Volcanic ash soil

Japanese: 火山灰土壌 - かざんばいどじょう（英語表記）volcanic ash soil

Soil whose parent material is volcanic ash. When ash, sand, and gravel released into the air during a volcanic eruption fall to the ground, they form a volcanic ash layer. Volcanic ash begins to form soil when volcanic activity ceases or declines, but during this short period of time, it does not develop into soil that fully reflects the effects of climate and vegetation (climatic soil or zonal soil), and remains in an immature state that retains the unique properties of the volcanic ash layer. Soil in this state is called volcanic ash soil because the retained properties are very distinctive, and the state is expected to last for quite a long time. Volcanic ash soil can be further divided into the following two types.

(1) Young volcanic ash soil is close to the eruption source and is therefore rich in coarse-grained scoria (rock scaup) and pumice (pumice derived from acidic volcanic rocks). It has been weathered very quickly and was formed over a period of less than several hundred years.
(2) Andosols (Ando soils or Andosols, volcanic ash soils in the strict sense) cover a wide area, even in non-volcanic areas far from the eruption source, and are made up of a loamy layer of fine volcanic ash that has weathered to a brown color.
[Shigeo Asami]

Agricultural Use

Volcanic ash soils are distributed throughout Japan, covering an estimated total area of more than 3.1 million hectares. They are particularly prevalent in Hokkaido, Tohoku, southern Kanto, and Kyushu. The surface layer usually contains a large amount of humus and is a distinctive black color, while the lower layer is light brown or brown. In soil series classification, most are classified as black soils, with some classified as humid black soils. Nearly half of field soils are black soils. About 10% of paddy fields are humid black soils. The properties vary depending on the age of the eruption, the parent material, and the degree of weathering, but generally they are lightly wind-accumulated soils that are easily blown away by the wind when dry, and are prone to frost pillars in winter. They have good drainage and are easy to cultivate, but the base is leached out, making them acidic, and aluminum ions that are harmful to plants are activated. This activated aluminum gives volcanic ash soils unique physicochemical properties, such as the fixation of phosphorus and the accumulation of humus. New soils are weakly acidic, but older soils are strong. Effective methods for improving the soil condition include the application of acidic correction materials such as quicklime (calcium oxide), the application of large amounts of phosphate fertilizer, and the application of organic matter such as manure.

[Yuki Koyama]

"Yamada Shinobu, 'Soil formation, classification, investigation and its use - with a focus on volcanic ash soil' (1968, Yokendou)" ▽ "Volcanic Ash Soil - Formation, Properties and Classification' (edited by the Japanese Society of Soil Science and Plant Nutrition) (1983, Hakuyusha)" ▽ "Volcanic Ash and Soil' (edited by Volcanic Ash and Soil Editorial Committee) (1983, Publishing Committee for Prof. Kurobe Takashi's Retirement Memorial Papers, Hakuyusha)" ▽ "Matsui Ken and Kondo Naruo, 'Soil Geography - Soils of the World and Japan' (1992, Asakura Publishing)" ▽ "Analysis of the effects of Volcanic Ash from Mt. Unzen and Mt. Fugen on soil and crops' (edited by Kyushu Agricultural Experiment Station, Ministry of Agriculture, Forestry and Fisheries)" ▽ "Soil Science and Archaeology' (edited by Kuma Kazutake and Nagatsuka Shizuo, supervised by the Japanese Society of Soil Science and Plant Nutrition/Pedologists Roundtable, 2001, Hakuyusha)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

火山灰を母材とする土壌。火山の噴火のときに空中に放出された灰、砂、礫(れき)が地表に落下すると、火山灰層を形成する。火山灰は火山活動の休止または衰退に伴って土壌生成を始めるが、その時間が短い間は、気候、植生の作用を十分に反映した土壌（気候的土壌または成帯性土壌）にまで発達せず、火山灰層のもつ特有な性質を保留した未成熟土壌の状態にとどまっている。あえてこの状態の土壌を火山灰土壌とよぶ理由は、保留された特有の性質が非常に特徴的で、かつその状態がかなり長く持続するとみられるからである。火山灰土壌はさらに以下の二つに大別される。

(1)噴出源に近く、したがって粗粒のスコリア（岩滓(がんさい)）やパミスpumice（酸性の火山岩に由来する軽石）に富み、風化年代のきわめて短い数百年以下の年月で生成された新期火山灰土
(2)噴出源より遠く離れた非火山地域をも広く覆い、細粒の火山灰が褐色に風化したいわゆるローム層を母材とするアンドソル（アンド土壌または黒ぼく土、狭義の火山灰土）
［浅海重夫］

農業利用

火山灰土壌は日本では全国的に分布し、総面積は310万ヘクタール以上とみられる。北海道、東北、南関東、九州にとくに多い。通常、表層は多量の腐植を含み特有の黒色を呈するが、下層は淡褐色または褐色である。土壌統による分類では大部分が黒ぼく土壌群に、一部が多湿黒ぼく土壌群に類別される。畑土壌では黒ぼく土がほぼ半分を占める。水田では約1割が多湿黒ぼく土である。性質は、噴出の年代、母材、風化の程度により異なるが、一般に風積性の軽い土で乾燥すると風で飛ばされやすく、冬には霜柱が立ちやすい。排水がよく耕うんが楽であるが、塩基が溶脱され酸性で植物に有害なアルミニウムイオンが活性化する。この活性アルミニウムにより火山灰土壌はリン酸の固定、腐植の集積など特異な理化学性を示す。酸性の度合いは新しいものは弱いが、年代の経ったものは強い。改良法としては、生石灰（酸化カルシウム）などの酸性矯正資材の施用、リン酸肥料の多投、堆厩肥(たいきゅうひ)などの有機物の施用が有効である。

［小山雄生］

『山田忍著『土壌の生成・分類・調査とその活用――特に火山灰土壌を中心として』（1968・養賢堂）』▽『日本土壌肥料学会編『火山灰土――生成・性質・分類』（1983・博友社）』▽『火山灰と土壌編集委員会編『火山灰と土壌』（1983・黒部隆教授退官記念論文集刊行会、博友社発売）』▽『松井健・近藤鳴雄著『土の地理学――世界の土・日本の土』（1992・朝倉書店）』▽『農林水産省九州農業試験場編『雲仙・普賢岳火山灰の土壌および農作物へ及ぼす影響の解析』（1993・農林水産省九州農業試験場）』▽『日本土壌肥料学会・ペドロジスト懇談会監修、久馬一剛・永塚鎮男編『土壌学と考古学』（2001・博友社）』