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VI. Experimental Observations and Future Quantity/Intensity Applications
V. P. EVANGELOU ET AL.
-Woodruff's predicled range of A G for
optimum K a v o l l o b i l i t y to p l a n t s
K;C mmol C'
Figure 16 The effect of increasing K + concentration on the uptake of K in 24 hours. The
concentration of Ca2+was 10 mmol, liter-' and the pH was 6. Modified from Mass (1969).
validity of Woodruff's observation that K availability could be described by the
free energy of K-Ca exchange as long as all other plant growth soil factors are
not limiting and are held constant (Beckett, 1972).
The use of Q/I in predicting K availability to plants in soils has been extensively tested in the past (Beckett, 1972; Bertsch and Thomas, 1985, and references therein). However, it has been shown that the Q/I relationship does not
enjoy universal application, because a single relationship for all soils between K
uptake by a given crop and ARK does not exist, perhaps due to the nature of the
soil components regulating ARK. Recall that the term ExK is related to
KG(CEC)(ARK) [Eq. (48)].By rearranging Eq.(48)and substituting the relationship AG = - RT In K G we can show that
RT ln[ARK] = RT In ExK
+ RT ln(l/KG) + RT ln(l/CEC)
Equation (88) reveals that when ExK represents an insignificant portion of the
CEC of a soil (Evangelou and Karathanasis, 1986), RT In[ARK]is determined
by three terms, namely, quantity of exchangeable K+, magnitude of K G , and
magnitude of CEC. The latter two components represent the PBCK of the soil.
Experimental evidence on the role of PBCK on K availability to plants was summarized by Khasawneh (1971). He pointed out that in the case of two soils with
the same K G and the same quantity of exchangeable K + ,but with one soil having
a CEC lower than the other, the ARK would be higher in the soil with the lower
CEC. Consequently, the soil with the lower CEC would allow greater K uptake
SOIL POTASSIUM QUANTITYDNTENSITY RELATIONSHIPS 2 17
by a given crop, assuming all other plant growth factors were held constant. A
similar argument could be made by varying the KG and holding the CEC constant. On the other hand, when two soils have identical ARK values but the CEC
and/or KG of one soil are higher than those of the other, then the quantity of
exchangeable K+ will be higher in the soil with the higher CEC and/or KG and
this soil will allow for a greater K uptake by a particular crop, if all other plant
growth factors were held constant. Similar data and arguments were presented
by Mengel (1982, and references therein). For the same reasons Beckett (1964a)
stated that ARK should be used as a comparative measure of K nutrition only for
soils of similar Ca status.
The above concepts demonstrate that soil K availability to plants is described
by the interrelationship between ARK and PBCK. Therefore, ARK could be related to K uptake by plants, assuming that it is able to predict soil PBC, during
the growth period of plant(s). Presently, such a prediction is not possible. For
example, in some soils ExK may represent a significant portion of the CEC and
therefore the relationship between ARK and ExK is a curvilinear one (Evangelou
and Karathanasis, 1986). In such case, the ability of a soil to supply K to plant
roots is rather complex. Le Coux and Summer (1968b) showed that during a K
uptake period of 30 days and employing Japanese millet as a test plant, soil PBCK
changed and, furthermore, there was no single ARK-K uptake relationship for
all three soils tested.
Grove et al. (1987) showed that the relationship between concentration ratio
for K (CRK) and relative yield of soybeans in three Kentucky soils was excellent,
whereas the relationship between extractable K by neutral, molar ammonium
acetate and relative soybean yield was relatively poor. Possible reasons for the
strong correlations between CRK and relative soybean yield for all three soils
include a fairly similar soil mineralogy and, consequently, similar Ca status or
PBCK values. It was of interest to note that two of the soils exhibited indistinguishable Q/I plots. For these two soils, as expected, the relationship between
relative yield and ammonium acetate-extractable K appeared similar to the relationship between relative yield and CRK . On the other hand, the third soil exhibited very low PBCK but a relatively large quantity of high-affinity K. These
results indicate that if soils have similar PBCK values, ARK or CRK alone can
effectively describe behavior of K uptake. However, as Rasnake and Thomas
(1976) pointed out, for many different soils, because different sites of adsorption
for K+ exist and because these sites vary drastically in their affinities for K + ,
single determinations of concentration or activity ratios or exchange coefficients
of soils are not sufficient for predicting K availability to plants. Rasnake and
Thomas (1976) reported that the K+ for six Kentucky soils changed in the range
of two- to sixfold from the period before cropping “Midland” bermuda grass
[Cynodon dactylon (L.) Pers] to after cropping (Table VI). The causes for these
large changes in the magnitude of the K + appear to be related to mineralogy and
V. P. EVANGELOU ET AL.
Gapon Exchange Coefficient of Several Kentucky Soils before and
Gapon coefficient (moI/Iiter)
Huntington sil (1)
Huntington sic1 (11)
"After Rasnake and Thomas (1976).
to the chemistry of the soil system along with physical processes such as wetting
and drying (Beckett, 1964b; Carson and Dixon, 1972, and references therein;
Knibbe and Thomas, 1972).
Natural soils are polycationic systems and for this reason Eq. (33) [or Eq.
(SS)] does not satisfactorily describe soil PBC, . In polycationic systems, ions
such as Mg2+,Na+, NH,', H 3 0 + ,H + , A13+ and the Al-hydroxy species need
to be taken into consideration in order to predict ARK and PBCK . At the present
time accomplishing such a task is impossible for a number of reasons. Exchange
constants of polycationic systems are not necessarily similar to binary exchange
constants (Lumbanraja and Evangelou, 1990, and references therein). Furthermore, ions such as H + and A13+and the numerous Al-hydroxy species appear to
act as soil potential-determining ions (Sposito, 1984b; Lumbanraja and Evangelou, 199l , and references therein). Currently, our understanding of such reactions at the level where we can accurately predict them is extremely poor, especially in polycationic systems. Even greater difficulties in describing the above
reactions are encountered when organic surfaces are involved.
Researchers have attempted to describe ARK in the presence of more than two
ions. For example, at the suggestion of Beckett (1964a,b, 1965), researchers
routinely include Mg2+and Ca2+when calculating ARK. Addition of Ca2+and
Mg2+ has thermodynamic justification because the two cations are nearly indistinguishable with respect to surface exchange reactions, although exceptions to
this have also been reported in the literature (Sposito and Le Vesque, 1985;
Sposito and Fletcher, 1985). Incorporation of Mg2+ into the calculation of ARK
is correct with respect to the mathematical treatment of exchange reactions; however, its implication with respect to K+ uptake by plants is a different matter. It
is well known that in some plants Mg2+uptake is highly controlled by K + levels,
SOIL POTASSIUM QUANTITY/INTENSITY RELATIONSHIPS
but the reverse is not always true (Stout and Baker, 1981). Furthermore, competitive plant uptake effects between Ca2+ and Mg2+ may also influence the
ARK-K uptake relationship (Khasawneh, 1971, and references therein).
Tinker (1964) included A13+ in calibrating ARK in a manner similar to that
used by Beckett (1964a,b) to include Mg2+.However, incorporation of AP+ into
the ARKas proposed by Tinker (1964) so that ARK = [K+/(Ca2+ Mg2+)”’
is strictly empirical. This may only allow one to evaluate the possible
influence of A13+on K + uptake in a particular study involving a soil or a number
of soils. In terms of modeling K availability in a soil system, A13+ inclusion in
the above manner needs experimental justification. Clay surface-adsorbed A1 and
its hydroxy species act as potential-determining ions, thus as pH changes and
dissolved A1 in solution changes, so does the apparent soil surface electric potential (Lumbanraja and Evangelou, 1991, and references therein). This affects
the magnitude of KG or Kv for all nonhydrolyzable exchanging metal-ion species
in the pH range studied.
The component that describes ion availability of a given ion is the electrochemical potential gradient of that ion between the solid phase and the solution
phase as well as between the solution phase and the root surface phase, or the
electrochemical potential difference between the soil surface phase and the root
surface phase. The Q/I approach allows us to estimate the difference in the relative chemical potential of an ion between the solid and solution phases. This
difference depends on the magnitude of the difference in the electrical potential
between the solid and solution phases. The latter difference cannot be calculated
nor can it be measured directly, and in a soil system it is highly transient depending on surface-specific reactions and solution ionic strength changes. This difficulty may add a major limitation to the potential of applying Q/I data obtained
under certain conditions [e.g., pH, ionic strength, Ca concentration, exchange
phase makeup, and soil solution anion type(s)] to a soil that will undergo a
number of changes during a single growing season.
The purpose of this review is to deal with the Q/I concept alone; however, it
is very difficult for one to separate plant nutrient availability effects due to soil
K chemistry from plant nutrient physiological effects (Haynes, 1980, and references therein). The Q/I concept does not deal with physiological effects, but it
appears to be influenced by them. One should be familiar with plant nutrition
concepts in order to have a better understanding of Q/I and its limitation in
predicting ion uptake.
Nevertheless, even if nutritional effects are assumed to play a limited role in
K uptake under a given set of experimental conditions, the ability of a soil to
replenish K + in the soil solution as described in this review is a highly complex
process and definitely not solely dependent on K-(Ca
Mg) exchange alone.
Additional factors controlling K availability, as discussed in this review, include
(1) hysteresis or exchange irreversibility effects, (2) anion effects, (3) multica-
V. P. EVANGELOU ETAL.
tion effects, (4) potential-determining ion effects, and (5) kinetic effects. There
is no doubt that these factors need further investigation in soils in order to predict
K availability. Furthermore, similar investigations must be carried out with respect to plant-root surfaces to determine the concentrations of particular ions
that the plant root surfaces encounter in soil and how the root surface interacts
with a particular soil, or soil solution (Wang et al., 1992).
In addition to understanding the interactive chemistry between soil and soil
solution and/or plant root surfaces, K availability is also influenced by the diffusivity of the soil medium, and consequently by the soil moisture content (Mengel, 1982, and references therein).
Aside from the fact that Q/I plays a fundamental role in understanding K
availability to plants, an additional purpose is its use in the environmentally
sound management of fertilizers involving soil surface-solution interactive components. These components determine soil solution composition and thus the
potential fate of plant nutrients, e.g., leaching, uptake, and transformations.
Recent breakthroughs in highly resistant ion-selective electrodes will allow
them to be attached directly to farm implements and thus used in monitoring/
regulating fertilizer applications. For example, a NO,- electrode of this type is
currently under field evaluation at the Princeton Experiment Station of the University of Kentucky and the preliminary results look promising (L. W. Murdock,
personal communication). Apparently, newly developed ion-selective electrode
methods for rapidly and accurately estimating soil Q/I relationships in soil suspensions (Wang, 1990; Wang et al., 1990) could also be used in the field for
monitoring/regulating the application of K’ , NH,+ , and/or Ca2+ plus Mg’+.
The ultimate future application of Q/I (as computing power becomes less costly
and understanding soils and plant root systems as polycationic systems advances)
would be in modeling soil-plant systems on a real-time basis.
The authors wish to dedicate this article to Drs. Woodruff and Beckett for their major contribution
to understanding potassium chemistry in soils and soil potassium availability to plants.
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