Thursday, 13 July 2006

Changes in Soil Surface Properties Under Acidification and Alkalization.

Grzegorz Jozefaciuk, Institute of Agrophysics, Doswiadczalna 4, Lublin, Poland

Soil acidification (humid climate) as well as alkalization (arid zones) lead to severe soil degradation, environment toxification and agricultural land devastation. Acidic and alkaline conditions alters soil properties due to composite effects of removal and alteration of organic and mineral soil components and changes in solution and exchangeable ions composition. Surface parameters become increasingly used for description and modeling of soil physical, chemical and biological processes as well as for quantitative analysis of soil typological and genetic properties. They have been used as highly sensitive indicators of soil degradation processes: acidification and alkalization, soil organic matter leaching and oxidation, soil pollution etc. This presentation concerns the alteration of selected surface parameters of soils as: variable surface charge, surface acidity, surface area, water vapor adsorption energy and surface fractal dimension under acid and alkaline conditions. These parameters, related mainly to a presence and kind of the finest soil particles (clay and fine silt), are important factors of acid-base soil buffering, ions exchange, microstructure (microaggregation), wettability, water retention at low moisture, sorption of organics and others. At the beginning the measuring methods and the way of calculation of surface parameters will be briefly outlined: water vapor adsorption isotherms (estimation of surface area, adsorption energy and fractal dimension) and back titration (estimation of variable charge and surface acidity). Some place will be given for interpretation of surface parameters. Next the effect of acid and alkaline treatments on the surface parameters will be shown on the background of changes in soil solid phase composition (organic matter, clay fraction and mineralogy). In acidification processes a decrease in variable charge of clay poor soils occurs and its increase in clay rich soils (counteracting the decrease in permanent charge) while the alkalization leads in general to the variable charge decrease. A decrease in surface area occurs most frequently in acidified and alkalized clay poor soils and increase in clay rich soils (as in pure minerals). Surface acidity decreases in acidified soils and in alkalized clay poor or organic soils. Alkalization of clay rich soils leads to increase in surface acidity. In acidified and alkalized soils the average adsorption energy usually decreases that indicates that water binding forces become lower. Alkalization of clay rich soils leads frequently to an increase of adsorption energy. Decrease in fractal dimension in acidified soils indicates smoothing of surfaces of soil fine fractions, whereas much less pronounced changes in surface build-up are observed under alkaline conditions. Hypotheses on the mechanisms of alteration of surface parameters will be placed and related to the removal of highly dispersed Fe and Al oxides (of large charge, area and roughness, medium surface acidity) in both acid and alkaline environments, destruction of mineral lattices and formation of amorphous silica (of large surface, small charge, low surface acidity and smooth surface) in acidic media, dissolution of organic matter (of high charge and area) and removal of outer silica layers (weakly acidic) from clay mineral surfaces and exposition of Al-layers (more acidic) in alkaline media. Some data comparing acid and base laboratory treatments and natural acidified and alkalized soils will be presented at the end.

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