Rice field - Suiden

Cultivated land filled with irrigation water for growing crops. The original meaning of the character "rice field" is a partitioned piece of land for growing grains, and there are "riku-ta" and "suida" (paddy fields). In China and the Korean peninsula, "riku-ta" is called "ta" and is used synonymously with "hata" (field), but in Japan, where paddy fields are primarily used for growing rice, they are abbreviated to "ta." In addition to rice, other crops grown in paddy fields include rush grass, water chestnuts, pearl barley, lotus, taro, taro, and Chinese makomo.

Paddy fields are surrounded by ridges and the surface is kept level to maintain an even water depth of 3 to 10 centimeters. To achieve this, the soil at the bottom of the paddy field is compacted, the topsoil is plowed, and the sides of the ridges are plastered to prevent the accumulated water from leaking underground. Irrigation water is let in through an inlet made by cutting part of the ridge, and excess water is discharged from a drainage outlet on the opposite side of the ridge. Irrigation water is supplied to and discharged from irrigation canals that run along the paddy fields, but it is often supplied to several fields in succession before being discharged into the canals.

Rice fields are generally created in low-lying, swampy areas around river basins and lakes, the bottom of alluvial fans, basins, and coastal deltas. In hilly areas, valley rice fields are created in the valleys using spring water, and on higher ground, rain-fed rice fields are created that depend solely on rainfall. Rain-fed rice fields are created in a stepped pattern on steep slopes, so they are also called "tanada" (rice terraces), and can be found in highlands such as the Sarashina region of Nagano Prefecture, famous for the "Tagoto no Tsuki" (Tageto Moon), as well as on the coast of Shikoku and various islands. They are also found in many parts of Southeast Asia.

Paddy fields are divided into dry and wet fields depending on the quality of drainage. Dry fields have good drainage, and when irrigation is stopped after rice cultivation, the fields become dry. For this reason, wheat and vegetables can be cultivated as second crops, and in this case they are called double-cropped fields. The soil of dry fields is permeable and has a good supply of oxygen, so even if a lot of manure is applied, there is no abnormal reduction of organic matter, and the roots of the rice are not damaged, so the rice grows well and can produce a high yield. Ill paddy fields have poor drainage and are constantly flooded, so the soil is reductive, so the productivity of rice is generally low, it requires a lot of work, and double cropping is not possible. If the soil remains reduced for a long time, the iron and manganese in the soil are reduced and run off into the ground. This is called a deteriorated paddy field, and the organic matter and fertilizer are reduced, resulting in the production of many harmful substances such as hydrogen sulfide. Rice roots create an iron coating to protect against these harmful substances, but in aged paddy fields this is not possible, and rice growth slows in the latter half of the year; these are also called autumn-fall paddy fields. This is seen in paddy fields with an abnormally high amount of organic matter, such as peat soil, black mud soil, and lakes and marshes, and is particularly common in warm southwestern regions where summers are very hot. Paddy fields where the irrigation water is too cold for rice to grow are called cold-water paddies and are common in highland areas. In this case, the first paddy field that receives the cold water is not planted, and the water temperature is raised and the water is used to irrigate the next paddy field. This first paddy field is called a warm-water paddy. In years of cold damage, paddy fields at the water outlet grow poorly, but these are used as warm-water paddies and are called sacrificial paddies. Other paddy fields with low productivity include leaky paddies with a lot of sand and gravel in the bed soil, which leak a lot, and salt-damaged paddy fields on the coast or undersea reclaimed land, which are damaged by residual salt.

In the past, Japanese rice paddies were roughly rectangular, but with irregular shapes, depending on the slope of the land and the location of the rivers that served as irrigation channels, and because they were done by hand, their size was limited to 10 ares (1 tan) at the largest and less than 3.3 square meters (1 tsubo). The more mountainous the rice paddies, the smaller and more irregular their shapes became. However, since the "Field Improvement Project" that has been implemented since 1963, the paddy fields have been divided into regular rectangles that are more efficient for using agricultural machinery, and the standard area is 30 ares (the improvement rate was 58.2% as of 2000), and irrigation and drainage facilities have also been installed, improving productivity.

In Japan, rice paddies were developed with the introduction of wet rice. The Toro ruins in Shizuoka Prefecture are known as the remains of a rice paddy from the Yayoi period, but in 1981 (Showa 56), a large rice paddy was discovered in a stratum believed to be about 2000 years old in Inakadate Village, Aomori Prefecture, and has become a new issue in the history of rice cultivation in Japan. The area of ​​rice cultivation in Japan reached 2.5 million hectares by the end of the Edo period, and 3 million hectares in the Taisho period. Currently, the area of ​​rice paddies has decreased due to production adjustments, conversion to housing land, and destruction, and was 1.7 million hectares in 2001 (Heisei 13).

The total area of ​​rice harvested in the world is about 150 million hectares (2001), which is a little over 10% of the total cultivated land area. About 90% of the total is concentrated in Asia, with India at about 44.5 million hectares, China at about 28.6 million hectares, Indonesia at about 11.8 million hectares, and other large areas in Bangladesh, Thailand, Vietnam, Myanmar, etc. Paddy fields in Southeast Asia are mostly along rivers and coastal deltas, and many of them are wet fields that are flooded during the rainy season. There are also many rain-fed rice fields in highlands, and in all cases the area per management is small and the productivity is low. In the United States, there are paddy fields in the southern part and California, and they are managed on a large scale with high mechanization. In America, paddy fields are wide strips with ridges along the contour lines, and rice is grown by direct seeding. They are dry fields and rotated with feed crops, making them highly productive. Paddy fields in the Lombardy plain in Italy, the Mediterranean coast of France and Spain, and Egypt have well-developed irrigation facilities and are rotated with feed crops, so they are also highly productive. Paddy field cultivation has also spread to areas at higher latitudes than before, such as the northeastern region of China and Heilongjiang Province.

[Hoshikawa Kiyochika]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

灌漑(かんがい)水をたたえて作物を栽培する耕地。田の本来の字義は禾穀(かこく)を栽培する区画された土地のことで、陸田と水田とがある。中国や朝鮮半島では陸田を田といい、畑と同義に用いているが、日本では水稲栽培がおもなので、水田を田と略称している。水田には水稲のほか、イグサ、クワイ、ハトムギ、ハス、サトイモ、タロイモ、マコモなどが栽培される。

　水田は周囲をあぜ（畦）で囲い、田面を水平にして3～10センチメートルの水深を均等に保つようにする。このために水田の土壌底部は底締(そこじ)めといって土壌を緊密にし、また表面の作土は代掻(しろか)きし、あぜの側面はあぜ塗りするなど、たたえた水が地下へ漏れるのを防ぐ。灌漑水は、あぜの一部を切ってつくった水口(みなくち)から入れ、それと反対側のあぜにある排水口から余剰な水を出す。灌漑水は水田に沿った灌漑水路から供給され、またそこへ排出されるが、田から田へといくつかの田に連続して供給されてから水路に出されることも多い。

　水田は一般に河川流域や湖沼周辺の低湿の土地、扇状地低部、盆地、海岸のデルタなどにつくられる。丘陵地などではその谷間に湧水(わきみず)などを利用した谷地田(やちだ)がつくられ、さらに高地では降雨だけに依存する天水田(てんすいでん)がつくられている。天水田は急斜面に階段状につくられるので、棚田(たなだ)ともよばれ、「田毎(たごと)の月」で名高い長野県更科(さらしな)地方などの高地のほか、四国の海岸や各地の島嶼(とうしょ)にみられる。東南アジアの各地にも多い。

　水田は排水の良否によって乾田(かんでん)と湿田(しつでん)とに分けられる。乾田は排水がよく、水稲の栽培が終わって灌漑を止めると畑状態となる。このため裏作としてムギ類や野菜を栽培することができ、この場合は二毛作田という。乾田の土壌は透水性がよく、酸素がよく供給されるので、堆厩肥(たいきゅうひ)が多く施されても有機質の異常還元がおこらず、水稲の根に障害を与えないので生育が優れ、高い収量をあげることができる。湿田は排水が悪く、常時湛水(たんすい)状態のため土壌は還元的になり、水稲の生産力は一般に低く、作業労力も多くかかり、二毛作もできない。土壌の還元化が長く続くと、土中の鉄やマンガンが還元されて地下へ流失してしまう。これを老朽化水田といい、有機質や肥料が還元されて硫化水素など有害物質も多く出る。イネの根は鉄の被膜をつくってこれら有害物質を防ぐのであるが、老朽化水田ではそれができず、イネの生育は後半になると衰えるので、これを秋落ち水田ともよぶ。泥炭土、黒泥土、湖沼地などで有機物が異常に多い水田にみられ、とくに夏に高温になる西南暖地に多い。また、灌漑水がイネの生育に低温すぎる田を冷水田(れいすいでん)といい高冷地に多い。この場合、冷水の入る最初の田は作付けをせず、水温を上昇させてその水を次の田に灌漑する。この最初の田を温水田(おんすいでん)という。冷害の年には水口の田は生育が悪いが、これを温水田として利用し、犠牲田とよぶ。ほかに、床土が砂土や砂礫(されき)が多くて水漏れの多い漏水田、海岸や海底干拓地で残留塩のため被害のおこる塩害田なども生産力の低い水田である。

　日本の水田は、かつては土地の傾斜ぐあいや灌漑水路の河川の位置に従って、ほぼ四角形ではあるが、ゆがんだ形につくられ、その大きさも人力による作業のために、大きくても1反（10アール）で小さいものは1坪（3.3平方メートル）に満たないものまであった。山地の田ほど小さく不定形であった。しかし現在は、1963年（昭和38）から実施されている「ほ場（圃場）整備事業」によって、水田の区画も農業機械を使用するうえで作業能率の高い正・長方形に、面積も30アールを標準区画とした水田基盤整備が進み（整備率は2000年現在58.2％）、また灌漑・排水施設も備えられて、生産力の向上が図られている。

　日本では水稲の伝来とともに水田が開かれた。弥生(やよい)時代の水田遺構として静岡県登呂(とろ)遺跡が知られているが、1981年（昭和56）、青森県田舎館(いなかだて)村に約2000年前とみられる地層から大規模な水田が発見されて、日本の稲作史の新しい問題点となっている。日本の水稲作付面積は、江戸末期までに250万ヘクタールに及び、大正時代には300万ヘクタールに達した。現在は生産調整や宅地等への転用、壊廃などで水田面積は減少し、2001年（平成13）では170万ヘクタールとなっている。

　世界の全水稲収穫面積は約1億5000万ヘクタール（2001）で、全耕地面積の10％強にあたる。アジア地域に全体の約90％が集中しており、インド約4450万ヘクタール、中国約2860万ヘクタール、インドネシア約1180万ヘクタールなどが多く、そのほかバングラデシュ、タイ、ベトナム、ミャンマーなども面積が広い。東南アジアの水田は河川沿岸、海岸デルタ地帯に多く、雨期には冠水するような湿田が多い。また高地には天水田が多く、いずれも1経営当り面積は小さく、生産力が低い。アメリカでは南部やカリフォルニア州に水田があり、高度に機械化された大規模経営である。アメリカの水田は等高線に沿ってあぜを設けた幅の広い帯状で、水稲は直播(じかま)き栽培され、乾田で、飼料作物などと輪作され生産力が高い。イタリアのロンバルディア平原、フランスやスペインの地中海沿岸、エジプトなどの水田は灌漑施設がよく整備され、飼料作物との輪作もされて、これらも生産力は高い。また中国の東北地区、黒竜江省などでも水稲栽培が広まるなど、水田は従来より緯度の高い地域にも拡大してきている。

［星川清親］

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