ALUMINUM-HUMUS COMPLEXES FOR ACCUMULATION OF ORGANIC-MATTER IN BLACK-COLORED SOILS UNDER GRASS VEGETATION IN BOLIVIA

Black-colored soils, equivalent to the Umbrepts suborder of USDA Soil Taxonomy, are distributed at altitudes ranging from 3,000 to 4,500 m u nder grass vegetation in the mountainous area surrounding the Cochabam ba basin of Bolivia. The soils had thick A horizons, as in the case of the Thick High-humic to Humic Andosols of Japan. The maximum value of organic C content of the A horizons was 157 g kg(-1) but the predomin ant clay minerals were clay mica, its weathering products, and kaolini te. Attempts were made to analyze the characteristics of organic matte r and the mechanism of its accumulation. Humic acids extracted by a 0. 5% NaOH solution belonged to the A type for 10 samples including Al, A 3, and C horizons collected at 3 sites, while those of the remaining t wo samples belonged to the B type. Amounts of Al extracted by the pyro phosphate solution ranged from 0.4 to 22.9 g kg(-1) and was positively and significantly correlated with the organic C content. The high deg ree of humification of organic matter was ascribed to the grass vegeta tion and the burning of these materials. Formation of Al-humus complex es was considered to contribute to the accumulation and stabilization of organic matter. Chlorite could be a source of Al for the formation of complexes with humus.