Pedological Problems concerning Volcanic Ash in Japan

Abstract

The authors tried to review scientific works on volcanic ash soils in Japan from the pedogenetical point of view.
Morphology: The soils consist of the thick humified loose A horizon and brownish structural B to BC horizon, though in younger soils A-C sequence is common, lacking B horizon. Neither any sign of podzolization nor laterization is recognized. Such a normal profile is often disturbed by denudation, accumulation of surface humic soil materials or by successive addition of recent pyroclastic fall.
Mechanical, chemical and mineralogical composition: Though younger soils mainly consist of sand and gravel in texture, most of matured soils are so highly weathered that the clay content often amounts to about one-half of the total soil mass. Chemical analysis reveals that matured soils have been suffered from severe weathering which results in marked loss of silica and bases leaving clay complex rich in hydrous sesquioxides. The principal mineral of the clay fraction in volcanic ash soils is allophane with some quantity of various layer silicates, gibbsite and other sesquioxide minerals. Allophane is considered to be formed by co-precipitation of hydrous silica and alumina gels both of which are regarded as end products of severe weathering of volcanic glass and plagioclase dominant in original volcanic ash. The genesis of allophane is mainly conditoned by such peculiar physical properties of the parent material as homogeneous fine grain size, extraordinally broad specific surface area and rapid permeability. And it seems that the genesis is accelerated by the warm and superhumid climate in Japan.
Accumulation of humus: The characteristic thick humus layer of volcanic ash soils contains very high amounts of organic carbon (8-30%), and its almost real black colour looks just like as in Chernozem. The raw material must be derived from grass vegetation seeing from the predominace of the phytolith or plantopal in the very fine sand fraction of the humus layer. The chernozemic black colour may be due to highly condensed and polimerized humic acid, which may have been formed in the rather aerobic soil condition of porous volcanic ash materials persisting even under humid monsoon climate. Nevertheless, judging from the fact that more content of the fulvic acid is determined than that of the humic acid in most cases, and that both acids are combined with sesquioxides or allophane and not with Ca, the humus of the soils should have also podzolic character which may be related to superhumid climate and aerobic soil condition. And allophane must be responsible for such a marked accumulation of humus in the volcanic ash soils.
Physical and chemical characteristics: The volume of the solid phase of the soils occupies only 20-30% of the whole soil mass, in other words, the porosity and water holding capacity are extremely high and apparent density is very low. It is the main reason why the volcanic ash soils are not only subjected from rapid weathering and specific humification, but also suffered from wind and sheet erosion, as well as frost damage. Clods of the soils are apt to be broken down into water-stable micro-aggregate (0.05-0.02mm in dia.), which is thought to be bound by loose hydrogen bond in the swelling water with Fe or Al bridge. The soils contain a lot of swelling and hygroscopic water, both of which may be mostly combined with allophane. Besides, such structural OH is released continuously by heating or an ion exchange treatments.
The reaction of surface horizon of the soils is generally acid based on humic acid, but very weakly acid to nearly neutral in subsoil in which allophane is dominant component in the clay fraction, in spite of extremely low degree of base saturation caused by severe leaching under the superhumid climate. The cation exchange capacity of the soils is rather high, though the determination tends to be ill-reproducible