Fjölrit RALA - 15.06.2004, Page 104

Application of diffuse reflectance spectroscopy to characterise
volcanic soil color and mineralogy
M.V. Sellitto, G. Palumbo, A. Di Cerce, C. Colombo1,
1 Dip. Scienze Animali Vegetali e dell’Ambiente, Unversitá delMolise, Gampobasso, Italy
Soil color is an important soil property that has been related to organic matter content of
surface horizons, soil moisture regime, and iron oxides content (Stoner and Baumgardner,
1981; Torrent et al., 1983). Color variation within a soil profile is related to its chemical,
mineralogical and hydraulic properties although the nature and strength of the relationship has
not been well defined (Baumgardner et al.,1985). Volcanic soils are generally very dark in the
upper horizons and the for this reason they were named “Andosols” (from the Japanese “An-
do”, which means An dark and do soil) while deep horizons consist of bright color varying
with the type of volcanic parent material and composition of weathering products. Volcanic
soils formed on fresh tephra show various colors ranging from white to black according of the
chemical composition and mineralogy of the tephra and organic matter content. Well
developed volcanic soils in temperate regions are yellow to reddish brown reflecting the
formation of hematite, goethite or ferrihydrite according to the drainage conditions. Poor
drainage is favourable to formation of goethite and lepidocrocite (with yellow color). Under
well drained conditions, volcanic soils show reddish brown colors indicative of hematitic
formation. Despite its pedological meaning, precise methods to measure soil color and soil
reflectance properties, have not been extensively studied. The normal method of measuring
soil color in the field requires visual matching of a sample with standard color chips (Soil
Survey Division Staff, 1993). This method is at best semiquantitative. However, it is limited
by the observer subjective knowledge and by the number of Munsell color chips
(Baumgardner et al., 1985). Visual measurement in situ was influenced by the lighting
condition that was related with geographic position. Direct full sunlight, sunlight filtered by
trees, low-angle sunlight, and indoor lighting may result in severe inaccuracy. These
limitations of visual measurement techniques result in poor correlation between soil color and
soil properties and reduce the application of soil color criteria in soil classification. The
specific objectives of this study are to compare field and laboratory soil color measurements
and to assess significant correlation between spectral measurements and volcanic soil
properties. Reflectance spectra were obtained using disturbed, < 2 mm, gently ground, air-dry
soil. Soil spectra were acquired using a Jasco 560 UV-visible spectrophotometer equipped
with an integrating sphere of 53 mm diameter, working in the 350-900 nm spectral range, and
with a spectral resolution of 0.5 nm. Barium sulphate (Merck DIN 5033) was used as white
standard, and CIE parameter (tristimulus coordinates) were calculated and converted to
Munsell notation (Torrent and Barron, 1993.). All the whole volcanic soil samples show
yellow color (form 0.1 to 3 Y and from 10YR to 7.5YR), with values ranging from 3.8 to 7.4,
and chromas ranging from 1.5 to 5.4 according to the spectrophotometric measurements. The
visual estimates of the Munsell color value and the spectrophotometer measurements are
moderately correlated (Fig. 1 a ). Discrepancies between the two measurements may originate
by the different lighting conditions and also because the two measurements were taken under
different moisture conditions. Visual estimates of Munsell value show more variation than
the corresponding spectrophotometer measurements (Fig. la). For example, visual estimates
of a Munsell value of 3 correspond to spectrophotometer Munsell values ranging between 3.5
to 7 . Munsell value decrease cuvilinearly as OM increased but different populations were
observed (Fig. lb). For example, Italian, Hungarian and Greek soils exhibit significant
curvilinear trends while Spain and France soil shown Munsell values that decrease more
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