Illinois Fertilizer Conference Proceedings January 23-25, 1995 Home 1995 Index Search

Continued Studies of the Effects of Iron Oxidation State on the Fate and Behavior of Potassium in Soils

Joseph W. Stucki and Siyuan Shen¹

Introduction

This study is a continuation of those that were initiated under FREC Project #58 on the effects of iron (Fe) oxidation state on the fate and behavior of potassium (K) in agricultural soils. In spite of many years of study, scientists still have a rather poor understanding of the behavior of K in soils, as evidenced by the lack of a reliable model for explaining the many contradictions in test-response relationships. The oxidation state is the chemical term which refers to the electrical charge borne by an element as either a free ion or within a compound. Iron is one element that may exist in different oxidation states, namely, +2 (ferrous iron, Fe²⁺) and +3 (ferric iron, Fe³⁺,). Iron is an abundant constituent of soil minerals and may exist in either of these states. Interestingly, changes in Fe oxidation state cause important changes in the chemistry of its host mineral. Earlier studies revealed that K fixation occurs in clay minerals as a result of Fe reduction from Fe³⁺ to Fe²⁺, and the first 3 years of this study showed that this effect depends strongly on the mineralogy of the clays, and that cyclic reduction and reoxidation (redox) events cause at least 25% of the K to be fixed irreversibly and about 18% of the Fe to be stabilized in the reduced state.

Objectives

The purpose for continuing these studies is to test the hypothesis that the effect of Fe oxidation state in the crystal structures of soil minerals is an essential factor in understanding the behavior of K in soils. The overall objective will be to establish from a physical-chemical level the role of oxidation state of Fe in soil minerals in determining the fate, behavior, and availability of K in agricultural soils. The study will provide information which will have direct relevance and significance for improving soil tests for K, in evaluating the effect of different K application strategies, and in assessing the true potential for K availability in soils. The specific objectives are: (1) to determine the extent of K fixation that occurs in the presence of competing cations, such as Ca²⁺, Na⁺, and NH₄⁺; (2) to measure the change in oxidation state that occurs during conventional methods of field sampling and K availability; (3) to assess the role of oxidation state in observed seasonal and spacial variations in available K in the soil; and (4) to discover why illite behaves differently from smectite relative to the effect of Fe reduction on K fixation.

Results and Discussion

This report addresses one aspect of objective 1, namely, the behavior of NH₄⁺. compared to K⁺; and Na⁺ in the clay as a function of Fe oxidation state. The amount of fixed NH₄⁺ in smectite SWa-1 increased with increasing Fe reduction (Figure 1), which behavior is similar to that of K⁺ and Na⁺ (Figure 2; Lear and Stucki, 1985; Khaled and Stucki, 1991); but the amount of exchangeable NH₄⁺ decreased (Figure 1), which differs from the behavior of K⁺ and Na⁺. By comparison, exchangeable K⁺ is only slightly affected by Fe reduction (Figure 2; Khaled and Stucki, 1991) and exchangeable Na⁺ increases (Figure 2; Lear and Stucki, 1985). These disparities may be the result of differences in hydration energy and mobility of these respective cations.

These results (Figures 1 and 2) further indicate that the increased charge due to Fe reduction was completely compensated by fixed NH₄⁺,moreover, the fixation process converted some of the initially exchangeable form to a fixed form. The fixation process for NH₄⁺ thus appears to be rather aggressive. This supposition is further borne out by the data from the reoxidation experiment. After reoxidation, most of the NH₄⁺ that became fixed during the reduction process remained fixed (Figure 3), even though the clay was almost completely reoxidized; and the net change in exchangeable NH₄⁺ was negative during the reduction-reoxidation cycle (Figure 3). Fixed NH₄⁺ is thus bound even more strongly than is fixed K⁺, because some of the fixed K⁺ in the reduced state reverts to the exchangeable form upon reoxidation.

Another notable observation is that the total amount of NH₄⁺ associated with the clay was greater in the reoxidized SWa-1 than in the original, oxidized clay (Figure 3), indicating that either reoxidation failed to restore completely the original layer charge or the clay initially was incompletely saturated with NH₄⁺.

Turning to the low-Fe smectite, Upton montmorillonite (API 25), reveals surprisingly different picture. In this clay the amount of fixed NH₄⁺ was small and insensitive to Fe reduction, whereas the exchangeable form increased steadily with increasing Fe(II) and decreased upon reoxidation. Total NH₄⁺ varied directly with the exchangeable form. This strongly contrasting behavior relative to the Fe-rich smectite (SWa-1) and relative to K⁺ may indicate that NH₄⁺ fixation is the consequence of specific-site interaction at the clay mineral surface rather than the cation passively becoming "trapped" between collapsing clay layers. This introduces (perhaps not surprisingly) another mineralogical factor that appears to be affecting the fixation of cations, i.e., the total Fe content or, perhaps, the source of layer charge in the clay layer. Variation also exists among different cations.

Acknowledgments

Tables and Figures Referenced

Figure 1. Distribution of ammonium in ferruginous smectite SWa-1 as affected by the oxidation state of structural Fe

Figure 2. Fixed and exchangeable sodium, potassium, and ammonium in oxidized and reduced ferruginous smectite SWa-1

Figure 3. Distribution of ammonium in ferrginous smectite SWa-1 after one reduction-reoxidation cycle

Figure 4. Distribution of ammonium in Upton montmorillonite (API 25) after one reduction-reoxidation cycle

Footnotes and References

¹Professor of Soil Physical Chemistry and former Graduate Research Assistant, respectively, Department of Agronomy, University of Illinois. Dr. Shen is now Research Associate, USDA Eastern Regional Research Center, Wyndmoor, Pennsylvania.

Khaled, E. M., and J. W. Stucki. 1991. Effects of iron oxidation state on cation fixation in smectites. Soil Sci. Soc. Am. J. 55: (In Press)

Lear, P. R., and J. W. Stucki. 1989. Effects of iron oxidation state on the specific surface area of Nontronite. Clays Clay Miner. 37:547-552.

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