2026 Volume 61 Issue 1 Pages 39-55
ドリップ・ファーティゲイション（以下DF）は，乾燥地域で広く導入されているが，日本のような多雨地域ではほとんど普及していない。本研究は，東京八王子の多湿条件下において，ブロッコリー（Brassica oleracea L. var. italica）およびダイコン（Raphanus sativus L. var. hortensis）を対象に，2020～2021年にかけて実施した。通常窒素量および倍量窒素量のDF区と，降雨による慣行区を組み合わせた6区を設定した。降水量が作物蒸発散量の160～500％に達する過湿条件下でも，DFはブロッコリー収量を顕著に増加させた。増収の要因は，花蕾重量および側枝重量の増加によるものであり，2020年秋作では通常窒素DF区は慣行区の2.4倍，2021年春作では1.15倍，2021年秋作では1.3倍の増収となった。倍量窒素DF区ではさらなる増収があった。一方，ダイコンでは根部収量に有意差は見られず，葉長や葉重の増加のみが確認された。DFにより常に土壌が湿っている条件下では，根の下方向の伸長が阻害される可能性が示唆された。以上より，多雨地域においてもDFはブロッコリーの収量向上に有効であるが，根菜類には過湿条件が不利に働く可能性がある。ただ蒸発散量に応じた精密な灌水制御をすることで，根菜類にも適用可能性が広がると考えられる。
Drip fertigation, the simultaneous application of water and nutrients through drip irrigation, has been widely adopted in arid and semi-arid regions but remains largely unexplored under humid climatic conditions, such as in Japan. Although annual rainfall is abundant, its irregular distribution often results in water stress, limiting crop productivity. This study evaluated the effects of drip fertigation on broccoli (Brassica oleracea L. var. italica) and Japanese white radish (Raphanus sativus L. var. hortensis) under humid conditions. Field experiments were conducted from 2020 to 2021 in Hachioji, Tokyo, with six treatments that combined fertigation at normal and double nitrogen levels with rainfed controls. Results revealed that drip fertigation significantly increased broccoli yields, even under excessively humid conditions, where total water supply exceeded 160–500% of crop evapotranspiration. Improvements in broccoli yield have been primarily attributed to increases in head weight and the production of side shoots. Fertigated plots using equal or smaller nitrogen inputs than those in rainfed controls achieved yield gains of 237%, 115%, and 131% in autumn 2020, spring 2021, and autumn 2021, respectively. Doubling nitrogen via fertigation further enhances yield, although excessive nitrogen increases the risks of nitrate accumulation and environmental pollution. In contrast, drip fertigation had no significant positive effect on the radish root yield. Although leaf growth was stimulated, the radish roots tended to shorten under continuous wet soil conditions, suggesting that vertical root development was inhibited. These findings indicated that drip fertigation is effective in enhancing broccoli yield in the humid climates of Japan, contributing to improved crop productivity with reduced nitrogen inputs. However, excess soil water may offset the potential benefits for root vegetables such as radish, underscoring the importance of precise irrigation scheduling. Advanced systems that balance irrigation with evapotranspiration may optimize the fertigation for root crops in humid regions.
