Cultivation effects on dispersible clay of soil aggregates

Abstract

The objective of this study was to examine the effect of long-term cultivation on clay dispersibility of four aggregate size fractions (2.0–9.5 mm, 0.85–2.0 mm, 0.25–0.85 mm, and < 0.25 mm) obtained from a Chernozemic soil by comparing two cultivated sites with an adjacent native prairie site. Aggregate size fractions (ASF) were subjected to increasing levels of ultrasonic energy and the amount of clay dispersed at each energy level was determined. Organic carbon, hexose carbon, soluble hexose C, total clay, cation exchange capacity (CEC), and exchangeable cations were measured for each ASF. Clay contained within prairie aggregates was held much more strongly within the aggregate and therefore showed greater stability towards dispersion by ultrasonic vibration. More energy was required to disperse one-half of the ASF clay under prairie than under cultivated soils (228–425, and 95–229 kJ L−1 for prairie and cultivated macroaggregates, respectively; 370–433, and 249–334 kJ L−1 for prairie and cultivated microaggregates, respectively). Clay dispersibility was significantly correlated with organic C, hexose C, soluble hexose C, non-hexose C, CEC, and exchangeable Ca and Mg but was not correlated with total ASF clay. Long-term cultivation of this soil resulted in a decrease in the energy required to disperse an equivalent proportion of clay from aggregates relative to that of the grassland soil. Thus, cultivation of these soils has resulted in aggregates which are more susceptible to clay dispersion and therefore prone to water erosion and surface crusting. Key words: Clay dispersion, aggregation, carbohydrate