reliance is placed on purchased inputs and overtime crops can maintain production goals with fewer inputs. This in tandem with greater short and long term yields makes rotation a powerful tool for improving agricultural systems.
Rotating crops adds nutrients to the soil. Legumes, plants of the family Fabaceae, for instance, have nodules on their roots which contain nitrogen-fixing bacteria. It therefore makes good sense agriculturally to alternate them with cereals (family Poaceae) and other plants that require nitrates. An extremely common modern crop rotation is alternating soybeans and maize (corn). In subsistence farming, it also makes good nutritional sense to grow beans and grain at the same time in different fields.
Pest control 
Crop rotation is also used to control pests and diseases that can become established in the soil over time. The changing of crops in a sequence tends to decrease the population level of pests. Plants within the same taxonomic family tend to have similar pests and pathogens. By regularly changing the planting location, the pest cycles can be broken or limited. For example, root-knot nematode is a serious problem for some plants in warm climates and sandy soils, where it slowly builds up to high levels in the soil, and can severely damage plant productivity by cutting off circulation from the plant roots. Growing a crop that is not a host for root-knot nematode for one season greatly reduces the level of the nematode in the soil, thus making it possible to grow a susceptible crop the following season without needing soil fumigation.
It is also difficult to control weeds similar to the crop which may contaminate the final produce. For instance, ergot in weed grasses is difficult to separate from harvested grain. A different crop allows the weeds to be eliminated, breaking the ergot cycle.
Soil erosion 
Crop rotation can greatly affect the amount of soil lost from erosion by water. In areas that are highly susceptible to erosion, farm management practices such as zero and reduced tillage can be supplemented with specific crop rotation methods to reduce raindrop impact, sediment detachment, sediment transport, surface runoff, and soil loss.
Protection against soil loss is maximized with rotation methods that leave the greatest mass of crop stubble (plant residue left after harvest) on top of the soil. Stubble cover in contact with the soil minimizes erosion from water by reducing overland flow velocity, stream power, and thus the ability of the water to detach and transport sediment. Soil Erosion and Cill prevent the disruption and detachment of soil aggregates that cause macrospores to block, infiltration to decline, and runoff to increase. This significantly improves the resilience of soils when subjected to periods of erosion and stress.
The effect of crop rotation on erosion control varies by climate. In regions under relatively consistent climate conditions, where annual rainfall and temperature levels are assumed, rigid crop rotations can produce sufficient plant growth and soil cover. In regions where climate conditions are less predictable, and unexpected periods of rain and drought may occur, a more flexible approach for soil cover by crop rotation is necessary. An opportunity cropping system promotes adequate soil cover under these erratic climate conditions. In an opportunity cropping system, crops are grown when soil water is adequate and there is a reliable sowing window. This form of cropping system is likely to produce better soil cover than a rigid crop rotation because crops are only sown under optimal conditions, whereas rigid systems are not necessarily sown in the best conditions available.
Crop rotations also affect the timing and length of when a field is subject to fallow. This is very important because depending on a particular region's climate, a field could be the most vulnerable to erosion when it is under fallow. Efficient fallow management is an essential part of reducing erosion in a crop rotation system. Zero tillage is a fundamental management practice that promotes crop stubble retention under longer unplanned fallows when crops cannot be planted. Such management practices that succeed in retaining suitable soil cover in areas under fallow will ultimately reduce soil loss.
Additional soil improvements 
The use of different species in rotation allows for increased soil organic matter (SOM), greater soil structure, and improvement of the chemical and biological soil environment for crops. With more SOM, water infiltration and retention improves, providing increased drought tolerance and decreased erosion. Soil aggregation allows greater nutrient retention and utilization, decreasing the need for added nutrients. Soil microorganisms also improve nutrient availability and decrease pathogen and pest activity through competition. In addition, plants produce root exudates and other chemicals which manipulate their soil environment as well as their weed environment. Thus rotation allows increased yields from nutrient availability but also alleviation of allelopathy and competitive weed environments.
Balancing the commitment to new crops or livestock with increased yield potentials and long term sustainability is the task of many farmers and agricultural scientists. With this research many new rotations have been developed and become widely accepted.
Risks of crop rotation include less overall profitability due to decreased acreage of the most valuable crop. Greater investment and lower relative efficiency in machinery used for different crops is also a possible outcome. More complex rotations require more crop species and livestock. This means the farmer must have additional skills and make more time and equipment investments initially. Also the more complex the system, the less flexible it becomes in terms of long term land management. Starting a rotation of a new crop may add profitability and farm resilience over time, but benefits are initially subject to being over-shadowed by volatile markets or high startup investments which can take time to overcome. Overall many farmers and agronomists agree finding a suitable rotation can benefit the overall productivity and sustainability of the farm.
See also 
- J.J Butt,Daily life in the age of Charlemagne, p.82-83
- Powell, J.M., William, T.O., (1993). "An overview of mixed farming systems in sub-Saharan Africa". Livestock and Sustainable Nutrient Cycling in Mixed Farming Systems of Sub-Saharan Africa: Proceedings of an International Conference, International Livestock Centre for Africa (ILCA) 2: 21–36.
- Unger PW, McCalla TM (1980). "Conservation Tillage Systems". Advances in Agronomy 33: 2–53.
- Rose CW, Freebairn DM. "A mathematical model of soil erosion and deposition processes with application to field data".
- Loch RJ, Foley JL (1994). "Measurement of Aggregate Breakdown under rain: comparison with tests of water stability and relationships with field measurements of infiltration". Australian Journal of Soil Research 32: 701–720.
- Carroll C, Halpin M, Burger P, Bell K, Sallaway MM, Yule DF (1997). "The effect of crop type, crop rotation, and tillage practice on runoff and soil loss on a Vertisol in central Queensland". Australian Journal of Soil Research 35: 925–939.
- Littleboy M, Silburn DM, Freebairn DM, Woodruff DR, Hammer GL (1989). "PERFECT. A computer simulation model of Productive Erosion Runoff Functions to Evaluate Conservation Techniques". Queensland Department of Primary Industries. Bulletin QB89005.
- Huang M, Shao M, Zhang L, Li Y (2003). "Water use efficiency and sustainability of different long-term crop rotation systems in the Loess Plateau of China". Soil & Tillage Research 72: 95–104.
- Anderson, R.L. 2005. Are some crops synergistic to following crops? Agron. J. 97:7-10
- Bullock, D.G. 1992. Crop Rotation. Critical Reviews in Plant Sciences, 11:309-326
- Francis, C.A. 2003. Advances in the design of resource-efficient cropping systems. Journal of Crop Production. 8:15-32
- Porter et al. 1997. Environment affects the corn and soybean rotation effect. Agron. J. 89:441-448
- White, L.T. 1962. Medieval Technology and Social Change. Oxford University Press
|Wikisource has the text of the 1920 Encyclopedia Americana article Rotation of Crops.|