Implication of zeta potential and surface free energy in the description of agricultural soil quality: Effect of different cations and humic acids on degraded soils

Here we present results of our study about the effect of natural organic matter in the adsorptions processes of commercial humic acid (HA) on the physicochemical properties, stability, and structure of an agricultural olive soil. Our soil belongs to the loamy texture class and gives a medium-low chemical fertility. Its organic carbon content is very low (0.40%), which could be explained by the Mediterranean climate type in which the soil developed. From the fine earth mineralogy, we conclude that smectite was the most abundant mineral in our soil. We first demonstrated that HA effectively adsorbs onto soil, mainly at basic pH. Electrophoresis measurements of soil particle suspensions as a function of pH in the presence and absence of humic acid clearly demonstrate the adsorption of negatively charged entities onto smectite in the soil. This effect is more significant when the soil contains natural organic matter, which confirms the mordant character of this matter in native soil. Analysis of surface free energy components leads to the conclusion that humic acid molecules impart a significant electron donor character to the soil particles, which in turn decreases their hydrophilicity. All collected data were used to interpret the final soil structure. Our results are consistent with fabric images obtained by scanning electron microscopy of natural samples that present the major aggregation state. Interpretation of these results can be explained by a combination of electrical and thermodynamic interaction, electrostatic forces, double-layer repulsion, van der Waals attraction, and hydrophilic/hydrophobic repulsion/attraction. (C) 2014 Elsevier B.V. All rights reserved.