Illinois
Fertilizer Conference Proceedings
January 25-27, 1993
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Joseph W. Stucki and Siyuan Shen1
In spite of many years of study, scientists still have a rather poor understanding of the behavior of potassium (K) in soils, as evidenced by the lack of a reliable model for explaining the many contradictions in soil test-plant response relationships. The hypothesis being tested in this study is that the iron (Fe) oxidation state in the crystal structures of soil minerals is an essential factor in understanding the behavior of K.
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. Oxidation and reduction (redox) is, therefore, the chemical process by which a change in the electrical charge of a chemical element occurs. Some of the elements in soils that are susceptible to redox changes are Fe, Mn (manganese), N (nitrogen), and S (sulfur). Such processes are commonly found in agricultural soils as a result of biological activity and alternate wetting and drying events. Iron is an abundant constituent of soil minerals and may be present in these minerals as either the +2 (ferrous iron, Fe2+) or +3 (ferric iron, Fe3+) oxidation state.
Changes in Fe oxidation state alters many important chemical properties of its host mineral. Recent studies showed that K fixation occurred in smectite clay minerals as a result of Fe reduction from Fe3+ to Fe2+. The present study is expanding those studies to provide new 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.
Redox reactions involving these elements, and particularly Fe, are responsible for many changes in the physical and chemical properties of soils (Yu, 1985). While the transformation of N to its various forms, ranging from nitrate (NO3-) to ammonium (NH4+), is probably the most studied redox process in soils, significant changes in oxidation state also occur in many soil minerals because of the presence of Fe in their crystal structures. The weathering of primary minerals is an example of an oxidation process in which Fe2+ is converted to Fe3+, and the minerals are converted to more expansive types which release various nutrients, including K+, to the soil solution. The in-place reduction of Fe3+ to Fe2+ in secondary minerals (vermiculite, montmorillonite, Illite) also occurs, and creates a climate in which some of the beneficial effects of weathering may be reversed, such as the fixation of K+. This is due to numerous changes which are invoked in the physical-chemical properties of the mineral phase of soils, including swellability in water (Stucki et al., 1984), electrical charge (Stucki and Roth, 1977; Lear and Stucki, 1985), and surface area (Lear and Stucki, 1989). These redox phenomena are of great importance to soil fertility because the availability of plant nutrients depends in large degree on the surface chemistry of the minerals. Since the oxidation state will vary with different environmental conditions, the associated properties also may change significantly throughout the year.
Recent studies have shown that the change in surface charge that occurs during
Fe reduction increases the ability of the mineral to fix interlayer cations,
including Na+ (Lear and Stucki, 1989), Ca2+, Cu2+, and Zn2+ (Khaled and Stucki,
1991), and K+ (Chen et al., 1987; Khaled and Stucki, 1991). Chen et al. (1987)
reported a rapid increase in the non-exchangeable form of K+ with Fe2+ content
of freeze-dried soil clays and standard reference clays, wherein the total fixation
capacity reached as high as 30% of the total exchange capacity. Khaled and Stucki
(1991) followed the distribution of K+ between exchangeable and non-exchangeable
fractions with increasing Fe2+ content of Upton montmorillonite gels (which
are rather typical of soil smectites) which were never dried, and found that
fixation increased steadily with increasing Fe2+ but that the exchangeable fraction
remained almost constant. They estimated that this could easily produce a K+
fixation capacity in the soil of about 900 lbs K2O/acre 6-inches. In other words,
the oxidation state of Fe in the mineral is potentially an extremely important
factor for determining ion fixation in soil clay minerals. Outside of the studies
mentioned above, this phenomenon has apparently been overlooked by soil fertility
research, but appears to be a vital factor which must be characterized fully.
The objective of this study was 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. Specific objectives were to:
The Fe(II) content and the amount of exchangeable (Ke ), total (Kt ), and fixed (Kf ) potassium after one reduction-reoxidation cycle were measured. Materials used were K-saturated samples of two reference smectite clays (SWa-1 and API 25), one reference Illite (Fithian), and the clay fractions from the A horizon of three Illinois soils (Drummer from Dekalb, Drummer from Urbana, and Cisne from Brownstown). Rates of sample color change under reducing and reoxidizing conditions for as many as 7 redox cycles were also determined.
Each sample was saturated with K+ by three washes of 1 M KCl solution, followed by four washes of high grade water to remove extra K+ in the solution. The sample was then resuspended in 10 ml of potassium citrate-bicarbonate buffer, which was made by mixing 1 part of 0.3 M potassium citrate with 8 parts of 1 M potassium bicarbonate, and placed in a water bath at 70 °C. Two hundred mg of sodium dithionite (Na2S2O4) was added to reduce the structural Fe in the sample. The reaction was allowed to proceed for 4 hr with N2 gas flowing continuously, then the sample was washed with 1 M deoxygenated KCl solution under inertatmosphere conditions. Excess salts were removed by 4 washings with deoxygenated, deionized H2O under an 02-free environment. A portion of the reduced sample was reoxidized by bubbling H2O-saturated 02 gas through the suspension for 0.5 hr.
Total Fe and Fe(II) were measured before and after reduction, and after reoxidation, using the method of Komadel and Stucki (1988). The samples were also analyzed in these respective states for K as follows. Total K in the clay sample is given by
Kt = Ks + Ke + Kf (1)
where Kt, Ke, and Kf are, respectively, the amount of K+ free in solution (not associated with the clay surfaces), clay exchanged, and fixed. The total K+ associated with the clay, represented by Kc, is the sum of Ke and Kf, viz.,
Kc = Ke + Kf (2)
If the clay is homoionically saturated with K+, then Kc equals the total negative layer charge, v , and Ke is the cation exchange capacity, ω, giving
ω = Ke (3)
v = Ke + Kf = ω + Kf (4)
The value of Ks, was determined by measuring the K+ concentration in the supernatant from the last H2O wash, using atomic emission spectroscopy (766 nm). Exchangeable K+ was obtained by washing the clay three times with -deoxygenated, 0.5 M MgCl2 solution and analyzing the combined supernatants for K. Total K was determined by atomic emission analysis of the fully digested sample, from which Kt was calculated. Kf was then calculated from Equation (1).
Smectites
The effects of one redox cycle on Fe(II) contents of six clay samples reduced and reoxidized are reported in Figure 1. Sample SWa-1 contained the most total Fe of all samples, and the Drummer soil from Urbana was highest among the soil clays. The Fe(II) content of these samples in their initial, oxidized (unaltered) state varied from less than 0.16 % of the total Fe in the smectites to more than 25% in Fithian Illite. The initial Fe(II) content of the soil clays was similar to smectite API 25, ranging from 3 % to 10% of their total Fe.
When the samples were reduced by sodium dithionite, the Fe(II) content exceeded 60% of total Fe in all samples except the Drummer soil from Dekalb which was reoxidized to some extent during the washing procedure. The levels of Fe(II) obtained are not necessarily the limiting case because longer treatments are known to reduce all of the Fe in SWa-1 (Komadel et al., 1990), which was only reduced 75 % by the treatment reported in Figure 1.
The 30-minute reoxidation treatment, i.e., bubbling O2 gas through the reduced suspension, failed to reoxidize the samples completely, as found by comparing the initial to the reoxidized Fe(II) contents in Figure 1. This phenomenon was common to all samples, although the ratio of reoxidized to initial Fe(II) varied slightly. Samples were thus somewhat resistant to complete reoxidation. This result differs from Na-saturated smectites which reoxidize completely with a 30-min. exposure to flowing O2 (Komadel et al., 1990). This indicates that structural Fe reduced in the clays is protected to some extent by the presence of K, presumably because of the collapse of superimposed clay layers. This same action could contribute to a fixation of K.
The total and fixed K contents of smectites increased when the clays were reduced, while the exchangeable potassium contents remained constant (bars 1-6 in Figure 2). This result is consistent with observations by Chen et al. (1987) and Khaled and Stucki (1991). But we found that the fixed potassium contents were still high for reoxidized smectites (compare bars labelled I and O, respectively, for the initial and reoxidized smectites SWa-1 and API 25 in Figure 2). This is related to the difficulty of reoxidizing the reduced smectite samples, as illustrated in Figure 1.
Illite
The Fithian Illite sample behaved very differently from the smectites. The amount of fixed K decreased upon treatment with the reducing agent, while Ke content increased (see bars 7-9 in Figure 2). The total potassium content also decreased. When the Illite was reoxidized, Kf increased and Ke decreased, while Kt changed only slightly. So, during one redox cycle, K was released from the Illite into its environment, whereas the smectite removed K from its surroundings. One could imagine, therefore, that a soil containing a mixture of Illite and smectite would either fix or release K during reducing conditions depending on which clay dominates; or, if these minerals are precisely balanced, no change in K availability would occur because the amount that one releases would be fixed by the other. Our hypothesis to explain the behavior of Illite is that as more Fe(II) is added to the octahedral sheet of the Illite during further reduction, the dipole moment of structural OH groups become canted more along the caxis and thereby create a weaker attractive (or stronger repulsive) force between the clay layer and the interlayer K ions. Release of K to soil solution is subsequently increased.
Soil Clays
Results from the soil clays -(Figure 2, bars 10-18) revealed that
the total K content of both Drummer soils remained constant during the redox
cycle, but varied slightly in the Cisne. The distribution of K between Ke
and Kf, however, fluctuated measurably, but less than in the
pure clay samples. The behavior of soil clays appeared to be closer to that
of Illite than of smectite, suggesting that the soil clays may be dominated
by Illite. X-ray powder diffraction revealed that Illite, indeed, comprised
a significant fraction of the constituents clays in the soil samples. The trend
for K behavior is, therefore, in the direction of greater availability of K
with the onset of reducing conditions. Subsequent redox cycling experiments
revealed that complete reoxidation of the Fe is not achieved from one cycle
to the next, suggesting that the amount of available K may also change with
additional cycles. Measurements of the distribution of K after various redox
cycles are currently in progress.
The reduction of Fe in the crystal structures of pure and soil clay minerals
alters the distribution of K between exchangeable and fixed states. In smectite
clay minerals, Fe reduction increased K fixation without changing the amount
available in exchangeable sites. In Illite, however, the reduction treatment
released K to the surrounding solution, possibly because of a change in electrostatic
force between K and structural OH groups. The behavior of soil clays is thus
highly dependent on the type of clay minerals that is present. If Illite dominates,
reduction may enhance K availability, whereas smectite domination will produce
greater K fixation during reducing conditions. Redox cycling definitely increases
the amount of residual Fe(II) in the clays, and may also permanently alter the
amount of available K.
1Professor of Soil Physical Chemistry and Graduate Research Assistant, respectively, University of Illinois, W-317 Turner Hall, 1102 South Goodwin Avenue, Urbana, IL 61801.
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. Stuckl. 1991. Effects of iron oxidation state on cation fixation in smectites. Soil Sci. Soc. Am. J. 55: (In Press)
Komadel, P., P. R. Lear, and J. W. Stuckl. 1990. Reduction and reoxidation of iron in nontronites: Rate of reaction and extent of reduction. Clays Clay Miner. 38:203-208. 4. Komadel, P., and J. W. Stuc1d. 1988. The quantitative assay of minerals for Fee+ and Fe3+ using 1,10-phenanthroline. III. A rapid photochemical method. Clays Clay Miner. 36:379381.
Lear, P. R., and J. W. Stuck!. 1985. Role of structural hydrogen in the reduction and reoxidation of iron in nontronite. Clays Clay Miner. 33:539-545.
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.
Stucki, J. W., and C. B. Roth. 1977. Oxidation-reduction mechanism for structural iron in nontronite. Soil Sci. Soc. Am. J. 41:808-814.
Stucki, J. W., P. F. Low, C. B. Roth, and D. C. Golden. 1984. Effects of oxidation state of octahedral iron on clay swelling. Clays Clay Miner. 32:357-362.
Yu, Tian-ren. 1985. Physical Chemistry of Paddy Soils. Springer-Verlag, New
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