University of Çukurova Departments of Soil Science and Archaeometry ANDOSOLS Dr. Selim KAPUR University of Çukurova Departments of Soil Science and Archaeometry Adana, TURKEY kapur@cu.edu.tr

Volcanism is not randomly distributed over the world. It is concentrated near plate boundaries where plate subduction or seafloor spreading takes place. Other occurrences are linked to deep mantle plumes that reach the Earth's surface at distinct `hotspots'. Figure 1 shows the geographic distribution of major volcanic regions. Landforms in volcanic regions are strongly influenced by the chemical and mineralogical composition of the materials that were deposited during eruptive phases. Volcanic rocks and magmas are grouped according to their silica contents in three main categories labeled `Rhyolite' (65-75% SiO2), `Andesite' (65-55% SiO2) and `Basalt' (55-45% SiO2). The mineralogical properties and chemical composition (notably the contents of K2O, Na2O and CaO) distinguish individual rock types. See Figure 2. This influences profoundly the character and morphology of volcanic phenomena.

MAJOR LANDFORMS IN VOLCANIC LANDSCAPES Figure 1. Volcanic regions of the world

The Reference Soil Group of the Andosols holds soils developed in volcanic materials. Common international names are: Andosols: FAO, Soil Map of the World, Andisols' :USDA Soil Taxonomy, Andosols' and `Vitrisols‘: France and `volcanic ash soils'. ANDOSOL: Black soils of volcanic landscapes From Japan an (black), and do (soil)

MAJOR SOIL FORMING LIMITING FACTOR Steep slopes, volcanic hazard, P fixation, thixotropy and structural instability

PARENT MATERIALS Volcanic ash, tuff, pumice, cinders and other volcanic ejecta TUFF PUMICE http://members.iinet.net.au http://www.athro.com

VOLCANIC EJECTA CINDER http://volcano.und.nodak.edu http://helios.bto.ed.ac.uk

Definition of Andosols Soils having a vitric@ or an andic@ horizon starting within 25 cm from the soil surface; and no diagnostic horizons (unless buried deeper than 50 cm) other than a histic@, fulvic@, melanic@, mollic@, umbric@, ochric@, duric@ or cambic@ horizon. Common soil units Vitric*, Silandic*, Aluandic*, Eutrisilic*, Melanic*, Fulvic*, Hydric*, Histic*, Leptic*, Gleyic*, Mollic*, Duric*, Luvic*, Umbric*, Arenic*, Placic*, Pachic*, Calcaric*, Skeletic*, Acroxic*, Vetic* , Sodic*, Dystric*, Eutric*, Haplic*.

HYDRIC ANDOSOL VITRIC ANDOSOL http://www.soils.wisc.edu/ VITRIC ANDOSOL HYDRIC ANDOSOL One or more layers within 100cm Water ret 1500kPa of 100% or more No andic horizon overlying a vittic horizon

VITRIC ANDOSOL (Central Anatolia) Dingil, 2003. Ph.D. Thesis

KEY TO ANDOSOL SOIL UNITS GELIC ANDOSOLS Andosols having permafrost within 200cm of the surface GLEYIC ANDOSOLS Andosols with gleyic properties within 100cm of the surface VITRIC ANDOSOLS Andosols lacking a smeary consistance or a texture which is silt loam or finer on the weighted average for all horizons within 100cm of the surface or both MOLLIC ANDOSOLS Andosols having a mollic A-horizon UMBRIC ANDOSOLS Andosols having an umbric A-horizon HAPLIC ANDOSOLS Other Andosols

VITRIC: A vitric horzion must: 1. Have 10% or more volcanic glass and other primary minerals in the fine earth fraction (250-50µm); and 2. Have ı. a bulk density less than 0.9kg dm3 or ıı. Alox + ½Feox more than 0.4%, or ııı. Phospahte retention more than 25%, and 3. Have a thickness of 30cm or more ANDIC: An andic horizon must have all of the following 1. A bulk density at field capacity (no prior drying) of less than 0.9kg dm3; and 2. 10 percent or more clay and an (Alox + ½Feox) value2 in the fine earth fraction of 2 percent or more; and 3. 70 percent or more phosphate retention; and 4. less than 10 percent volcanic glass in the fine earth fraction; and 5. a thickness of 30 cm or more.

ENVIRONMENT PROFILE DEVELOPMENT Undulating to mountainous, humid, semi-arid?, arctic to tropical regions with a wide range of vegetation types. PROFILE DEVELOPMENT A-C or A-B-C profile. Rapid weathering of porous volcanic material resulting in accumulation of stable organo-mineral complexes, and minerals such as allophane, imogolite (Al2 SiO3 (OH), and ferrihydrite

Allophane Imogolite http://www.uky.edu http://www.chem.umass.edu Allophane commonly occurs as very small hallow rings or spheres having diameters of approximately 35 - 50 Å. This morphology is characteristic of allophane, and can be used in its identification. Ferrihydrite are hydrous iron oxides. Fe2O3.2FeOOH.2.6H2O. Secondary mineral in an oxidizing environment, strongly dependant on pH. Functions as allophane. The organically bound Fe is most probably ferrihydrite –Fe. Imogolite occurs as very small tubes having inside diameters of 10 Å and outside diameters of 20 Å. These tubes may be several µm in length, and often form bundles of two to several hundred tubes. Occasional branching of tubes may occur.

REGIONAL DISTRIBUTION

Topographic sequence of associated soils ANDOSOLS CAMBISOLS/LUVISOLS VERTISOLS

GENESIS Allophane Volcanic glass Presence of andic (rich in allophane) and vitric (rich in volcanic glass) Allophane Volcanic glass http://www.mindat.org Dingil, M. 2003. Ph.D thesis)

Development depends on rapid chemical weathering of porous, permeable fine grained volcanic minerals + organic matter eg. Hydrolysis of microcline and augite yielding sufficient Al and Fe. KAlSi3O8 + 2 H2O = K+ + Al3+ + 3 SiO2 + 4 OH- microcline CaFeSi2O6 + 2 H2O = Ca2+ + Fe2+ + 2 SiO2 + 4 OH- augite Fe2 and Al3 form stable complexes with humus. However, Fe precipitates eventually to Ferrihydrite

Aluminum alone protects organic matter against BIODEGRADATION by developing Al-Humus complexes with high metal/organic ratio of limited mobility This induces accumulation of organic matter in top soil ie carbon sequestration developing a melanic surface horizon The liberated silica in the weathering products partly yield allophanes and imogolite.

Thus Andosols are of binary composition indiacting the competition between Al humus complexes and formation of allophane. Allophane stays stable in weakly acid and neutral conditions whereas the Al-humus complexes are dominant in more acid environments. The clay contents of Andosols changes over time particularly in the subsoil as allophane and imogolite are transformed to halloysite, kaolinite and at extreme acid conditions to gibsite. Eventually an Andosol may grade into a Luvisol or Podzol depending on precipitation

Morphology Typical (?) Andosols have an AC or ABC profile with a dark Ah-horizon (20 - 50 cm thick) on top of a brown B- or C-horizon.  The average organic matter content of the surface horizon (melanic) is between 5-6% but the darkest profiles may contain more.  The surface horizon is very porous, very friable, and has a crumb or granular structure.  Smeary consistence or a texture which is silt loam or finer and feels greasy within 100cm. It may become almost liquid when rubbed, presumably because of sol-gel transformations under pressure (thixotropy) in Vitric Andosols.

Hydrology Mineralogy Excellent drainage because of high porosity Gleyic properties at shallow ground water Stagnic in paddy fields Mineralogy X-ray amorphous materials' of allophane and imogolite, and/or humus complexes of Al and Fe together with opaline silica. Besides primary minerals, ferrihydrite, (disordered) halloysite and kaolinite, gibbsite and various 2:1 and 2:1:1 layer silicates and intergrades can be present.

Physical Characteristics Good aggregate stability Resistant to water erosion But difficult to disperse for texture analysis Low bulk density, typically (?) less than 0.9g/cm3 at some cases of high hydration is as low as 0.3g/cm3 The quantity of available water is generally higher than other mineral soils because of the high water content at the permanent wilting point (1500kPa) Excessive air drying or severe drought conditions develop irreversible deterioration in water holding capacity, ion exchange capacity, soil volume and cohesion of soil particles. Ultimately!, particles fall apart to a fine dust which is susceptible to wind erosion.

Chemical Characteristics High exchange properties Charge dependent on pH and electrolyte concentration due to high contents of soil organic matter and allophane Figure 2 illustrates the variation of charge by pH. Halloysite and montmorillonite are dominantly permenantly charged Base saturation (BS) values are variable due to the variable charge properties. BS low in strongly leached Andosols of the humid tropics except in young and dry region Andosols These characteristics are attributed to the active Al already present in humus complexes as well as exhangeable, interlayer and as allophane and imogolite

Figure 2. NH4+ and Cl- retention curves measured in 0. 01 M NH4Cl (0 Figure 2. NH4+ and Cl- retention curves measured in 0.01 M NH4Cl (0.1 M NH4Cl for montmorillonite). (a) montmorillonite; (b) halloysite; (c) allophane 905 (Al:Si=2:1, containing some imogolite); (d) allophane PA (Al:Si=1:1). Wada & Okamura, 1977)

MANAGEMENT High potential for agricultural production Fertile at unleached conditions at profiles formed on intermediate and basic volcanic ash Active Al is a drawback for phosphate availability –fixation. This may be remediated via liming and addition of silica, organic material and phosphate fertilisers Easy to till with good rootability and water storage except in strongly hydrated cases Sugarcane, tobacco, sweet potatoes (tolerant to low P levels), tea, vegetables, wheat and horticulturalş crops are suitable for the tropic and sub-humid areas. On steep slopes they are best kept under forests or well-managed pastures. In low lands best used for paddy rice cultivation which bears a problem of development of dense hardpans due to the development of Fe and Mn oxides