Illinois
Fertilizer Conference Proceedings
January 27-29, 1992
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Siyuan Shen and Joseph W. Stucki1
This is a three-year project, which officially began in July, 1991, to study
the effects of iron oxidation state on the fate and availability of potassium
in agricultural soils. In spite of many years of study, scientists still have
a rather poor understanding of the behavior of potassium 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 (Fe) is one element which 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 be present in either of
these states. Interestingly, changes in Fe oxidation state cause important changes
in the chemistry of its host mineral. Recent studies have shown that potassium
(K) fixation occurs in clay minerals as a result of Fe reduction from Fe3+
to Fe2+. The proposed study will expand 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.
The hypothesis to be tested in this study is that the effect of Fe oxidation state in the crystal structures of soil minerals is an essential factor in understanding the behavior of potassium (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:
Oxidation and reduction (redox) is the chemical process by which a change in the electrical charge of a chemical element occurs. Some of the elements in soils which are susceptible to redox changes are Fe, Mn (manganese), N (nitrogen), and S (sulfur), and such processes are commonly found in agricultural soils as a result of biological activity and alternate wetting and drying events. 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., 1984c), 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 nonexchangeable 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 (Figure
1). By interpolating the data in Figure l, notice
that with only 20% of the Fe reduced (% Fe(II) to Total Fe = 20), the amount
of fixation is about 10 meq/ 100 g of clay. In a soil having a clay content
of 15 % by weight, of which perhaps 2/3 would be smectite, the K+
fixation capacity would be about 9001bs 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.
Each of three soils will be separated into five fractions, namely, clay, silt, sand, whole soil, and whole soil minus the organic matter. Each fraction will then be treated with an appropriate reducing agent and with K+, then the resulting level of fixation will be determined under the influence of variables such as moisture content and time of exposure. The effect of cyclic reduction, reoxidation, re-reduction, etc., will be determined.
In order to accomplish objectives 1 and 2, the experimental program will follow the general outline given in Figure 2. The amount of K+ fixation will be determined at various levels of Fe reduction and reoxidation along the dotted-line path shown in the base plane of the figure. This will establish the effects of both Fe reduction and several redox cycles.
Objective 3 will be accomplished by submitting five different soil fractions, namely, clay, silt, sand, whole soil, and whole soil minus organic matter, from three different soils to the process shown in Figure 2.
The first two parts of objective 4 will be achieved by varying the levels of
K+ added to the sample, ranging between 25% and a great excess of
the exchange capacity. Some samples will be allowed to stand for extended time
periods in order to gauge the effect of time on fixation events. The third part,
namely drying, will be tested simply by drying selected samples either by air,
oven, or freeze-drying methods prior to K+ determinations. To fulfill
the fourth part of objective 4, three reduction methods will be compared. The
first and most extreme method will be chemical treatment with sodium dithionite
(Na2S2O4) buffered to Ph 8 with a solution
containing both potassium citrate (K3C6H7O5)
and potassium bicarbonate (KHCO3) (methods described by Stucki et
al., 1984a; Chen et al., 1987; Komadel et al., 1990). The second method will
utilize Fe reducing microorganisms which are native to agricultural soils. Sterile
soil suspensions will be prepared with a nutrient broth deficient in Fe and
incubated for different lengths of time to achieve differing levels of microbial
reduction (see method described by Stucki et al., 1987; Komadel et al., 1987).
The third method will involve simply incubating the soil or soil fraction for
various lengths of time in a submerged environment. Reoxidation will be achieved
by purging the system with oxygen gas.
This initial period of the project has been devoted almost entirely to sample preparation. Soil samples were obtained from experimental fields near Urbana, Dekalb, and Brownstown, and the clay, silt, and sand size fractions were separated from each. After separation, each fraction was freeze dried. Reference smectite and illite clays were also fractionated and prepared for reduction by sodium dithionite (Na2S2O4).
A long-term experiment was begun in which soils will be kept under water-logged conditions for several years, in preparation for tests of their level of reduction and K-fixation capacity after prolonged submergence. These samples were prepared and set aside for future analysis.
The first experiments will focus on the smectite clays and the clay fractions of the soils, and will include tests of cyclic redox effects on layer collapse and K fixation. These studies are scheduled to begin in January, 1992. Evaluation of the levels of K fixation will consist of measuring the exchangeable and total K in each sample after redox treatment. The spacings between clay layers will also be measured using both conventional and synchrotron X-ray methods. The conventional X-ray work will be performed in the Department of Agronomy at the University of Illinois; whereas the synchrotron measurements will be obtained on beam line X-12A at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, Utpon, New York, under a recently approved General User Proposal submitted to NSLS by the Principal Investigator and his research group. The synchrotron work is highly experimental and on the leading edge of soils research technology, so the outcome is uncertain. But the potential benefits are that, by virtue of the much greater beam intensity and choice of X-ray energies of the synchrotron, the quality of data should be much higher. This will improve confidence in the interlayer spacings observed in the Kfixed samples, which is fundamental to understanding the mechanism by which K is retained in the soil minerals.
Microorganisms which reduce large amounts of structural Fe (up to 60 % reduction) in smectites have been obtained from another project, have been inoculated into reference clays. They will also be used with the soil clays in this project to compare with results from dithionite-reduced samples..
Figure 1. Effect of Fe2+ on exchangeable and non-exchangeable K+ in montmorillonite
1Graduate Research Assistant and Professor of Soil Physical Chemistry, respectively, University of Illinois, W317 Turner Hall, 1102 South Goodwin Avenue, Urbana, IL 61801.
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.
2. 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)
122
3. Komadel, P., P. R. Lear, and J. W. Stucki. 1990. Reduction and reoxidation
of iron in nontronites: Rate of reaction and extent of reduction. Clays Clay
Miner. 38:203-208.
4. Komadel, P., J. W. Stucki, and H. T. Wilkinson. 1987. Reduction of structural
iron in smectites by microorganisms. p. 322-324. In E. Galan, J. L. Perez-Rodriguez,
and J. Cornejo (eds.) Proc. Sixth Meeting of the European Clay Groups, Seville,
1987. Sociedad Espanola de Arcillas, Sevilla.
5. Komadel, P., and J. W. Stucki. 1988. The quantitative assay of minerals for Fe" and Fe' using 1,10-phenanthroline. III. A rapid photochemical method. Clays Clay Miner. 36:379-381.
6. Lear, P. R., and J. W. Stucki. 1985. Role of structural hydrogen in the
reduction and reoxidation of iron in nontronite. Clays Clay Miner. 33:539-545.
7. 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.
8. Stucki, J. W., and C. B. Roth. 1977. Oxidation-reduction mechanism for structural
iron in nontronite. Soil Sci. Soc. Am. J. 41:808-814.
9. Stucki, J. W., D. C. Golden, and C. B. Roth. 1984a. The preparation and
handling of dithionite-reduced smectite suspensions. Clays Clay Miner. 32:191-197.
10. Stucki, J. W., D. C. Golden, and C. B. Roth. 1984b. The effect of reduction
and reoxidation on the surface charge and dissolution of dioctahedral smectites.
Clays Clay Miner. 32:350356.
11. Stucki, J. W., P. F. Low, C. B. Roth, and D. C. Golden. 1984c. Effects
of oxidation state of octahedral iron on clay swelling. Clays Clay Miner. 32:357-362.
12. Stucki, J. W., P. Komadel, and H. T. Wilkinson. 1987. The microbial reduction
of iron(III) in clay minerals. Soil Sci. Soc. Am. J. 51:1663-1665.
13. Yu, Tian-ren. 1985. Physical Chemistry of Paddy Soils. Springer-Verlag,
New York. 217 p.
