Fjölrit RALA - 15.06.2004, Page 138

Thus, it is considered that organically complexed A1 controls A1 concentration in soil
solution of nonallophanic Andosols. Exchangeable A1 estimated by 1 M KCl extraction is
thought to be easily released into soil solution. Therefore, exchangeable A1 should be related
to organically complexed Al. To confirm this hypothesis, we investigated the relationship
between A1 saturation (exchangeable A1 / effective CEC) and Alp contents of nonallophanic
Andosol A horizons from Noshiro City, Akita Prefecture. A significant, positive correlation
was observed between A1 saturation and Alp content (r = 0.714, p < 0.001) (Fig. 2). In these
soil horizons, aqueous A1 is considered to be equilibrated as follows: humus complexed A1 <=»
aqueous A1 <=> exchangeable Al.
The above results indicate that a portion of Al-humus complexes are labile and are easily
altered by rather simple chemical treatment such as liming. Figure 3 shows the pyrophosphate
extractable A1 (Alp) of limed and unlimed soil samples. A large decrease of Alp value with
liming was observed (decrease rate of 9 - 43%). The decrease of Alp values cannot be
explained only by the disappearance of the KCl extractable Al. It is strongly indicated that
liming reduces significant amounts of organically complexed A1 as well as exchangeable Al.
The increase in the cation exchange capacity at pH 7 after Uming further suggested that the
carboxyl group was partly liberated from A1 complexation.
Fig. 1 Plot of equilibrium A1 soiubility versus pH for Hiyamizu A horizon (Aomori Prefecture), Noshiro
A horizon (Akita Prefecture) and Hubbard Brook Bhs horizon (New Hampshire, USA).
The solubility of synthetic gibbsite is indicated by the dotted hne.
Fig. 2 Relationship between A1 saturation
(KCl-Al/ECEC) and Alp content of
Noshiro soils from Akita Prefecture.
Fig.3 Amounts of pyrophosphate-extractable A1
from unlimed and hmed soils samples.
114