Crop rotation - Rinsaku

The practice of planting crops in a regular cycle. Continuous cultivation of the same crop in the same field reduces soil fertility and yields. Crop rotation was developed in ancient European agriculture to prevent this and to secure feed.

In the early days, the land was divided into grain fields and grasslands for livestock feed, and when the fertility of the grain fields was lost, the grasslands were turned into fields and replaced. Later, as the proportion of grain cultivation increased, the cultivated land was divided into three sections, one section was left fallow, and the other two sections were planted with spring-sown wheat and autumn-sown wheat, and these were rotated annually. This changed to a three-field crop rotation, and then further developed into an improved three-field system in which legumes were planted in the fallow land to actively increase soil fertility. A farming method called the grain system was also implemented, in which fields were divided into several sections, and grains and pasture were regularly rotated in each section every few years, and soil fertility was maintained using livestock. From the 19th century onwards, fallow land was abolished, and a crop rotation system was developed in which legumes and forage crops were planted in a fixed order with grains to rationally prevent soil fertility and to enhance the production of livestock feed. A typical example of this is the Norfolk method of crop rotation, which became popular mainly in Germany. However, in recent years, it has become possible to maintain and increase soil fertility by applying fertilizer, so crop rotation has become more important for streamlining and stabilizing management, such as averaging and properly allocating labor input throughout the year and dispersing risks from disasters. Crop rotation is also effective in preventing the outbreak of diseases and pests and the increase of weeds that can result from continuous cropping. This is because changing the type of crop results in different types and ecology of pests, diseases, and weeds. Another important benefit of incorporating grassland into crop rotation is the prevention of soil erosion.

In Japan, crop rotation is practiced in field farming with the main aim of efficient land use and maintaining soil fertility. There are various types, such as a two-year rotation of potato-wheat-soybean, or a three-year rotation of potato-wheat-clover-corn. Generally, legume crops are introduced in between to increase soil fertility. Recently, with the development of livestock farming, more and more crop rotations are being implemented, with feed crops as the main crop and grains and vegetables added to it. Crop rotation is not necessary for paddy rice cultivation, as there are no problems with continuous cropping. However, paddy field rotation, in which paddy fields are converted to fields every few years to cultivate pasture grass and field crops, is effective in controlling weeds and soil nematodes, and is also effective in improving the soil, increasing the yield of both paddy rice and field crops. Therefore, the significance of crop rotation is also important in future paddy field management.

[Hoshikawa Kiyochika]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

作物を一定の順序に周期的に交替させて作付けすること。同じ畑に作物を連作すると地力が低下して収量が減る。輪作はこれの防止と、飼料の確保の両目的から、古くヨーロッパの農業において発達した。

　初期には土地を穀物畑と家畜飼料用の草地とに二分し、穀物畑の地力が失われてくると草地を畑にして交替させていた。その後、穀物栽培の割合が増えるにつれて、耕地を三区分し、一区は休閑し、他の二区に春播(ま)きムギおよび秋播きムギを作付けて、年々これらを交替させる三圃(さんぽ)式輪作にかわり、さらに休閑地にもマメ科植物を植えて積極的に地力を増す改良三圃式に発展した。また畑をいくつかに区画して、各区に穀物と牧草とを数年おきに規則的に交替させ、家畜を媒介として地力を維持する、穀草(こくそう)式とよばれる農法が行われた。19世紀以降は、休閑地を廃して、マメ科作物や飼料作物を穀物と一定の順序で交替に作付けて、地力の消耗を合理的に防止し、家畜飼料の生産も強化した輪栽(りんさい)式農法が発達した。その代表的なものが、ドイツを中心に普及したノーフォーク式輪作であった。しかし近年は地力の維持・増強は肥料を施すことで可能となったため、輪作はむしろ労力投下の年間平均化や適正配分、災害による危険の分散など経営の合理化や安定のための意義が強くなった。輪作はこのほか、連作による病気や害虫の発生、雑草の増加を防ぐことに効果がある。作物の種類がかわると病害虫・雑草の種類や生態が異なるためである。また草地を輪作に組み込むことによって、土壌侵食を防ぐことも重要な効果としてあげられる。

　日本の畑作地では土地の有効利用と地力維持を主眼に輪作が行われ、ジャガイモ―コムギ―ダイズによる2年輪作、ジャガイモ―コムギ―クローバー―トウモロコシの順の3年輪作など、さまざまな形式がある。一般には途中にマメ科作物を組み込んで地力の増加を図っている。最近では畜産の発達に伴って、飼料作物を主軸にして、これに穀物や野菜を組み込む形式の輪作も多くなってきている。なお、水田稲作は連作障害がないので、輪作は必要がない。しかし数年ごとに水田を畑に転換して牧草や畑作物を栽培する、田畑輪換(でんぱたりんかん)は、雑草の防除や土壌線虫の防除などに効果があるうえ、作土の土壌改良に有効で、水稲作にも、畑作にも増収となる。したがって今後の水田作経営においても、輪作の意義は重視されている。

［星川清親］

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