Saturday, 15 July 2006

Water Dispersible Clay and Zeta Potential as Affected by Sodicity and Management Practices in Andic Soils.

Silvia Armas-Espinel1, Carlos M. Regalado2, and Jose M. Hernandez-Moreno1. (1) Univ de La Laguna, Dept. Edafologia & Geologia, Ave. Astrofisico F. Sanchez, s/n, La Laguna, 38204, Spain, (2) ICIA, Aptdo.60, La Laguna, 38200, Spain

Variable charge soils show a high structural stability. The authors have previously shown that in Andic soils containing both non crystalline materials and layer silicates the aggregation effect of the former did not counterbalance soil structure deterioration under certain combinations of exchangeable cations distributions, salinity and clay mineralogy. The aims of this work are to study the influence of management practices on clay dispersivity and zeta potential and to relate the dispersion to some soil properties. To this end, a clay dispersion test was conducted on natural and cultivated andic soils from the Canary Islands with a range of amorphous materials content (quantified by Al and Fe extracted with ammonium oxalate, Alo+1/2Feo). Samples of 2 g of soil were shaken with 40 mL of distilled water for 30 min. After the appropriate settling time, 10 mL of suspension were pipetted and oven dried, and the amount of clay determined gravimetrically. In order to study the influence of sodium and electrical conductivity (EC) in clay dispersion, the soils were treated with different SAR/EC combinations. Suspension stability was estimated by measuring the zeta potential of the clay suspension (Zetasizer Nano, Malvern). Particle size, EC and pH of the suspension were also determined. Soils with similar or lower amounts of (Alo+1/2Feo) under cultivation yielded more dispersible clay (2-6%) than their natural analogues (0.1-1%). Increasing SAR resulted in an increase in the dispersed clay in the more diluted solutions. The cultivated andic soils dispersed from 5 mmol/L (at SAR 25 and 50), while the less andic soil (non cultivated) dispersed only from 1 mmol/L (at SAR 50 and 100). The higher stability of this soil, compared with its homologous cultivated soils, may be attributed to the negative effects of specific anion sorption (phosphate fertilizers) on the structural stability. No dispersion was observed in the natural Andisols for the SAR value studied (SAR 50). Zeta potential (absolute values) decreased with increasing (Alo+1/2Feo) from -40 to -15 mV. The latter value corresponded to Andisols in which clay fractions obtained by sonication gave Z-potential values near zero or even positive at the same suspension pH. The Z-potential values increased with increasing SAR values and with decreasing salinity concentration C. An inverse relationship was obtained between Z-potential (absolute values) and the particle average diameter. Particle average diameter decreased significantly with SAR values and with decreasing C values, especially in the less cultivated andic soils. Chemical and optical analysis suggested that particle diameters at least greater than 0.8 Ám corresponded to soil aggregates.

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