Fjölrit RALA - 15.06.2004, Side 57

Complex exchange properties of soils from a range of European volcanic
areas
M. Madeira1, F. Monteiro1, E. García-Rodeja2 & J. C. Nóvoa-Monoz2
1 Departamento de Ciéncias do Ambiente, Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa,
Portugal; 2 Departamento de Edafología y Química Agrícola, Fac. Biología, University ofSantiago de
Compostela, Campus Sur s/n, 15782 Santiago de Compostela, Spain.
Most Andisols contain large amounts of pH-dependent charge, which iníluence the majority
of the reactions which control the ability of the soil to retain cations and anions. The influence
of the pH-dependent charge components on the cation exchange capacity (CEC) and the anion
exchange capacity (AEC) is of considerable importance for the successful management of
Andisols. The CEC measured by the 1 M NH4OAC at pH 7 is commonly used and has become
a standard reference to which other methods are compared. The values of the sum of bases by
1 M NH4OAC at pH 7 are used to establish criteria for taxonomic separation of Andisols at the
sub-group level.
Hence a study on a wide range of Andisols and related soils from European volcanic areas
was conducted to determine (1) the CEC and the AEC, (2) the base exchange cations and the
effective cation exchange capacity (ECEC), and (3) the influence of various components of
the soil colloidal systems on CEC and AEC values. Seventeen pedons of Andisols (of the
COST 622 reference soils of Europe; Soil Resources of European Volcanic Systems) selected
from representative volcanic areas of Italy, Portugal (Azores), Iceland, Spain (Tenerife),
Greece, France and Hungary were used. Determinations were done on the fine earth fraction
(<2mm) of air-dried samples. The results are expressed on an oven dry basis (105°C). The
CEC was determined by the 1 M NH4OAC method (SM), using continuous leaching of 5 g of
soil with 100 mL of 1 M NH4OAC. The leachate was used to determine the exchangeable
bases, measured using the atomic absorption spectroscopy. The CEC was also measured by
the compulsive exchange method (CE), and AEC was also measured by the same method
(Gillman & Sumpter, 1986). The ECEC was calculated by taking the values of the sum of
bases plus the 1 M KCl extractable Al. For presentation, data of Andisols and non-Andisols
are shown separately.
The CEC values of Andisols measured by the SM showed a wide variation (17.64-89.30
cmolc kg"1) and were higher in the Andisols very rich in organic C than in the other soils.
Those values were positively correlated with the organic C content (r=0,82; p<0.01); the
correlation between CECsm and Al0 content was not observed. In the other soils (non-
Andisols), CECsm values varied between 4.64 and 62.64 cmolc kg"1. The values were weakly
correlated with organic C content (r=0.58; p<0.05), and strongly correlated with the Al0
content (r=0.83; p<0.01). The ECEC values (as the sum of basic exchangeable cations and
extractable Al) of Andisols were positively correlated with the organic C content (r=0.77;
p<0.01) and negatively with Al0 content (r=-0.72; p<0.05).
For Andisols, CECce values were much lower than those obtained by the SM, especially in
Andisols very rich in organic C. Similar trend was observed for non-Andisols. The values by
the two methods were not correlated. Those values varied from 0.20 to 21.55 emolc kg”1, for
Andisols, and from 1.30 to 26.77 emolc kg'1, for non-Andisols. CECce values were not
correlated with Al0 and organic C contents. Values of Andisols were positively correlated
with ECEC values obtained by the SM (r=0.70; p<0.05); this correlation was also observed
for the non-Andisols (r=0.69; p<0.01).
The difference between CECsm and CECce (ACEC) increases with increasing content of a
variable charge constituents (i.e. organic matter and allophanic constituents). The values of
Andisols showed a wide variation (about 10 to 68), and the highest values were observed in
40