Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2004
Publication Date: 7/1/2004
Citation: Dontsova, K.M., Norton, L.D., Johnston, C.T., Bigham, J.M. 2004. Influence of exchangeable cations on water adsorption by soil clays. Soil Science Society of America Journal. 68(4):1218-1227.
Interpretive Summary: The problem addressed why previous work found soils behave differently with the element Magnesium (Mg) versus those with Calcium (Ca) added, with respect to water entry and erosion. This work was conducted to try to determine why at a very basic molecular level. We studied soil clays from two Midwestern USA soils typical of the corn-belt in the USA where water entry problems are severe. The methods used included determining the amount of water that could be held by the soil clay with differing amounts of various elements attached to them. We found, using spectroscopic and gravimetric techniques, that Mg interacted with water differently than Ca as water content increased. The Mg treated clay held the water more tightly than did the high Ca clay, meaning less water was available. This also explained why Mg clays were dispersed and resulted in low water intake rates compared to Ca clays. The impact of this research is that this fundamental understanding of how soil clays (the reactive part of soil) interact with water can change the way we manage soils to reduce runoff and erosion while at the same time improving crop yield through greater available water and soil structure stability.
Technical Abstract: The interaction of water with the clay fractions (<2um) from two Midwestern soils was studied using FTIR spectroscopy and gravimetric methods. The soil clay fractions were obtained from a Blount loam (fine, illitic, mesic Aeric Epiaqualfs) and a Fayette silty clay loam (fine-silty, mixed, mesic, superactive Typic Hapludalfs). These clay fractions were exchanged with Mg*2+, 50:50 Ca*2+:Mg*2_, Ca*2+, Na*+ and K*+ to determine the influence of the exchangeable cation on their water sorption behavior. Water sorption isotherms and Fourier Transformed Infra Red (FTIR) spectra of the clays were collected simultaneously using a gravimetric-spectroscopic cell. Overall, the amount of water sorbed by the samples increased as the ionic potential of the exchangeable cation increased and was strongly correlated to the hydration energy of the cations (P>F<0.0001). The position of the H-O-H bending band (v2 mode) also increased with increasing ionic potential of the exchangeable cation indicating strengthening of water H-bonds. In addition, it was observed that the position of this band decreased with increasing water content for the Mg-exchanged clays compared to an overall increase for the Ca-exchanged samples. X-ray diffraction patterns indicated an expansion of the phyllosilicate clay minerals as the water activity increased; however, no differences were observed between the Ca- and Mg-exchanged samples. This study shows that the molecular properties of water on Ca- and Mg-exchanged soil clays are similar to that on specimen clays and provides new insight about the role of exchangeable cations in soils.