Illinois Fertilizer
Conference Proceedings
January 29-31, 1996
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Joseph W. Stucki and Dongfang Huo
Previous work (Chen et al., 1987; Khaled and Stucki, 1991; Shen and Stucki, 1994) revealed that the oxidation state of Fe in the crystal structure of clay minerals in the soil greatly influences the amount of K that is fixed or made unavailable to plants. Shen and Stucki (1994) estimated, very conservatively, that this process could produce a fluctuation of more than nine hundred pounds of K20 per acre, either into or out of the pool of plantavailable K at any given time. But this effect depends on the type of clay mineral that is present -- in smectite a strong direct correlation exists between Fe(II) (reduced Fe) and K fixation, whereas if the clay mineral is illite the correlation is weaker and inverse. One factor that these studies failed to consider, however, is the effect of other cations in the soil, such as Ca2+, Mg2+, and NH4+ , on the extent of K fixation under oxidizing or reducing conditions. The above-referenced studies were limited to the condition where only one cation (in most cases K) was present, but one would expect the presence of other cations to alter the amount of K that is fixed because Khaled and Stucki (1991) showed that the amount of fixation of any cation depends inversely on its hydration energy (Figure 1). If two or more cations are present simultaneously, a competition for exchange sites on the clay will ensue. If the correlation illustrated in Figure 1 is generally true, then one would predict that K+ will always win in these competitions and, thus, will be the dominant fixed cation at any degree of Fe reduction. Experimental evidence to support this statement, however, is missing. Because the multiple-cation condition is the one that exists in the soil, where many different cations and chemical compounds are present, a knowledge of the actual outcome of this contest is of fundamental importance to understanding the behavior of K in the soil. The purpose of this study was, therefore, to determine how K fixation is altered when other cations are present, as compared to the single cation condition.
Duplicates of three levels of Fe reduction of Na saturated ferruginous smectite (SWa-1) clay were prepared to study multi-cation competition in the interlaminar spaces between clay layers. About 30 mg of clay sample in a reaction tube was reduced in a citratebicarbonate buffer (pH of about 8) with 100 mg of sodium dithionite (Na2S204) at 75 °C. The time of reduction under these conditions was varied, giving results at 0, 0.33, and 4 hr of reaction.
Aqueous Na-clay suspensions were exchanged twice with an 02-free solution of K and Ca, prepared with a concentration ratio of 0.5 M K+/0.25 M Ca2+. These suspensions were shaken for about 1 hr for each exchange to get better exchange or replacement of Na+ by the K+ and Ca2+. The exchange was followed by two washings with O2-free 0.005 M K+ /0.0025 M Ca2+ solution to remove excess salts. The final supernatant was saved for Ca and K analysis to determine the equilibrium outer solution concentration.
The sediments after the final centrifugation were resuspended in 5 ml of O2-free water and freeze-dried. During drying, the samples were inadvertently reoxidized so only the total exchange condition was evaluated. This limitation was imposed because a change in oxidation state alters the distribution of cations between exchangeable and fixed phases. It also changes the total layer charge and, consequently, the distribution of cations between exchanged and non-exchanged states. But because reoxidation occurred in the dried state, the total amount present in the dried sample would still reflect the total distribution of K and Ca on the clay surface in the reduced state. Studies are in progress in which the reduced state will be preserved during drying and the distribution between exchangeable and fixed states determined.
The dry samples were digested using 12 ml of 3.6 N H2SO4 and
1 ml of 48% HF in boiling water for 40 min. During digestion the solution was
mixed by hand-shaking. After digestion, one 20-mL aliquot of 10% H3BO3 was
added to each tube to remove the excess HF, and water was added to bring the
final volume to 50 mL. Digestate solutions were analyzed by ICP and the results
are given in Table l.
The solutions were initially prepared in a 1:1 equivalent ratio (0.5:1 molar ratio) of Ca:K, but after reaction with the clay this ratio decreased slightly, indicating that some preference for Ca was manifested by the clay (see first two rows in Table 1).
The selectivity of the unaltered (oxidized) clay was for Ca2+, giving an equivalent ratio of Ca to K exchanged on the clay of about 4:1 (2:1 molar ratio). This means that the number of negative charges on the clay that were neutralized by Ca2+ was four times that of those neutralized by K+ in the unaltered clay. This selectivity is easily explained by the greater charge density, i.e., charge-to-radius ratio, of Ca2+ as compared to K+ . It is also consistent with the fact that the unaltered clay shows little tendency for collapsed layers, even in the presence of K+ (Khaled and Stucki, 1991; Shen and Stucki, 1994).
Reduction of structural Fe for a period of 0.33 hr decreased the selectivity for Ca2+ sharply (Table 1; Figure 2) to an equivalent ratio of about 0.5:1 (0.25:1 molar ratio). This large effect of structural Fe oxidation state is unlikely to be the result of changes in coulombic forces, even though the layer charge and surface charge density of the clay increase upon Fe reduction (Lear and Stucki, 1989), because such forces would favor Ca2+, not K+, as observed in the unaltered sample. How, then, is this explained? Another factor to consider is the tendency for clay layers to collapse around the K+ ion. This tendency increases as structural Fe is reduced, and clearly favors K+ over Ca2+ as the cation becomes fixed in the mineral interlayer region (Figure 1). The change in selectivity from Ca2+, to K+ must, therefore, be attributed to the collapse of clay layers in response to the Fe reduction phenomenon, as reported earlier by Khaled and Stucki (1991).
Continued reduction for as long as 4 hr resulted in a minor reversal in the
selectivity trend (Table 1; Figure
2). The greater reduced smectite exhibited slightly less preference for
K+ over Ca2+ than its lesser reduced partner. This phenomenon
is difficult to explain based on current understanding of the cation fixation
processes because no studies comparing the fixation of different cations have
been carried out at such large Fe(II) contents. Other studies, including fourier-transform
infrared and extended X-ray absorption fine structure (Stucki, Gates, Manceau,
and Huo, unpublished), however, indicate that as the clay becomes reduced to
such levels a transformation within the clay crystal occurs. This change involves
a loss of structural OH groups and migration of Fe from one octahedral site
to another. The net result of these two processes is that the charge distribution
and oxygen orbital configurations at the surface of the clay layer likely are
modified significantly. These are the factors that are believed to govern layer
collapse. So, although direct evidence is lacking, a reasonable hypothesis
is that the trends for layers to collapse and fix interlayer cations could
be reversed at greater levels of reduction. Indeed, this is what Shen (S. Shen,
Ph.D. thesis, University of Illinois) observed when studying the effects of
redox processes on the fixation of K by illite and illitic soils -- Fe reduction
decreased rather than increased K fixation! Illite naturally has a much more
collapsed structure than smectite. The significance of this observation of
a change in the direction of Ca selectivity over K upon Fe reduction may be
far-reaching in understanding the net result that occurs in the field, where
illite and montmorillonite are both present, Ca and K are both present, and
all are together competing for lowest energy states.
The selectivity of soil clay minerals for K+ is greatly decreased
when cations with greater charge density, such as Ca2+, are present.
As structural Fe in the clay is reduced to Fe(II), this selectivity trend is
reversed until K+ is preferred over Ca2+ at the exchange
sites on the clay. These observations are explained by two phenomena, namely,
coulombic attraction between the clay surface and the cation (which favors
cations with greater charge density), and attraction of one clay layer for
another to produce a collapsed interlayer state (which favors cations with
lesser charge density). When Fe is oxidized, the coulombic process dominates;
when reduced, the layer-collapse mechanism is dominant. The significance of
these results for Illinois agriculture is that the distribution of soil K between
plant available and non-available states depends not only on the oxidation
state of Fe and the clay mineralogy, but the extent of these effects is compounded
by the suite of other cations that are present.
Table 1: Effect of Ca2+ competing for K+ in oxidized and reduced ferruginous smectite (SWa-1)
Chen, S. Z., P. F. Low, and C. B. Roth. 1987. Relation between potassium fixation and the oxidation state of octahedral iron. Soil Sci. Soc. Am. J. 41:82-86.
Khaled, E. M., and J. W. Stucki. 1991. Effects of iron oxidation state on cation fixation in smectites. Soil Sci. Soc. Am. J. 55:550-554.
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.
Shen, S., and J. W. Stucki. 1994. Effects of iron oxidation state on the fate and behavior of potassium in soils. Chapter 10. p. 173-185. In Havlin, J. L., J. Jacobsen, P. Fixen, and G. Hergert (eds.) Soil Testing: Prospects for Improving Nutrient Recommendations. SSSA Special Publication 40. Soil Science Society of America, Madison.
