Fjölrit RALA - 15.06.2004, Síða 40

Icelandic volcanic soils
Olafur Arnalds
Agricultural Research Institute (RALA), Reykjavik Iceland
Controlling factors and classification
Icelandic soils, recently reviewed by Amalds (2004), are predominantly Andosols, including
many soils of deserts which are termed Vitrisols according to the RALA classification. Wind
erosion is extremely active in Iceland, which leads to massive redistribution of vitric materials
from desert surfaces throughout Iceland. Tephra deposition is frequent. The soil surface is
generally rising as eolian materials and tephra are continuously being added to the surface.
There is a clear gradient in soil properties from the active volcanic rift-zone and desert areas
to the Tertiary landscapes of the east, north and west Iceland, as is indicated in Fig. 1. Rate of
eolian and tephra deposition controls the pH, with sufficient release of bases to maintain the
pH in areas of rapid/frequent deposition, while pH becomes lower in areas of less deposition.
Soil pH controls to a large extent both allophane formation and accumulation of metal-humus
complexes. Allophane does not form when pH is < 4.9 (Fig. 2a), but it occurs in subsurface
horizons at such low pH because of redistribution of soil materials, and possibly changes in
pH in time caused by environmental change. Much more carbon is accumulated in soils of
low pH than in soils with higher pH (Fig. 2b).
These relationships are used to separate Icelandic Andosols into main classes of Icelandic
soils, which are indicated by the shades in Fig. 1. The classes are: Vitrisols (V), Brown
Andosols (BA, freely drained Andosols, < 12% C), Gleyic Andosols (GA; Andosols of
wetlands, < 12% C), Histic Andosols (HA, organic Andosols of wetlands, 12-20% C), and
also Histosols (H, >20% C). Cryosols, Arenosols, and Leptosols also occur. Soil map of
Iceland can be obtained from www.rala.is/desert. Icelandic soil class names are shown with
italic letters in this abstract.
Morphology and properties
The Andosol/Histosol mantle over the 10 000 year old glaciated surfaces is commonly 0.5-2
m thick. The Andosols often show distinct layering as a result of tephra deposition and eolian
events. Tephra layers are sometimes prominent, especially near the active volcanic belt. The
soils are very friable, lack cohesion, and some soils show thixotropic properties. Bulk density
is characteristically low and more related to carbon content than allophane as is high 15 bar
water retention (30-200%). The physical properties of Icelandic soils magnify cryoturbation
resulting in clear cryogenic features.
The mineralogy of the soils is dominated by basaltic rock minerals and clay minerals
characteristic of Andosols. Smectites and other phyllosilicates have been found in horizons
influenced by sediments from the Tertiary basalt stacks (Kleber and Amalds, unpubhshed).
Undisturbed Icelandic soils are generally rich in OC, which can extend to >2 m depth with
erratic distribution. There is a clear trend in OC which relates to the pH of the soils and
distance from sources of tephra materials (Fig. 1). Soil reaction (H2O) ranges from 4
(Histosols, Histic Andosols) to >7 (Vitrisols), also depending on vitric inputs to the soils.
pH(KCl) is 0.3-1.5 unit lower, with d pH closely related to pH (H2O).
Vitrisols
Typical Vitrisols, which are mainly associated with desert surfaces, have different character
than Andosols under vegetation. Carbon and clay contents are low, they are coarse with
higher BD, but lower water retention and CEC than typical Andosols. Their extent in Iceland
27