毎月恒例の水田勉強会を開催。

水田は，植物生育に必須のリン酸やカリウムを自前できる特異な生態系。相対的に窒素の重要度が高く，窒素施肥は稲作の主要な技術となってきた。しかし，水稲体内の窒素のうち大半は土壌由来であり，実は米つくりにとって土壌こそが重要だと言える。まさに「米つくりは土作り」たる所以。この土壌による窒素供給能を評価する指標が可給態窒素。1984年に定められた地力増進法でも可給態窒素の向上が重要な目標となっている。それは，この指標が営農努力で改善可能であることを暗に示す。ところが，可給態窒素の変動が地形の生み出す複雑な水門環境の影響を受けている可能性があるという。この指標には人為の及び難い特性があるということか？興味深い。

日本の大豆単収は諸外国と比べて低い水準。とりわけ，東海と九州では低収が顕著で，ここ数年に限っては減収傾向に。日本では加工適性に優れた品種フクユタカのモノカルチャー状態。フクユタカを栽培する場合，過繁茂による倒伏を避けることや，梅雨時期の播種を避けるため，生育期間が短縮して収量が低下してしまう。増収に転じさせるには，倒伏しにくい，発芽時の加湿に強い，早播きに耐えうる，そもそも多収性，といった品種への切り替えや，梅雨時期の播種作業を可能にし，播種後の大豆生育を良好に維持させる播種機の開発が重要。技術はできた。残るは生産現場と市場が動くかどうか。

関谷（国際資源植物学）
「水田土壌の可給態窒素に与える地形の影響」
渡邊様（中日本農研））
「東海地域における大豆の安定多収に向けた早播適性品種の選定と不耕起播種機を用いたその栽培技術の開発」

Monthly Rice Paddy Study Session

Rice paddies are unique ecosystems that can naturally provide essential phosphoric acid and potassium for plant growth. Nitrogen is relatively more important, and nitrogen fertilization has become a key technology in rice cultivation. However, most of the nitrogen in rice plants comes from the soil, indicating that soil is actually crucial for rice production. This is precisely why “rice cultivation is soil building.” The indicator used to evaluate this nitrogen supply capacity of soil is available nitrogen. The Land Fertility Enhancement Act established in 1984 also sets the improvement of available nitrogen as an important goal. This implicitly suggests that this indicator can be improved through farming efforts. However, there is a possibility that fluctuations in available nitrogen are influenced by complex hydrological environments created by topography. Does this mean that this indicator has characteristics that are difficult to control by human intervention? Interesting.

Japan’s soybean yield per unit area is low compared to other countries. This is particularly evident in Tokai and Kyushu regions, which have shown a decreasing trend in recent years. In Japan, there is a monoculture of the Fukuyutaka variety, which has excellent processing suitability. When cultivating Fukuyutaka, avoiding lodging due to overgrowth and avoiding sowing during the rainy season shortens the growing period, resulting in reduced yields. To increase yields, it is important to switch to varieties that are less prone to lodging, resistant to moisture during germination, tolerant to early sowing, or simply higher yielding, as well as to develop seed drills that enable sowing during the rainy season and maintain good soybean growth after sowing. The technology has been developed. The remaining question is whether production sites and markets will adopt it.

Sekiya (International Resource Plant Science): “The Impact of Topography on Available Nitrogen in Rice Paddy Soils”
Watanabe (Central Japan Agricultural Research): “Selection of Early Sowing Suitable Varieties for Stable and High Soybean Yields in the Tokai Region and Development of Cultivation Techniques Using No-Till Seeders”