Paddy field soil - Suiden Dojo

This is the most important rice-growing soil in Japan, and its area has decreased since 1970 (Showa 45), to 2.51 million hectares (2009). Because rice is grown under flooded conditions, paddy fields are often found on flat alluvial land near rivers and lakes where a large amount of water is available, and they have a great flood control effect, but there are also many paddy fields on plateaus, hills, and mountain slopes. Paddy field soil is strongly affected by flooding, so it has many different characteristics from field soil. The first is the reduction of the cultivated soil. Because it is flooded for a considerable period of the year, the soil is oxygen-deficient except for the very surface layer, and iron, manganese, and nitrogen are also reduced. Therefore, when a cross section is observed during the drainage period, it is dark green or gray in color and contains iron deposits. Below that, there are layers rich in iron and manganese. Because harmful filamentous fungi have difficulty thriving in reduced conditions, paddy fields do not suffer from the same kind of problems associated with continuous cropping that occur in upland fields. Secondly, a hard plow bed layer several centimetres thick exists just below the cultivated soil, which prevents water leakage and makes flooding easier. Thirdly, flooding controls weeds and also helps maintain soil fertility through the action of algae that grow in the water on the paddy field surface. However, a drawback is that only crops that can adapt to reduced soil conditions can be grown in paddy fields.

[Yuki Koyama]

"Japanese Paddy Field Soils" by Kawase Kinjiro, Yokoyama Eizo, and Matsui Makoto (1972, Kodansha)" ▽ "Paddy Field Soil Science" edited by Kawaguchi Keizaburo (1978, Kodansha)" ▽ "Paddy Field Soil Science" edited by Yamane Ichiro, Iimura Koji, and others (1982, Rural Culture Association)" ▽ "Paddy Field Soils and Phosphorus - Supply Capacity and Fertilization" edited by the Japanese Society of Soil Science and Plant Nutrition (1984, Hakuyusha)" ▽ "Latest Soil Science" edited by Kuma Kazutake (1997, Asakura Shoten) ▽ "Soil and Production Environment" by Hasegawa Kazuhisa (2002, Yokendou) ▽ "Silicate and Crop Production" edited by the Japanese Society of Soil Science and Plant Nutrition (2002, Hakuyusha)" ▽ "Nitrogen Mineralization and Fertilization in Paddy Field Soils" POD Edition (2003, Hakuyusha) edited by the Japanese Society of Soil Science and Plant Nutrition "New Soil Microorganisms 10: Progress and Prospects of Research" edited by the Japanese Society of Soil Microbiology (Hakuyusha, 2003)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

日本の農耕地でもっとも重要な水稲栽培土壌で、その面積は1970年（昭和45）以降減少し、251万ヘクタール（2009）である。水稲は湛水(たんすい)（水を張った）状態のもとで栽培されるため、水田は多量の水が得られる河川や湖沼に近い平坦(へいたん)な沖積地に多く分布し、その治水効果は大きいが、台地や丘陵地、山腹の斜面などにある水田も少なくない。水田土壌は湛水の影響を強く受けるので、畑土壌とは多くの点で異なった特徴がみられる。第一は、作土の還元である。年間かなりの期間水を張った状態にあるため、土壌はごく表層を除いて酸素不足の状態となり、鉄、マンガン、窒素なども還元された形態へと変化する。したがって、落水期にその断面を観察すると暗緑色ないし灰色を呈し、鉄の沈積物を含んでいる。またその下には、鉄に富む層とマンガンに富む層がある。還元状態では有害な糸状菌の生息が困難なので、水田では畑で問題となるような連作障害がおこらない。第二は、作土のすぐ下に数センチメートル程度の厚さの硬い鋤床(すきどこ)層が存在することで、この層の働きにより漏水が防止され、湛水が容易になる。第三は、湛水により雑草が防除され、また、田面水中に生える藻類の働きなどで地力が維持されることなどである。しかし、土壌の還元状態に適応できる作物しか栽培できないのが欠点である。

［小山雄生］

『川瀬金次郎・横山栄造・松井慎著『日本の水田土壌』（1972・講談社）』▽『川口桂三郎編『水田土壌学』（1978・講談社）』▽『山根一郎編、飯村康二他著『水田土壌学』（1982・農山漁村文化協会）』▽『日本土壌肥料学会編『水田土壌とリン酸――供給力と施肥』（1984・博友社）』▽『久馬一剛編『最新土壌学』（1997・朝倉書店）』▽『長谷川和久著『土壌と生産環境』（2002・養賢堂）』▽『日本土壌肥料学会編『ケイ酸と作物生産』（2002・博友社）』▽『日本土壌肥料学会編『水田土壌の窒素無機化と施肥』POD版（2003・博友社）』▽『日本土壌微生物学会編『新・土の微生物10　研究の歩みと展望』（2003・博友社）』

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