Investigating Flocculation Capacities of Different Cations and Their Implications for Soil Structure and Sustainability

Abstract

A laboratory exercise was developed for the study of the effect of three cations (Al, Ca, and Na) on the flocculation and dispersion of soil particles. The exercise was designed for Soil Science undergraduate students in the second-course in the Agricultural Engineering and Environmental Sciences department at the University of Vigo. The students’ exercise consisted of the addition of different concentrations of NaCl, CaCl2, or AlCl3 to a kaolinite water suspension and to an agricultural soil water suspension (a more complex system). The most relevant results showed that for the same cation concentration (equiv L-1), the effects on increased flocculation of colloidal material followed the sequence Al > Ca > Na, for both kaolinite and agricultural soil; that is, aluminum is the cation with the highest flocculation capacity, while Na is the cation with the lowest one. These differences are important for the sustainability of agricultural soils, since flocculation in soil systems is a key factor to sustain a suitable soil structure, and therefore, for suitable air and water fluxes in soils. Moreover, the differences in flocculation capacities of different cations should be taken into account in the fertilization programs during the selection of fertilizers (in relation to composition), in order to avoid irreversible problems for the soil structure. The experimental results obtained by students were highly satisfactory, showing experimental values similar to those previously found by the teachers. According to postlab surveys, the students were highly satisfied and in favor of the laboratory exercise developed from both sides, the didactic point of view and the importance of the exercise for the understanding of soil science concepts.