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crop rotation[′kräp rō′tā·shən]
the scientifically based alternation of different crops in the same field, over a regular sequence, to help restore or improve soil fertility; a significant aspect of any system of crop cultivation. Crop rotation involves a variety of agricultural techniques, such as tillage and the use of fertilizers and chemical pesticides; it also involves land improvement measures, such as irrigation, drainage, and the application of chemicals for soil improvement. The period during which crops and fallow alternate on a given field is called the rotation period. The order in which crops and fallow succeed each other is called the rotation. Several crop rotations, integrated optimally on a single farm, make up a crop rotation system.
Scientific basis of crop rotation. Long before crop rotation became a science, it was observed, in actual practice, that crop yields decline if the same crop is grown continuously in the same place, especially if no fertilizers are added. Subsequently, the study of plant characteristics and their effect on soil characteristics led to a scientific explanation of what had been observed in practice, thus demonstrating the need to alternate crops. Many scientists have made contributions to modern views on crop rotation, including A. Thaer, J. von Liebig, the German agricultural chemist H. Hellriegel, J. Boussingault, V. V. Dokuchaev, P. A. Kostychev, K. A. Timiriazev, D. N. Prianishnikov, V. R. Vil’iams, and N. M. Tulaikov. Similarly, the work of scientific research institutes in the USSR and the work of the venerable scientific research institutions in Western Europe and the USA has gained world renown—for example, the work of the Rothamsted Experimental Station in Great Britain, the Institute of Agriculture and Horticulture at the University of Halle, now in the German Democratic Republic (GDR), and experimental stations on the island of Ask0. Denmark, and in Montana, Minnesota, Illinois, Iowa, and Ohio in the USA.
Synthesis of the data amassed by world science has led to the modern theory of crop rotation.
The chemical basis of crop rotation is related to plant nutrition. Some plants require more nutrients, others less. The root systems of some plants are better able to take nutrients from deep soil layers and from poorly accessible chemical compounds. Legumes have good atmospheric nitrogen-fixing properties and can thus enrich the soil. The soil nutrient balance can be regulated by adding fertilizers as required by the crop under cultivation. However, fertilizers are more effective in a crop rotation, since the alternation of different plants assures the full utilization of the fertilizers. If legumes are included in a crop rotation, the consumption of fertilizers is considerably reduced.
After the harvesting of various kinds of crops, plant matter remains in the soil in varying quantities. Thus, different plants have different effects on the physical properties of the soil, including the soil’s structure and resistance to water and wind erosion. If crops are properly selected and rotated and if organic and mineral fertilizers are applied, it is possible to control the formation and decomposition of organic matter in the soil and to attain a self-sustaining balance of organic matter in the soil. If perennial grasses or grain crops are planted in strips that alternate with strips of row crops, there is less danger of soil erosion.
When some crops are planted twice in succession on the same field, biological factors produce an adverse effect more quickly than other factors do, an effect that primarily takes the form of more weeds, more pests, and more plant pathogens. This, together with chemical and physical factors, depletes the soil. Crop rotation prevents soil depletion and other ill effects. According to the principles of crop rotation, for example, certain crops, such as flax, should not be planted twice in succession on the same field, and different plants that are attacked by the same diseases should not be sown twice in succession on the same field.
Crop rotation is economically advisable, since it makes rational use of the land, assures steady high yields, and permits greater diversification; because different crops are planted, tilled, and harvested at different times, agricultural work is spread out more evenly over the year.
Crops. In any given field, the previous year’s crop or pure fallow is called the “predecessor.” Predecessors are classified by the extent to which they influence soil properties and basic crop yields.
Perennial leguminous grasses, such as clover, alfalfa, and sainfoin, and mixes of such grasses and cereals are good soil fertility builders in irrigated and well-watered regions, excellent predecessors for all crops except legumes, provided they attain good development. They usually precede more valuable and productive crops, such as wheat, cotton, flax, millet, corn, or potatoes. Their beneficial effect lasts three to five years. Perennial leguminous grasses are of less value as predecessors if they do not attain adequate development.
Legumes such as lupine, vetch, pea, chick-pea, and peavine shade the soil, improve soil structure, and suppress weeds, provided they attain good development. They are good predecessors for all spring and winter crops except legumes. Their beneficial effect lasts at least two years.
Row crops, such as potatoes, beets, corn, sunflowers, and cotton, have various biological characteristics. They must be cultivated in certain ways; for example, they require frequent interrow tillage, which removes weeds and helps retain soil moisture. Their specific requirements, therefore, result in the growth of useful soil microflora and in improved plant nutrition. Second plantings of some row crops, such as corn, cotton, and sugar beets, are possible, especially if the crops are irrigated and free of disease; potatoes can be planted a second year in succession, but not more, in a special vegetable-potato rotation. Row crops are good predecessors for all winter grains, flax, and hemp. Silage corn, silage beans, and early potato varieties are adequate predecessors for winter crops. The beneficial effect of row crops lasts two years.
Winter grains, such as rye, wheat, and barley, tiller well, shade the soil, and suppress many kinds of noxious plants. They are harvested before other crops are and thus create favorable conditions for moisture accumulation in the postharvest period. Fertilized winter grains are good predecessors for row crops, perennial grasses, winter cereals, legumes, and flax. In the Kuban’, the Ukraine, and parts of the Chernozem Zone of the USSR, that is, in areas where there is no danger of root rot, winter crops can follow winter crops in succession.
Industrial fiber crops sown by means of broadcasting—such as flax and hemp—are hard on the soil, removing moisture and nutrient elements from the top soil layer. Hemp assimilates poorly soluble phosphorous compounds well. With optimum technology, industrial fiber crops are satisfactory predecessors for cereals and row crops.
Spring spike crops and groat crops sown by means of broadcasting—such as wheat, barley, oats, millet, and buckwheat—remove roughly similar amounts of nutrient elements from the soil, provide little shade, and are often infested with weeds. They are satisfactory predecessors for other plants of the same group and for row crops. In Siberia and Kazakhstan, for example, spring wheat sown on plast (seePLAST) or strip fallow is a good predecessor; in the European USSR, spring wheat is a good predecessor after perennial legumes or leguminous grasses and cereals.
True fallows, both bare and early, and strip fallows retain the moisture from spring and summer rains well, help control weeds, stimulate useful microbiological soil activity, and increase the supply of soil nutrients. They are excellent predecessors for winter crops in all parts of the USSR, especially in arid and semiarid regions, and excellent predecessors for spring wheat in Siberia and Kazakhstan. Their beneficial effect lasts at least two to three years. Occupied fallows are often used as predecessors for winter and spring grain crops on weed-free soils in regions with adequate precipitation.
Classification. The crop rotation classification used in the USSR since 1968 distinguishes three types of crop rotation: field, feed, and special crop rotations. In field crop rotations, most of the rotation area is in grains, potatoes, and industrial crops, and in feed crop rotations, more than half is in feed crops. In special crop rotations, crops are grown that require special conditions and techniques, for example, such crops as vegetables, tobacco, hemp, cotton, and rice. The three types are further subdivided according to the crops and fallows involved and the ratio between them into grain-fallow rotations, grain-fallow-row crop rotations, grain-grass rotations, grain-row crop rotations, grass-field crop rotations, grass-row crop rotations, green-manure rotations, grain-grass-row crop rotations, and row-crop rotations.
In grain-fallow rotations, grain crops alternate with true fallow; 50–70 percent of the rotation area is in grains. Such crop rotations are used in the arid regions of Northern Kazakhstan and in the steppe region of Siberia.
In grain-fallow-row crop rotations, grain crops alternate with true fallow and row crops. At least half the rotation area is in grains. Such crop rotations are common in steppe and forest-steppe regions throughout the southern and southeastern USSR.
In grain-grass rotations, most of the rotation area is in grains, and the rest in annual and perennial grasses. Such crop rotations are found in the nonchernozem zone of the RSFSR. In flax-growing regions, they include one flax field, thus forming a grain-flux-grass rotation.
In grain-row crop rotations, at least half the rotation area is in grains, which alternate with row crops. Such crop rotations are typical of humid regions, such as the Northern Caucasus, the Central Chernozem Zone, and the forest-steppe region in the Ukrainian SSR.
In grass-field crop rotations, more than half the land is in perennial grasses, and the rest of the arable land in annual field crops, such as grains, flax, and annual grasses. Such crop rotations are used primarily in the nonchernozem zone and in irrigated regions.
In grass-row crop rotations, row crops alternate with perennial grasses that occupy at least two fields. Such crop rotations are practiced on floodplains and reclaimed wetlands. In the Union republics that grow cotton, they include alfalfa-cotton rotations.
In green-manure rotations, crops such as lupine are raised to provide green manure in occupied (green-manure) fallows. Such crop rotations are used on loams, sandy loams, and sandy soils.
In grain-grass-row crop rotations, or nonfallow rotations, grains, row crops, and legumes alternate. Here, no more than half the rotation area is in grain crops, and it is therefore possible to grow different plants on every field every year without leaving any true fallow. Such crop rotations are common in the nonchernozem zone of the RSFSR, in the forest-steppe regions of the European USSR, and on irrigated lands in arid regions.
In row-crop rotations, at least half the rotation area is in row crops planted in wide rows, planted as single seeds at the corners of a square, or planted in clusters at the corners of a rectangle. Such crop rotations are introduced in damp regions in the Northern Caucasus and the Ukrainian SSR.
In the USSR. In the USSR, republic land-allocation planning institutes, in consultation with kolkhoz and sovkhoz specialists, draw up the farm’s crop rotation plan at the same time they draw up its general organizational-economic plan. In so doing, they study the farm’s climatic, soil, and hydrological features. Then, using soil maps and agrochemical cartograms, they divide all the farm’s cropland into several production categories; at the same time, they examine and evaluate all agricultural land in general.
The results of this preparatory work are used to draw up two plans: an intrafarm land-allocation plan and a land conversion plan, that is, a plan that envisages the conversion of less valuable land into more productive land. Then, in accordance with the principal agricultural development indexes and the state plan for the sale of agricultural products, it is decided what crops are to be planted in what areas. In so doing, several variant plans are prepared; each is evaluated in terms of plant production per hectare (ha) of cropland, in terms of total feed production and production of each kind of feed, in terms of protein yield, in terms of the use of tractors and other agricultural machinery, and in terms of the farm’s haulage capacity. Using the optimum variant, the number of crop rotations and the rotation area are established, the number of fields and the sequence of crops are determined, and the crop rotation pattern is laid out over the farm’s total land area. In each crop rotation, the leading crop is given priority.
The proposed crop rotation system is then discussed at a conference of specialists from the farm and planning organization and at a sovkhoz production conference or meeting of kolkhoz representatives, whichever is relevant. After corrections have been made, the plan is submitted to higher agricultural bodies for approval. Once approved, the plan is translated into actual land areas, that is, the farm’s lands are allocated as provided in the plan, and the boundaries of each field and crop rotation are delimited. After the land has been allocated and the relevant documents have been drawn up, the crop rotation is considered “introduced.” After about two to three years, the crop rotation is usually considered “established,” that is, each crop is in the fields to which it is assigned, within properly delimited boundaries.
In the USSR, as of Nov. 1, 1974,196.2 million ha of cropland, or 93 percent of all cropland, was in “introduced” crop rotations; 157.4 ha, or 75 percent of all cropland, was in “established” crop rotations.
Abroad. From the 18th to mid-20th centuries, grain-grass-row crop rotations were the rule in Western Europe, the USA, and Canada. They have now given way to nonfallow grain-crop rotations in most grain-growing regions, except in the wheat-growing regions of the USA and Canada, where two-field and three-field fallow-grain rotations are common, except in regions of intensive animal husbandry, where feed rotations are common, and except in suburban vegetable farms, where specialized rotations are common.
Because of the intensification of crop cultivation, there has been a general tendency toward increased specialization and shorter rotation periods. In the eastern part of Great Britain, where the Norfolk rotation has been in use for more than a century, grain crops have come to occupy more than 70 percent of the cropland on 27 percent of the farms since the late 1960’s. In the GDR and in the Federal Republic of Germany and other countries in Western Europe, crop rotations with various catch crops, such as undersowing crops, stubble crops, and winter crops, have come into broader use, which has made it possible to plant up to 30 percent more in the same rotation area and, in regions with abundant precipitation, to protect the soil from destruction. In the USA, Canada, and several European countries, soil-conserving crop rotations are introduced in order to control soil erosion. Small private farms are limited to a single crop rotation with a minimum number of crops.
In Asia and Africa, crops are alternated on a yearly cycle; that is, crops are chosen so that their moisture requirements coincide with seasonal variations in soil moisture. In Burundi, for example, the following alternation is common: rice from November to May, peanuts from July to September, rice from November of one year to May of the next year, and fallow from May to November. In other rice-growing countries, rice is alternated with legumes, for example, in Madagascar with sweet clover or rice and vetch. Short rotations are used with peanuts, such as peanuts-millet, peanuts-sorghum, and fallow-peanuts-sorghum. In tropical regions the land is in field crops for two to three years and in long fallow for four to five years.
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