|Abstract or Summary
- Extractable forms of Al and Fe were studied in five soils from
the Coast Range, and in two soils from the Willamette Valley in
Exchangeable Al was determined in a way which permitted a
comparison of three different methods for its determination. Values
for exchangeable Al obtained by the method of lirrited leaching by Lin
and Coleman (1960) agreed closely with those obtained by the method
of Skeen and Sumner (1965) which consists of successive extractions
and extrapolation to obtain the exchangeable Al content. Values for
exchangeable Al measured by the method of exhaustive leaching proposed
by Dewan and Rich (1970) exceeded the Coleman values in different
amounts depending on the amount of exchangeable Al present, and
on the final slope of the extraction curves. Steep slopes were found in
coastal soils containing amorphous aluminosilicates, indicating that these soils have a strong supplying power for exchangeable Al, From
the shape of the Al extraction curves it was concluded that two different
forms of Al were extracted by the method of successive extractions,
namely, a readily exchangeable form, and a form that is slowly released
and which presumably results from mineral solution of amorphous
oxides or from Al-organic complexes.
In soils with a high base saturation good agreement between both
the Coleman and the Rich values for exchangeable cations with the
neutral salt CEC was observed. In soils in which Al is the dominant
cation of the exchange complex, the neutral salt CEC was from 2 to 3
meq lower than the Coleman value for exchangeable cations. These
differences were attributed to errors in the CEC determination due to
hydrolysis. The Rich values for exchangeable cations exceeded the
neutral salt CEC by amounts as high as 10 meq/100g. It was therefore
concluded that Rich's method to determine exchangeable Al furnishes
values that are not compatible with other standard methods.
The ratios of humic to fulvic acid extracted by pyrophosphate
were higher in grassland soils than in forest soils. The total amounts
of Al and Fe correlate fairly well with the organic carbon extracted
by pyrophosphate. The contents of Al and Fe were much higher in
fulvic acid than in humic acid, but this was ascribed to the formation
of soluble complexes of previously organically bound Al and Fe with
pyrophosphate appearing in the fulvic fraction. It was concluded that pyrophosphate extractable Al and Fe represent a rough estimate of
the amounts of Al and Fe involved in cation bridges.
The ratio of Al or Fe to organic carbon extracted by pyrophosphate
increased with depth. This could be due either to a change in
organic matter composition, or a result of increased dissolution of
inorganic amorphous oxides in lower horizons.
In the top horizons of soils high in organic matter the absolute
amounts of pyrophosphate extractable Al and Fe were much higher
than in soils low in organic matter. However, the metal to organic
carbon ratios in the top horizons were fairly similar. This suggests
that the proportions of organic matter bonded to inorganic surfaces
by cation bridges have similar contents of polyvalent cations. It was
therefore concluded that differences in organic matter accumulations
should be explained in terms of the kinds and amounts of inorganic
surfaces available for organic matter adsorption in a soil. In previous
work,the coastal soils with high organic matter accumulations have
been shown to contain amorphous alumino-silicates. These minerals
are unique in that they combine negative charges and an aluminous
surface with a high specific surface area. Thus, they favor the formation
of all the known types of organo-mineral bonds, except anion
The oxalate and dithionite extractions did not yield much information,
and were difficult to interpret due to uncertainties regarding their presumed specificity for pedogenetic oxides. However, dithionite
in conjunction with pyrophosphate appears to be useful for the chemical
identification of spodic horizons.
The classification of the Knappa and the Nehalern profiles did
not correspond to the official series classification. The Knappa profile
was classified as a Typic Dystrandept instead of a Pachic Haplumbrept.
The Nehalern profile was found to be a member of the Fluventic.
Hapludoll instead of the Fluventic Haplumbrept subgroup.