Graduate Thesis Or Dissertation
 

Extractable forms of Al and Fe in acid Western Oregon soils

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  • Extractable forms of Al and Fe were studied in five soils from the Coast Range, and in two soils from the Willamette Valley in Oregon. 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 exchange. 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.
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