Iyachi

When the same or taxonomically closely related crops are cultivated continuously, the growth of the crops is poor and the yield is reduced or nonexistent. This phenomenon is called "avoidable soil" or "avoidable soil phenomenon". Although the degree of averse soil is different, many crops have a tendency to have averse soil, and especially flax, peas, burdock, watermelon, tomato, eggplant, etc., the yield is significantly reduced. In fields where these crops have been cultivated, it is better not to cultivate the same crop for several years to about 10 years. However, generally, there are almost no averse soils for crops of the Poaceae family, and rice grown in paddy fields does not show any averse soils at all. The cause of averse soils has not been fully elucidated, but it is thought that several factors often have a complex influence on each other. The factors include the depletion of specific nutrients in the soil, the spread of pathogens, nematodes, and pests, the secretion of growth-inhibiting substances from the roots of the crops, toxins in the remaining roots, and changes in the physical properties of the soil. Soil disinfection and soil addition are carried out as countermeasures against averse soils, but since neither of these provide a fundamental solution, it is preferable to avoid continuous cultivation and rotate with other crops.

[Hoshikawa Kiyochika]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

同一または分類学的に近縁な種類の作物を連作した場合、作物の生育が悪くなり、収量が減少もしくは皆無となる現象を忌地または忌地現象という。忌地は、程度の差はあるが多くの作物でみられ、とくにアマ、エンドウ、ゴボウ、スイカ、トマト、ナスなどは著しく減収する。これらの作物を栽培した畑では、数年から10年くらいは同一作物の栽培をしないほうがよい。しかし、一般にイネ科の作物はほとんど忌地がなく、水田作のイネはまったく忌地を示さない。忌地の原因は十分に解明されてはいないが、いくつかの要因が複雑に影響しあっていることが多いと考えられる。その要因として、土壌中の特定の養分の消耗、病原菌や線虫、害虫の蔓延(まんえん)、作物の根からの生育阻害物質の分泌、残根中の毒素、土壌の物理性の変化などがあげられている。忌地の対応策として土壌消毒や客土などが行われているが、いずれも根本的な解決策とはなっていないため、連作を避け、他の作物と輪作をすることが望ましい。

［星川清親］

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