Wednesday, January 14, 2009

Crop Rotation

Crop rotation or Crop sequencing is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons for various benefits such as to avoid the build up of pathogens and pests that often occurs when one species is continuously cropped. Crop rotation also seeks to balance the fertility demands of various crops to avoid excessive depletion of soil nutrients. A traditional component of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals and other crops. It is one component of polyculture. Crop rotation can also improve soil structure and fertility by alternating deep-rooted and shallow-rooted plants.

1 Method and purpose
2 History
3 Effects on soil erosion
4 References

Method and purpose
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Crop rotation avoids a decrease in soil fertility, as growing the same crop repeatedly in the same place eventually depletes the soil of various nutrients. 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, rices followed by cottons. By crop rotation farmers can keep their fields under continuous production, without the need to let them lie fallow, and reducing the need for artificial fertilizers, both of which can be expensive. Rotating crops add nutrients to the soils.

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. A 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.

Crop rotation is a type of cultural control that 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 popluation 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.

This principle is of particular use in organic farming, where pest control may be achieved without synthetic pesticides.

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 labor.

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 choosing a crop rotation.

Old crop rotation methods were mentioned in Roman literature, and referred to by several civilizations in Asia and Africa. During the Muslim Agricultural Revolution of the Islamic Golden Age, Muslim engineers and farmers introduced a new modern rotation system where land was cropped four times or more in a two-year period. Winter crops were followed by summer ones, and in some cases there was a crop in between. In areas where plants of shorter growing season were used, ie.spinach and eggplants, the land could be cropped three or more times a year. According to some sources, in parts of Yemen wheat yielded two harvests a year on the same land, as did rice in Iraq. Scholars such as Andrew Watson have written of a Muslim agricultural revolution as the Islamic world made significant progress in developing a more "scientific" approach based 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 encyclopaedias, with surprisingly great precision and details, were produced.

From the end of the Middle Ages until the 20th century, the three-year rotation was practiced 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. The fact that suitable rotations made 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.

George Washington Carver pioneered crop rotation methods in the United States by teaching southern farmers to rotate soil depleting crops like cotton with soil enriching crops like peanuts and peas.

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. Some disadvantages of this type of monoculture have since become apparent, notably from the perspective of sustainable agriculture and the risk of catastrophic crop failure.

Effects on 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. For example, wheat stubble consistently leaves a significant mass of plant residue after harvest. Wheat production supplemented with no till or reduced till management systems can typically yield 90% post-harvest soil cover with up to 15 months of stubble retention.

The amount of stubble mass retained over time governs whether a crop will be successful in controlling erosion. Crops with little stubble mass retained over time should not be planted following a plant production system with similar characteristics. Sunflowers for example typically produce less than 40% soil cover after harvest with very little stubble remaining after cultivation. This leaves a significant percentage of the soil susceptible to erosion. However, when sunflower crops are rotated with wheat crops in production, the soils are less prone to erosion because the high-stubble producing wheat crops are followed by the low-stubble producing sunflower crop. A corn – soybean crop rotation in a no till system works similarly. Corn plants leave substantial residue mass after harvest. Soybeans, a relatively low-residue producing plant, following corn will have sufficient cover from the previous crops corn residue to limit soil losses. It is important to avoid mono-cropping low-stubble producing plants when attempting to reduce soil loss.

The additional crop residue added by rotation with crops with substantial biomass will also enhance soil structure. Stubble cover will 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 sewn optimal conditions, whereas rigid systems are 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 regions 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.

^ Andrew M. Watson (1974), The Arab Agricultural Revolution and Its Diffusion, 700-1100, The Journal of Economic History, Vol. 34, No.1, The Tasks of Economic History, pp. 8-35.
^ al-Hassani, Woodcock and Saoud (2007), Muslim heritage in Our World, FTSC publishing, 2nd Edition, pp.102-123.
^ Unger, P.W., and McCalla, T.M. “Conservation Tillage Systems.” Advances in Agronomy. Vol. 33. pg. 2-53. 1980.
^ Rose, C.W., and Freebairn, D.M. “A mathematical model of soil erosion and deposition processes with application to field data.” Soil Erosion and Conservation. Pg. 549-557. 1985.
^ a b Sallaway, M.M., Lawson, D., and Yule, D.F. “Ground cover during fallow from wheat, sorghum, and sunflower stubble under three tillage practices in central Queensland. Soil and Tillage Research. Vol. 12. pg. 347-364. 1988.
^ a b c Carroll, C., Halpin, M., Burger, P., Bell, K., Sallaway, M.M., and Yule, D.F. “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. Vol. 35. pg. 925-939. 1997.
^ Loch, R.J., and Foley, J.L. “Measurement of Aggregate Breakdown under rain: comparison with tests of water stability and relationships with field measurements of infiltration.” Australian Journal of Soil Research. Vol. 32. pg. 701-720. 1994.
^ Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R., and Hammer, G.L. “PERFECT. A computer simulation model of Productive Erosion Runoff Functions to Evaluate Conservation Techniques.” Queensland Department of Primary Industries. Bulletin QB89005. 1989.
^ Huang, M., Shao, M., Zhang, L., and Li, Y. “Water use efficiency and sustainability of different long-term crop rotation systems in the Loess Plateau of China.” Soil & Tillage Research. Vol. 72. pg. 95-104. 2003.

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