Illinois Fertilizer
Conference Proceedings
January 27-29, 1997
| Main Index | 1997 Index | Search |
|
|
| Main Index | 1997 Index | Search |
Joseph W. Stucki1 and Dongfang Huo2
Potassium is one of the most important plant nutrients in soils, and has thus been studied extensively (Potash and Phosphate Institute, 1980; and Munson, 1985). But in spite of much study and diligent efforts, the fundamental chemical and physical phenomena which govern its fate, movement, and availability to plants in soils have yet to be fully characterized (Bertsch and Thomas, 1985). Soil tests for K often fail to reveal the true fertilizer demand in the field, resulting in frequently unreliable and inefficient fertilizer recommendations (Munson, 1980). Many factors contribute to this problem, but perhaps the one factor which makes the solution to the problem so elusive is that K, which in the soil phase is usually distributed among soluble, exchangeable, fixed, and insoluble forms, becomes redistributed among these forms in an unpredictable manner. In other words, the fate and movement of K in the soil have yet to be well understood. Any unreliability in differentiating between the available and non-available forms, and uncontrolled or unrecognized transitions of K among its forms between the time of testing and the time the plant "sees" the soil will produce an erroneous result. The purpose of this study is to not only understand better the fate and behavior of K in the soil, but to determine the energy differences among the different forms of K in the soil, thence to devise a useful method for separating these forms in a soil test.
Oxidation and reduction (redox), the terminology used to describe the chemical
process which changes the electrical charge of a chemical element, may account
for much of the variability observed. Some of the elements in soils which are
susceptible to redox changes are iron (Fe), manganese (Mn), nitrogen (N), and
sulfur (S), 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
Fe(II) is converted to Fe(III), and the minerals are converted to more expansive
types which release various nutrients, including K+, to the soil
solution. The in-place reduction of Fe(III) to Fe(II) 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.
Early studies in our laboratory revealed 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) observed a
rapid increase in the nonexchangeable form of K+ with Fe(H) 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 nonexchangeable
fractions with increasing Fe(II) content of Upton montmorillonite gels (which
are rather typical of soil smectites) which were never dried, and found that
fixation increased steadily with increasing Fe(H) but that the exchangeable
fraction remained almost constant. Calculations revealed that this could easily
account for the fixation of as much as 1000 lbs K20/acre. In other
words, the oxidation state of Fe in the mineral is an extremely important, potential
factor for determining ion fixation in soil clay minerals. Outside of the studies
cited above, this phenomenon has apparently been overlooked by soil fertility
research, but appears to be a vital factor which must be characterized fully.
More recent results confirmed the previously observed, strongly positive relationship
between structural Fe(II) in smectites and K fixation, and that with successive
redox cycles K fixation and structural Fe reduction become increasingly irreversible.
Reduction of the Fe increases fixed K in smectite from about 2.5 to about 35
meq/100 g. Reoxidation decreases K fixation to only about 25 meq/100 g. The
amount of Fe(H) that resists reoxidation increases to about 18% of the total
Fe. This process could account for the formation of illite over time as the
soil experiences cyclic redox events.
In the present study we investigated the impact of competing cations on the
selectivity of clay surfaces for K, and how reduction and reoxidation of structural
Fe affects that selectivity. This is a fundamentally important component to
understanding the partitioning of K between fixed and exchangeable forms because
it does not exist in the soil alone, but must compete with other cations for
clay surface sites. Multi-cation competition in the interlayer of clay minerals
has been discussed by Sposito and Fletcher (1985) and Sposito et al.
(1983). In general, clay surfaces exhibit overwhelming preference for divalent
over monovalent cations. Does that same trend continue when the oxidation state
of the Fe is changed?
Duplicates of several 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 citrate-bicarbonate buffer (pH of about 8) with 100 mg of sodium
dithionite (Na2S204) at 75 °C. The time
of reduction under these conditions was varied between 0 and 4 hr of reaction.
Samples were also reoxidized by bubbling oxygen through their aqueous suspensions
for 12 hr.
Aqueous Na-clay suspensions were exchanged twice with an 02-free
solution of K and Ca, prepared with a concentration ratio of 0.25 M Ca2+/0.5
M K+ (1:1 equivalent ratio); 0.5 M
Ca 2+/0.5 M K+ (2:1 equivalent ratio), and 0.125
M Ca2+/0.5 M K+ (1:2 equivalent ratio). These suspensions
were shaken for about 1 hr for each exchange in order to attain complete replacement
of Na+ by the K+ and Ca2+. The exchange was
followed by two washings with 02free 0.005 N K+/Ca2+
solution to remove excess salts. The final supernatant was saved for Ca and
K analysis to determine the equilibrium outer solution concentration.
Some sample sets were reoxidized prior to final washing by bubbling 02
through them for 12 hr. Then the exchangeable K+ and Ca2+
was measured by washing three times with 1 M ammonium acetate solution.
The amount of fixed cation was determined by the residual of that cation found
in the sample after complete digestion, as described below.
Samples were freeze-dried and 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.
In the unreduced clay we confirmed that the surface strongly favors the divalent
over the monovalent cation, as shown by the first point of curves A and C in
Figure l, wherein the exchange complex
was 80% occupied by Ca2+ and only 20% by K+ when the Ca2+:K+
equivalent ratio in the exchanger solution was 1:1. 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).
When structural Fe(III) in the clay was reduced to Fe(II) by dithionite, the
cation distribution on the exchange complex shifted to more than 60% K+
and less than 40% Ca2+ (compare curves A and C in Figure
1). The selectivity for K+ over Ca2+ increased with
increasing ratio of K:Ca in the exchanger solution. 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. Instead, it is the result
of the collapse of clay layers around the K+ ion. This tendency for
layers to collapse in the presence of K+ as layer charge increases
is well known in clays, and is attributed to its low hydration energy (i.e.,
the cation is dehydrated in the clay interlayer region) and its ability to "key"
into the hexagonal holes of the planar clay surface. Other studies in our laboratory
have also discovered significant rearrangements in the location of Fe in the
clay crystal during the reduction process, which could give rise to profound
changes in the interlayer attractive forces and hydration energy of the bare
surface. All of these factors could influence the interaction of the clay surface
with its exchanged cations.
Reoxidation of the reduced sample shifted the selectivity back in favor of Ca2+
(curves B and D in Figure 1), but failed
to restore it to the original selectivity of the unreduced (oxidized)
system. Calcium is still strongly favored in the reoxidized clay, even though
not to the extent as in the unreduced system. This represents the completion
of one redox cycle. One could speculate that after many such redox cycles,
the trend could cause the selectivity to miIrate more toward K+.
The extent of selectivity for Ca2+ is also related directly to
the initial Ca2+:K+ equivalent ratio in solution (Figure
2a)(Figure 2b). Notice that the amount
of exchangeable Ca2+ in the reoxidized
clay is always greater than exchangeable K+, but the difference between them
decreases as the Ca2+:K+ equivalent ratio decreases.
This is interesting because if the surface selectivity is a result of surface
properties alone, the phenomenon should be independent of the solution.
These results clearly show that other principles are operating besides surface
site selectivity. Examples of such principles include simple phase equilibrium,
effect of divalent cations on electrical double layer thickness, and effect
of different valent cations on layer collapse. The extent of reversibility may
also be affected by the initial degree of reduction and the ionic strength in
the reoxidizing medium.
The extent of cation fixation was also measured in the reoxidized samples, using
ammonium acetate as the exchanger salt. By this method more than 99% of the
interlayer cations were recovered (Figure 3),
whereas exchange with Mg2+ (Khaled and Stucki, 1991) discriminated
more precisely. This raises doubts with regard to our laboratory methods for
measuring K in its different forms. Why did ammonium replace or exchange such
a large fraction of the interlayer cations when other studies indicate that
a significant amount should have been fixed in the reoxidized sample? One possibility
is that the measurements of fixed cations in the present study are in error,
even though they were replicated. The fact that the total exchangeable K+
and Ca2+ is similar to values found in other studies, but is low
if taken to be the total layer charge, suggests that a significant fraction
of non-exchangeable cations must be present. Why were they not observed in this
study? This is a matter of on-going investigation.
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.
Upon reoxidiation, the trend is partially reversed. 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.
Figure 1. Cation Selectivity by Oxidized, Reduced and Reoxidized Smectite
Figure 2a. Effect of Ca2+:K+ Equivalent Ratio in Solution on Cation Selectivity
Figure 2b. Effect of Ca2+:K+ Equivalent Ratio in Solution on Cation Selectivity
Figure 3. Total Exchangeable and FIxed Cations in Oxidized, Reduced, and Reoxidized Smectite
1Joseph W. Stucki is Professor, Dept. of Natural Resources and Environmental Sciences. 2 Dongfang Huo is graduate student in professor Stucki's laboratory
Bertsch, P. M., and Grant W. Thomas. 1985. Potassium status of temperate region
soils. p. 131162. In R. D. Munson (ed.) Potassium in Agriculture. Soil
Science Society of America, Madison.
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.
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.
Munson, R. D. 1980. Potassium availability and uptake. p. 28-66. In Potassium
for Agriculture -- A Situation Analysis. Potash and Phosphate Institute,
Atlanta.
Munson, R. D. (ed.) 1985. Potassium in Agriculture. American Society
of Agronomy, Madison. 1223 p.
Potash and Phosphate Institute. 1980. Potassium for Agriculture - A Situation
Analysis. Potash and Phosphate Institute, Atlanta. 216 p.
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.
Sposito, G., and P. Fletcher. 1985. Sodium-calcium-magnesium exchange reactions
on a montmorillonite soil: II. Calcium - magnesium exchange selectivity. Soil
Sci. Soc. Am. J. 49:1160-1163.
Sposito, G., K. M. Holtzclaw, C. Jouany, and L. Charlet. 1983. Cation selectivity
in sodium-calcium, sodium-magnesium, and calcium-magnesium exchange on Wyoming
bentonite at 298 K. Soil Sci. Soc. Am. J. 47:917-921.
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 York. 217 p.
