99

Unique Properties of Volcanic Ash Soils
Masami NANZYO
Graduate School of Agricultural Science, Tohoku University
1-1, Tsutsumidori-Amamiyamachi, Aobaku, Sendai, 981-8555 Japane-mail address: nanzyo@bios.tohoku.ac.jp
Abstract
Various kinds of soils can be formed from volcanic ash depending on the individual set of soil
forming factors at different sites. Among these soils, Andisols or Andosols show unique properties
mostly due to abundant noncrystalline materials such as allophane, imogolite, Al-humus complexes,
ferrihydrite and so on. Highly porous structures made of aggregated noncrystalline Andisol materials
have a light and fluffy nature, accommodating large amounts of both plant-available and hygroscopic
water. They show many other unique physical properties different from other soils. Moreover,Andisols have unique chemical properties, including aluminum-rich elemental composition, large
amount of humus accumulation in A horizons, variable charge characteristics, high phosphate retention
capacity, high affinity for multi-valent cations, high KCl-extractable and water-soluble Al in
nonallophanic Andisols at an acidic to weakly acidic pH range, and so on. Finally, topics related tobiological activity in volcanic ash soils are discussed regarding P supply from apatite and K supply frombiotite for crops in fresh volcanic ash, effects of KCl-extractable and water-soluble Al on soil-borne
diseases, adsorption characteristics of viruses on Andisols and P tropistic root growth of Brassica plants
in P-deficient Andisols.
Key words: Andisol, Andosol, elemental composition, noncrystalline material

1. Introduction
Volcanic eruptions are mostly violent andhazardous. The land around volcanoes is intermittently attacked by pyroclastic flows, volcanic ash
deposition, lahar deposition and so on. These volcanic ejecta or tephras contain various silicates andother minerals of different sizes such as volcanic glass,feldspars, quartz, hornblend, hypersthene, augite,
magnetites, biotites and apatites. After tephra deposition, soil formation starts, the tephra's elemental andmineralogical composition changes and volcanic ash
soils or Andisols having unique properties are formed.
Many plant nutrient elements are provided to the soil
environments during soil formation.
The word “Andisols” is used regarding soils thathave unique and distinctive properties from other soils
as mentioned by Takahashi and Shoji (this issue).
The expression “volcanic ash soils” means any soilthat is derived from tephras. Thus, volcanic ash soils
in this sense show a wider range of properties thanAndisols. Slightly or little weathered tephras are
sandy or gravelly Entisols. Very old and highly weathered volcanic ash soils may no longer be Andisols
but Ultisols or Oxisols (Ugolini and Dahlgren, thisissue). Although a soil name is given for an individual pedon or a soil profile, the word “Andisol” is also

used here for soil samples taken from horizons thathave Andic soil properties.
Some unique physical properties of Andisols are
directly visible to the eye and sensible to the touch.
Surface soils are rich in humus and dark in color, soil
clods are light, fluffy and easy to break into small
pieces. Drainage or water permeability is so highthat farmers can do field work on the next day afterrain. Water from volcanic areas is mostly transparent
due to poorly dispersible clays in the soils at a neutral
pH range. These physical properties are suitable forgrowing various upland crops (Shoji and Takahashi,
this issue).
Volcanic ash soils or Andisols have also distinctive
chemical and mineralogical properties although they
are not visible directly. Chemical and mineralogicalcharacteristics of Andisols are reflected in their
physical properties and biological activities and affectthe utilization of these soils. The unique chemical
properties of Andisols are basically due to their
aluminum-rich elemental composition, the highly
reactive nature of their colloidal fractions and theirhigh surface area. The aluminum-rich elemental
composition of these soils is obtained after leaching of
Si, Na, Ca and so on during Andisol formation.
The natural vegetation of Andisols is similarly
abundant as in other soils under humid climatic

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M. NANZYO

conditions. However, when agricultural crops areplanted in newly reclaimed Andisols, they show
inferior growth to that in young alluvial soils. The
reasons for this are often their low content ofplant-available nutrient elements, especially P, andsome micronutrients, sometimes their high toxic Al
content, the highly sorptive properties of the nutrient
ions and stabilization of soil organic N.
Ameriolation of these shortcomings is more or lessconducted in agricultural Andisols in many volcanic
countries.
In this paper, the unique mineralogical, physical,
chemical and biological properties of volcanic ash
soils, mainly Andisols, are introduced and discussed.
Review articles or chapters on Andisols in textbooks
have been written by Buol et al., (1997), Kimble et al.,(2000), and Ping (2000) and by Harsh et al. (2002) onallophane and imogolite after a monograph of Andisols was published by Shoji et al. (1993).

2. Mineralogical Properties
Morphological properties of matured Andisol profiles are quite different from those of new airborne ash
deposits, lahar deposits, pyroclastic flow deposits andso on. There is no horizon development as in Andisols in the new tephra deposits. However, one caneasily understand that the new tephra deposits will be
converted to Andisols over time by seeing the buried
multi-sequum profile (see Fig. 5a of Takahashi andShoji, this issue) underlying the new tephra deposit.
It is easy to see that new tephras are different from
Andisols in many aspects. Among them, mineralogical properties are the basis for understanding thedifferences in physical, chemical and biological properties between new tephras and matured Andisols.
2.1 Primary minerals and their roles in the soil
environment

Fig. 1

The major primary minerals in fresh tephras arevolcanic glass, plagioclase, quartz, pyroxenes, opaqueminerals, hornblende, biotite, olivine and so on. The
relative abundance of these minerals in fresh tephras
is schematically summarized in Fig. 1 as a function ofrock type (Shoji, 1983; Shoji et al., 1993). Mineralogical composition of tephras is also dependent on
particle size: the content of crystalline minerals suchas feldspar and heavy minerals tends to be more orless high in the fraction coarser than 0.1 mm in diameter and that of volcanic glass tends to be abundantin the particle size fraction finer than 0.1 mm indiameter (Yamada and Shoji, 1975).
The most abundant mineral in volcanic ash is
volcanic glass. The type of volcanic glass is largely
divided into noncolored and colored volcanic glass.
Rhyolitic, dacitic and andesitic tephras contain noncolored volcanic glass while basaltic tephra contains
colored volcanic glass. Colored volcanic glass has alower SiO2 content and a higher refractive index.
The morphology of volcanic glass also varies depending on chemical composition, gas content of magma
and type of volcanic eruption (Yamada and Shoji,1983).
Not only major minerals but also minor minerals
are important in volcanic ash because minor minerals
contain some essential elements for plants. For example, apatite contains phosphorus and tourmaline,
boron. Although K can be supplied from various
minerals, biotite releases K very easily.
2.2 Secondary minerals and their role in soil
environment
2.2.1 Allophane
A hollow spherical structure was proposed forallophane. Allophane has a diameter of 3 to 5 nm
under a transmission electron microscope and its
atomic Si/Al ratio ranges between 0.5 and 1 (Fig.

Relationship between tephritic material rock types and their mineralogy (adapted from Shoji et al., 1993.
Copyright Elsevier, with permission).

Unique Properties of Volcanic Ash Soils

Fig. 2

101

Transmission electron micrographs of secondary minerals. (1) Allophane and imogolite, (2) opaline silica, (3)halloysite and (4) chloritized 2:1 minerals (courtesy of S. Shoji).

2-(1)). According to selective dissolution analysis
using acid oxalate and pyrophosphate solutions, allophane with atomic Si/Al ratio of 0.5 is more common
than that with about 1 (Shoji et al., 1993). As in theimogolite structure, a monolayer of gibbsite sheet isthought to be located outside and silicate, inside of thehollow spherical structure. There are several deficit
holes in the spherical structure. Water molecules andcations such as NH4+ or Ca2+ appear to pass throughthe deficit holes. Allophane shows variable charge
characteristics and high selectivity for divalent cations,and is highly reactive with phosphate (Wada, 1989).
2.2.2 Imogolite
Imogolite was named after a Japanese local soil
called “Imogo” in Hitoyoshi, Kumamoto Prefecture,
where this mineral was first found. It has an atomic
Si/Al ratio of 0.5 and a tubular structure with an innerdiameter of 1 nm and outer diameter of 2 nm (Fig.
2-(1)). Imogolite tubes are more visible under atransmission electron microscope than unit particles ofallophane. Imogolite has a nesosilicate structure.
Imogolite occurs in volcanic ash soils mostly as a
mixture with allophane. Imogolite is less reactivewith phosphate than allophane (Henmi et al., 1982).
2.2.3 Pedogenic opaline silica
Pedogenic opaline silica has a thin ellipsoidal
shape with a diameter of up to several µm (Shoji and
Masui, 1971). Pedogenic opaline silica can be morphologically distinguished from biogenic opaline
silica such as plant opal or diatoms (Fig. 2-(2)).
While biogenic forms have their own shapes inheritedfrom biological cells, pedogenic opals show a rather
simple morphology. They are found in the A horizons of Andisols relatively younger than several thousands of years (Shoji et al., 1993). The formation
process of opaline silica is thought to be concentrationand precipitation of silica released from volcanic glassby evaporation or freezing of soil water. Complexation of Al by humus appears to prevent Si from reacting with Al. Chemical functions of opaline silica are
not outstanding.
2.2.4 Kaolin minerals
Halloysite (Fig. 2-(3)) is frequently found in volcanic ash soils in various amounts. Basal spacing of
halloysite is 1 nm or 0.7 nm depending on the soil
environment. Halloysite (1 nm) has a monolayer ofwater at the interlayer position of the 1:1 aluminosilicate layer. Dehydration of the interlayer water irreversably reduces the basal spacing to 0.7 nm. It
shows various morphologies such as curled plate,lath-like plate, tubular, and spheroidal. Halloysite isformed under semi-dry climatic conditions where silicate concentration in the soil solution is high. Volcanic ash soils having all of allophane, imogolite andhalloysite are rather common in humid areas although
their composition varies widely. Halloysite also occur in the lower Andisol horizons where percolation ofsoil water is slow and water content is high. Under
such conditions, SiO2 content in the soil water appears
to be greater and suitable for halloysite formation.
Halloysite-rich soils often show high CEC andselectivity for K+ and NH4+ (Takahashi et al., 2001).

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M. NANZYO

2.2.5 2:1 minerals and chloritized 2:1 minerals
Chloritized 2:1 minerals (Fig. 2-(4)) are a group of
2:1 minerals that have polymerized hydroxy Al ionsand exchangeable Al ions in their interlayers. Theseminerals are stable under acidic soil conditions ataround pH 5. Although these minerals are the major
clay minerals in the A horizons of nonallophanic
Andisols, properties of humus complexed with Al
often appear strongly compared with those of chloritized 2:1 minerals, because, the Al-humus complex
content is abundant and polymerized hydroxy Al
interlayering reduces CEC occupying negatively
charged sites, reduces swelling and increases water
permeability. Strong treatment using hot citric acidis employed to remove hydroxy Al polymers fromnatural chloritized 2:1 minerals. However, an artificially prepared hydroxy Al-montmorillonite complex
reacts with phosphate to release hydroxy Al polymers
from their interlayers (Saha et al., 1998), suggesting
diversity of the polymerized hydroxy Al interlayerings.
2.2.6 Ferrihydrite
Poorly crystalline to virtually noncrystalline ferrihydrite is a major secondary iron mineral in Andisols.
Ferrihydrite has a brownish color in Bw horizons ofAndisols. Sorption of silicate, phosphate or organic
material by ferrihydrite retards crystallization to goethite or hematite (Bigham et al., 2002). The content
of ferrihydrite can be estimated by multiplying 1.7
times the oxalate-extractable Fe (Feo). Poorly crystalline ferrihydrite has a large surface area and highphosphate retention capacity.

3. Physical Properties
Matured Andisols typically have a low bulk density of 0.9 or lower and are lowest among mineralsoils. The low bulk density of Andisols can easily berecognized if one picks up an air-dried soil clod by

Fig. 3

hand. Only organic soils or organic horizons havelower bulk densities than Andisols. The low bulk
density is due to high porosity caused bywell-developed aggregate structures made ofnoncrystalline minerals. Figure 3 shows that bulk
density values decrease with an increase in the
allophane content of Andisols with low humus content.
The particle density of allophane is almost the same asthat of other layersilicate clay minerals (Wada, 1989).
Thus, allophane itself is not the reason for the low
bulk density of Andisols. Accumulation of a large
amount of humus, making highly porous aggregates,
is also an important reason for low bulk density innonallophanic Andisols. Figure 4 clearly shows thatbulk density decreases with increasing humus content.
Similarly, allophanic Andisols with low allophane
content have a low bulk density if they contain asubstantial amount of humus as shown in Fig. 3.
Although the particle density values of humus areusually lower than 1.5 (Blake and Hartge, 1986) andthe humus content of matured Andisols are mostly lessthan 30%, their typical bulk density is still 0.9 orlower. Using Ti content as an immobile element
during Andisol formation, the soil volume increase
due to humus accumulation was strongly suggested as
an important factor in the low bulk density(Nieuwenhuyse and Van Breemen, 1997).
Pore size distribution in Andisols shows a wide
range. With advancement in formation of noncrystalline materials, the percentage of macropores decreases and that of micropores increases. However,matured Andisols still have a macropore percentage ofaround 10% due to their aggregated structure. Thus,
they have macropores and micropores which contribute to air permeability and retention of plant-available water, respectively. Further, high porosity contributes to quick gravitational drainage of water in thesoils.
The high stability of aggregates means low dispersibility of colloids in Andisols. Not only is humus

Relationship between the oxalate-extractable Si and bulk density of allophanic Andisols (adapted from Shojiet al., 1993. Copyright Elsevier, with permission).

Unique Properties of Volcanic Ash Soils

Fig. 4

Relationship between organic C content and bulk densities of nonallophanic Andisols (adapted from Shoji etal., 1993. Copyright Elsevier, with permission).

Fig. 5

Effect of pH and phosphate fertilizer application on the dispersion of allophanic clays (prepared from Nanzyo
et al., 1992).

digestion using hot H2O2 needed to disperse noncrystalline clays, but also ultrasonic treatment, pH adjustment and repeated washing with deionized water.
The effect of pH on dispersion of allophanic Andisols
reflects their variable charge characteristics. Allophanic clays have a point of zero charge at about
neutral pH and they hardly disperse around this pH.
On the other hand, allophanic clay disperses underacidic and alkaline pH conditions developing positive
and negative charges, respectively (Fig. 5).
Hydraulic conductivity of an allophanic Andisol withlittle humus decreases with addition of dilute HNO3
due to dispersion of the allophanic clay at very low
pH of 3 to 4, but not with dilute H2SO4 (Ishiguro andNakajima, 2000), possibly due to the coagulative
effect of the divalent anion on the positively charged
colloids. The size of the domain consisting of allophanic particles in the aqueous dispersion system ishundreds of nm, dependent on pH, and is much larger
than a unit particle of allophane (Karube et al., 1996).
If field moist soil samples are used, their clay

103

content obtained by this procedure almost coincides
with that calculated using the oxalate-extractable Al(Alo) and Feo content. When allophanic Andisols are
air-dried, their colloidal fraction irreversibly aggregates and the sand-sized fraction increases andclay-sized fraction decreases. Surface soils may
experience drying under natural conditions.
Therefore, the dispersion method is often ineffective
for determining the clay content of surface soils thatexperienced natural drying. Instead, the inorganic
noncrystalline materials content can be calculated using Alo and Feo contents. Other irreversible changes
with air-drying are decreased liquid limit and plastic
limit, water retention, and so on (Shoji et al., 1993).
Dispersibility of allophanic clay is also affected byheavy application of phosphorus fertilizer as shown in
Fig. 5 (Nanzyo et al., 1992). This suggests that
positive charge development is perturbed by phosphorus fertilizers due to sorption of phosphate at
positive charge sites. Dispersion under alkaline pH
conditions is little affected by phosphorus fertilization.

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M. NANZYO

Fig. 6 Total content of elements in Andisols (adapted from Nanzyo et al., 2003).

4.

Chemical Properties

In accordance with changes in mineralogical properties during the formation of Andisols from fresh
tephras, changes in elemental composition of the soils
also take place. The mineralogical changes are reflected in changes in soil chemical properties. The
unique chemical properties of Andisols are mainly
caused by their noncrystalline secondary minerals andAl-humus complex.
4.1 Elemental composition
The ranges and means of 57 total elemental analyses in allophanic Andisols are shown in Fig. 6 on alogarithmic scale. The element content was highlyvariable and the maximum/minimum values ranged
between 2 and 300. The maximum/minimum value
for Si, Al and Fe were rather narrow ranging between
2 and 4. The mean content of the 12 major elements(C, N, Na, Mg, Al, Si, P, K, Ca, Ti, Mn and Fe) was
more than 1 g kg–1 and the others were less than these.
The Oddo-Harkins’ law that the content of theeven-numbered atomic elements is greater than thoseof the neighboring odd-numbered ones was clearly
demonstrated for the lanthanum series elements, theatomic numbers of which proceed from 57 (La) to 71(Lu) (Yamasaki et al., 2001).

Many factors such as the rock type of the tephras,
noncrystalline material contents, and biological activity can be involved in high maximum/minimum
values of the 57 element contents in volcanic ash soils.
The relationship between oxalate-extractable Si (Sio)and the total content of the 57 elements is shown in
Fig. 7 (Nanzyo et al., 2003). A positive and highcorrelation was observed both between Sio and thetotal Al content and between Alo and the total Al
content. In contrast, the total Si content decreased
with increase of both Sio and Alo. These results arecompatible with the previous report that Al remains insitu (Ugolini, 1988), Si is leached out andallophane-imogolite and/or Al-humus are formed
during the weathering of volcanic ash (Shoji et al.,1993).
Alkaline and alkaline-earth elements such as Na, K,
Ca and Sr significantly decreased with an increase in
the Sio, Alo or Feo content (Table 1). The first transition metals, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu
correlated positively with the noncrystalline material
contents. Thus, these elements were residually enriched in the volcanic ash soils. Feo correlated morestrongly with these metals than Sio and Alo (Table 1)suggesting that these elements accumulate with ferrihydrite rather than with allophane-imogolite during
Andisol formation.

Unique Properties of Volcanic Ash Soils

Regarding the total content of the lanthanum series
elements, the correlation coefficients with both Sio and
Alo gradually increased with their increasing atomic
number. Among the elements shown in Fig. 7, La,
Th, and U appear to show rather poor correlation with
Sio. However, the contents of these elements are105

strongly dependent on the rock type of the parent
tephras. Basalttephras have only a small content ofthese elements. On the other hand, dacitic tephras
have higher content and stronger correlation with Sio.
Yttrium showed a significant correlation with the noncrystalline material contents (Table 1).

Fig. 7 Relationship between oxalate-extractable Si (Sio) and total element content in Andisols (adapted from Nanzyo et al., 2003).
Table 1

Correlation coefficients between content of oxalate-extractable Si (Sio), Al (Alo) or Fe (Feo) and those of individual
elements of volcanic ash soils in Japan. A coefficient of 0.33 or more for 95 samples and 0.37 or more for 74 samples(shown in boldface) is significant at the 0.1% level.
Sio
Alo
Sio
1.00
Alo
0.93 1.00
Feo 0.37 0.46
95
n*
95*: number of samples.
V
0.18
0.17
0.58
95

Feo

Li
0.02
-0.04
1.00 -0.16
95
95

Be
C
N
Na
Mg
0.49 -0.45 -0.46 -0.39 0.00
0.46 -0.19 -0.24 -0.56 -0.02
0.10 0.10 0.16 -0.43 0.02
95
95
95
95
95

Al
Si
P
K
Ca
0.80 -0.02 -0.32 -0.21 -0.32
0.73 -0.27 -0.22 -0.30 -0.47
0.48 -0.61 0.12 -0.47 -0.10
95
95
95
95
95

Sc
0.39
0.36
0.58
95

Cr
0.11
0.24
0.48
95

Fe
0.41
0.41
0.72
95

Ga
0.17
0.14
0.42
74

Ge
Rb
Sr
0.36 -0.04 -0.33
0.38 -0.07 -0.51
0.63 -0.27 0.05
74
95
95

Y
0.47
0.42
0.39
95

Zr
0.50
0.40
0.15
95

Ce
0.41
0.37
0.09
95

Pr
0.39
0.37
0.19
95

Eu
0.49
0.43
0.52
95

Gd
0.51
0.46
0.44
95

Sio
Alo
Feo
n

Ti
0.49
0.49
0.79
95

Sio
Alo
Feo
n

Nb
Mo
Ag
Cd
0.34 -0.11 -0.10 -0.25
0.30 -0.19 -0.09 -0.30
0.03 -0.06 0.17 0.03
95
86
74
74

Sn
Sb
Cs
Ba
0.29 -0.04 -0.04 -0.10
0.14 0.01 -0.02 -0.21
0.25 -0.03 -0.14 -0.05
33
33
95
95

Sio
Alo
Feo
n

Tb
0.52
0.49
0.41
95

Tm
0.53
0.50
0.24
95

Dy
0.48
0.41
0.26
95

Ho
0.54
0.50
0.43
95

Mn
0.34
0.30
0.42
95

Er
0.53
0.46
0.37
95

Co
0.27
0.31
0.71
95

Yb
0.51
0.42
0.22
95

Ni
0.14
0.24
0.48
95

Lu
0.56
0.54
0.30
95

Cu
Zn
0.08 -0.10
0.11 -0.14
0.74 0.04
95
95
La
0.29
0.29
0.04
95

Hf
Ta
W
0.52 0.33 0.16
0.43 0.27 0.12
0.17 -0.03 -0.08
95
95
95

Nd
0.41
0.38
0.27
95

Sm
0.44
0.39
0.26
95

Tl
Pb
Bi
Th
U
0.14 -0.03 -0.31 0.28 0.17
0.08 0.01 -0.30 0.26 0.12
0.24 -0.02 -0.11 -0.11 -0.12
92
95
92
95
95

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M. NANZYO

4.2 Organic constituents
Large amounts of humus are stored in the A andburied A horizons of Andisols. Organic C content ofAndisols ranges between 0 and about 200 g kg–1. An
important reason for the high humus accumulation isstabilization of humus by complexation with Al. In
the buried A horizons, weathering of glass continues
while fresh organic matter is not supplied anymore.
In these buried A horizons, it appears that easily
decomposable organic matter no longer remains andthe humus is fully complexed with Al. Stabilizationof humus by complexation with Al is strongly
suggested by the linear relationship between
pyrophosphate extractable Al (Alp) and organic C
content. The atomic ratio of Alp to organic C isabout one to thirteen.
The color of the humus and vertical distribution oforganic C content are highly dependent on vegetation.
Black humus in many cases accumulates under grassy
vegetation of C4 plants and it leads to formation of amelanic epipedon. Contribution of C4 plants to formation of melanic epipedon was reconfirmed by thedelta 13C value of the humus. It is well known thatC3 plants tend to select 12C while C4 plants show little
selection. Figure 8 shows typical delta 13C values of
C3 and C4 plants and vertical distribution of
humus-rich horizons from a Melanudand. As thepresent vegetation of this site is forest, the delta 13C
value is low in the uppermost A horizon. But, thelower humus-rich horizons showed higher delta 13C
values indicating significant contribution of C4 plants
to black humus formation (Yoneyama et al., 2001).
Black humus is different from others in the characteristics of the humic acid fraction. It was classified
as an “A type humic acid” according to Kumada

Fig. 8(1981).
Figure 9a shows a comparison of theelemental composition of humic acids prepared fromblack humus-rich horizons of Andisols and those fromvarious soils in the world. While the C content of
the black humus is almost in the same range, the O
content tends to be higher and the H content tends tobe lower than those of the other soils. These
differencies in elemental composition indicate thatthe humic acids from black Andisols are in a moreoxidized state than those from the other soils.
Figure 9b shows the functional group content ofhumic acids from the black Andisols and those fromother soils in Japan. Humic acids from the black
Andisols tend to have larger amounts of carboxyl andcarbonyl groups and lower amounts of the alcoholichydroxyl group than those of the other soils. Thus,
the functional group composition also shows that thehumic acids from black Andisols are in a more oxidized state than those of other soils. The molar ratioof carboxyl groups to organic C is one to ten. This
was also validated by infrared spectroscopy(Yonebayashi and Hattori, 1988) and proton and 13C
nuclearmagnetic
resonance
spectroscopy(Yonebayashi and Hattori, 1989; Yonebayashi, 1993).
The melanic horizon enriched with the A-typehumic acid is separated from the others using melanic
index (Honna, 1988). Using visible range absorption
spectroscopy of the alkaline extract of the melanic
epipedon, almost straight lines are obtained between
the logarithm of the absorbance and wavelength.
The melanic index is the ratio of absorbance at
450 nm to 520 nm and the A-type humic acid has amelanic index of 1.7 or less.
P-type humic acid is found in the dark brown
humus formed under forest vegetation. It has the absorption maximum at 450 nm and a minimum at 520

Vertical distribution of δ 13C value of an Andosol (Imaichi), (after Yoneyama et al., 2001). C3 and C4 plants
show δ 13C values of -27 and -13, respectively. The δ 13C value of the uppermost A horizon of this site is -26
and is due to current C3 forest vegetation. The underlying horizons have δ 13C value of -19 indicating
significant contribution of C4 plants.

Unique Properties of Volcanic Ash Soils

Fig. 9107(a) Elemental composition of humic acids from Japanese black Andisols (Yonebayashi and Hattori, 1988) andfrom various soils in the world (Schnitzer, 1978). (b) Functional group content of humic acids fromJapanese black Andisols and other soils from Japan (Yonebayashi and Hattori,1988).

Fig. 10 An approximate formula of an Al-humus complex at pH7 (adapted from Nanzyo and Shoji, 1993).

nm in the visible range absorption spectra due to somecompounds including perylene quinone (Kumada,1981) and shows a melanic index higher than 1.7.
In accordance with the relationships described in
the discussion above, an attempt was made to formulate a partial structure of the Al-humus complex at
pH7. The ratio of the amount of carboxyl groups toorganic carbon content was 4 to 40. The ratio of Al
complexed with humus to the organic C content was 3
to 40. The ratio of the amount of negative charge tothe organic carbon content at pH 7 is 1.1 to 40. Thus,
the approximate formula of the Al-humus complex is
proposed as shown in Fig. 10. Every 40 organic C
include four carboxyl groups, three of which are complexed with Al and one dissociated to have a negative
charge. Chelate type complexes may also be included in the Al-humus complex. Al was tentatively
shown using monomeric forms in this formula.

4.3 Physicochemical properties of Andisols
Allophane, imogolite and Al-humus are basicallythe major components characterizing Andisols. Theyshow variable charge characteristics, high phosphate
retention capacity, high affinity for multi-valent cations and a contrasting effect on KCl-extractable andwater-soluble Al at an acidic pH range. Ferrihydrite
also has variable charge characteristics and a high P
retention capacity.
The variable charges are negative and positive
charges the amounts of which are dependent on pH
and salt concentration in the equilibriating liquid
phase.
Andisols have a large amount ofvariable-charge colloids.
Functional groups
contributing to variable negative charges are carboxylgroups of humus and silanol groups of allophane andimogolite. The amount of negative charge increases

108

M. NANZYO

with an increase in salt concentration and pH values.
Variable positive charges are due to the protonatedhydroxyl groups bound to aluminum of allophane andimogolite and those bound to iron minerals. The
amount of positive charge increases with an increasein salt concentration and a decrease in pH value.
However, in the allophanic A horizons, positive
charges appear to be masked by carboxyl groups ofhumus and only the variable negative charge develops.
Similarly in the humus-rich nonallophanic A horizons,
only variable negative charges develop due to humus
and chloritized 2:1 minerals. Both positive andnegative charges develop in the allophanic Bw
horizons of Andisols.
The amount of variable negative charge tends to beoverestimated if it is compared with actual field conditions under humid soil moisture regimes. Because
salt concentrations of soil solutions are mostly low in
the field due to rainfall and soil pH is mostly weakly
acidic while the amount of negative charge or cationexchange capacity is determined using an ammonium
acetate solution of 1 mol L–1 at pH 7, values that are
higher than those in the field.
The variable negative charge shows high selectivity for multi-valent cations such as Ca2+, Mg2+, Ba2+and heavy metal cations. Selectivity for Ca2+ andMg2+ further increase with sorbed P content inAndisols (Uhm et al., 1994). KCl-exchangeable Alcontent, mostly in the form of Al3+ (Saigusa et al.,1980), is abundant in the uncultivated or unlimed
nonallophanic Andisols and low in allophanic
Andisols except those under highly acidified conditions such as tea garden soil in Japan.
Matured Andisols show high phosphate retention
capacity. The reactive components in Andisols areallophane, imogolite, Al-humus complex, ferrihydrite,
KCl-extractable Al, and polymeric hydroxy Al ions in
the interlayer of chloritized 2:1 minerals. The
reactive Al and Fe in these components are called
active Al and active Fe, respectively (Wada, 1980).
The phosphate retention capacity depends on thecontent of these components in Andisols. Reactivityof F–, arsenate, and so on with active Al and Fe
resembles those with phosphate. These anions react
not only with positively charged sites but also neutral
sites, substituting some ligands such as -OH
and -Si(OH)3 and are more reactive with active Al andFe than are Cl–, NO3–, ClO4–, SO42–.
Large amounts of P fertilizers are applied to Andisols in intensive agriculture. However, the P recovery
rate by crops is low due to fixation by the Andisols
and most of the P accumulates in the plow layer ofAndisols.
The active Al in Andisols appears to retard development of low Eh (reduction potential) conditions tosome extent. Andisols are mostly located on uplands
but they are used in a small percentage for paddy
fields in Japan. Decrease in Eh values is slower inAndisols than in lowland paddy soils. (Shoji et al.,1993).

5. Properties of Volcanic Ash Soils Related
to Biological Activities
Time zero of Andisol formation is the deposition of
new volcanic ash. Several years after the eruption of
Mt. Usu, Japan in 1977, bacterial flora started to grow(Yoshida, 1993), followed by growth of fungal flora.
Regarding N fixers, anaerobic ones started to growvery early after the tephra deposition and appeared tohave contributed to the initial organic matteraccumulation, possibly because anaerobic N fixers cangrow even with a very poor organic C source.
Subsequently, the number of aerobic N fixers increased about 10 years after the eruption. Growth ofyeasts was poor for about 15 years after the eruption
at the Mt. Usu site.
Microbial activity is not very high in unameliorated Andisols due to P deficiency, acidic to weakly
acidic pH and high stability of organic matter (Nishio,1996). Addition of phosphates to Andisols with pH
higher than 5 increases the number of aerobic bacteria,
actinomycetes and fungi and enhances soil respiration.
When the number of microbes was compared in uncultivated lands, Andisols had significantly more actinomycetes and anaerobic bacteria than other soils inJapan. Similar results were also obtained in cultivated lands.
Although its solubility is low (Engelstad et al.,1974), the total phosphorus content of fresh tephras is
generally higher than that of uncultivated alluvial soils.
The major form of phosphorus in fresh rhyolitic toandesitic tephras is apatite (Kawasaki, et al., 1992;
Nanzyo et al., 1997; Nanzyo and Yamasaki, 1998).
This mineral plays an important role in crop production without phosphorus fertilization and in revegetation of areas with thick deposition of volcanic ash.
Figure 11 shows SEM images of apatite from new(Fig. 11a) and old (Fig. 11b) lahar deposits (older than
500 years) in the Central Plain of Luzon. The former
has a smooth surface while the latter has a rough surface and partial dissolution. Paddy soil formed fromthe old lahar deposit showed a lower content of total
and acid-soluble P in the plowed layer than in the lower horizons. These findings suggest that apatite significantly contributed in this area to rice production.
The old volcanic ash soils in the Central plain of
Luzon are not Andisols but Ustepts or Ustipsamments
possibly due to high Si concentration in the soil
solution resulting in formation of layer silicates suchas halloysite rather than allophane and imogolite. In
these soils, apatite is not readily transformed into
insoluble aluminum phosphate.
Biotite is also a minor component of volcanic ash,but, it can readily release K, one of the major essential
plant nutrients, from an interlayer site resulting information of trioctahedral vermiculite (Nanzyo et al.,
1999; 2001). Figure 12 shows changes in K content
of biotite particles during rice cultivation using

Unique Properties of Volcanic Ash Soils

109

a

b

AP

AP
Fig. 11 SEM images of apatite (AP) particles separated from 1991 Mt. Pinatubo volcanic ash (a: adapted from
Nanzyo et al., 1997) and a lower horizon of a paddy field soil (b: lahar deposit older than 500 years).

Fig. 12

Changes in K2O content of Pinatubo biotite with lowland rice cultivation. Bars show standard deviation of
measured means. Different letters by the symbols indicate significant difference at the 0.05 probability
level according to the t-test (adapted from Nanzyo et al., 1999).

Al conc. in the water extract ≧0.3 mg L–1
0.3>Al conc. in the water extract ≧0.2 mg L–1
Al conc. in the water extract <0.2 mg L–1
Fig. 13

Relationship between pH(H2O) and log(OC/Sio) of 3 groups of soils showing different Al concentrations in
the water extract (adapted from Nanzyo et al., 2000).

Pinatubo ash from the 1991 eruption.

The K content

110

M. NANZYOof the biotite particles decreased with cumulative daysof rice cultivation.
Allophanic and nonallophanic Andisols show contrasting effects on the behavior of KCl-extractable andwater soluble Al (Takahashi and Dahlgren, 1998).
These forms of Al are toxic to many sensitive crops,
but show a suppressive effect against potato scab
(Mizuno et al., 1998) and bean root rot (Furuya et al.,1999). Content of these forms of Al is high in nonallophanic Andisols and low in allophanic Andisols.
High Al concentration of 0.3 mg L–1 or more in thewater extract is considered to provide resistance topotato scab. Although water-extractable Al is negatively correlated with soil pH values, soils with low tohigh levels of water-extractable Al are included between pH(H2O) levels of 5 and 6 (Fig. 13). The im-

Fig. 14 SEM images of weathered Kanuma pumice with tobacco mosaic virus (TMV) adsorption (i) and acidoxalate-treated Kanuma pumice with TMV addition (ii). Scale = 1 µm (adapted from Toriyama et al., 1995).

(i)

a
b

(ii)
Fig. 15(i) Development of main and lateral roots of B. pekinensis in a P-deficient soil. The dark spherical objects
are the DCPD gel beads covered with the lateral roots. (ii) Cross section of one of the dark sphericalobjects in (i). Soil is located outside the root-covered DCPD gel bead. There is no soil between the roots(a) and the DCPD gel bead surface (b) (adapted from Nanzyo et al., 2002).

Unique Properties of Volcanic Ash Soils

portant factors affecting water-extractable Al levels inthis pH region were organic C content and Sio. The
soils showing high water-extractable Al levels, contained a larger amount of organic C and a smaller
amount of Sio than those with low water-extractable
Al levels.
Allophanic clays adsorbed tabacco mosaic virus
because adsorption of the virus was not observed after
removal of allophanic clays with acid oxalate treatment (Fig. 14). Although the virus was not adsorbed
by an A horizon soil of a nonallophanic Andisol, thevirus ceased multiplication suggesting the soil had ananti-virus effect (Toriyama et al., 1995).
Under P deficient conditions in an uncultivatedAndisol, Brassica roots show almost P-tropistic
growth (Nanzyo et al., 2002). The thin lateral rootsof Brassica pekinensis, L., completely encircled
DCPD gel beads (a Ca-alginate gel in which a
CaHPO4 · 2H2O powder was suspended) and no soil
area remained between the roots and the DCPD gel
beads (Fig. 15). This morphological plasticity ofBrassica roots is suitable for improving the P recovery
rate because soil a few mm thick with a high P retention capacity between plant roots and P fertilizer
reduces the recovery rate of P fertilizers by crops in aP-deficient soils (Nanzyo et al., 1996). P-preferential root proliferation was also observed for
Fagoppyrum esculentum Moench.

Acknowledgements
The author thanks Emeritus Professor S. Shoji and
Mr. T. Sato, Graduate School of Agricultural Science,
Tohoku University, for providing valuable commentson the manuscript and operating SEM-EDX analysis,
respectively. This work was partly supported by aGrant-in-Aid for Scientific Research (No. 12660055)
from the Ministry of Education, Science, Sports andCulture of Japan.
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(Received 5 November 2002, Accepted 20 December 2002)