Tải bản đầy đủ - 0trang
XIV. Empirical Methods for Characterizing Soil Redox
REDOX CHEMISTRY OF SOILS
alters organic and mineral redox and acidity characteristics so that behavior of a dried sample will change markedly with time during dry storage
and while it is returning to its metastable moist state if water is added back
to the dry soil (Bartlett and James, 1980). Rarely will stored dried soil
samples [designated “lab dirt” for emphasis (Bartlett and James, 1979)]
oxidize added Cr(II1). And rarely will a field-moist soil sample fail to
oxidize Cr(II1). After drying, a soil will readily reduce Cr(V1) when it is
remoistened, whereas, in a continuously moist soil,Cr( VI) may persist for
years in the presence of stable humic and fulvic acids.
Moist soil samples are most suitable for handling if near field capacity
moisture. It is difficult to get moist samples to pass through a 2-mm sieve,
but a 4-mm polyethylene sieve is generally suitable and has the advantage
of preserving some of the soil crumb structure and aeration status of field
soils during storage before analysis. Time is saved by presieving through an
old tennis racquet and mixing in the field. Samples should be mixed
individually before sieving. Samples will remain most stable if they are
stored in double, 25-pm-thick polyethylene bags with moist paper towels
between the layers in a refrigerator at 4°C. Freezing and freezedrying
cause soils to change during experimentation almost as much as airdrying
does. Drying in the sunlight is an unconditional never, unless you are
mimicking a field condition and have considered its implications. Microbial activity appears to become stabilized after 2 or 3 months at 4”C, and
most soil samples seem to reach an internal metastable equilibrium. It is
safe at this time to transfer samples to tight heavy-walled plastic bags or
garbage containers with lids to prevent all moisture loss. Samples should be
kept in semidarkness, but temperatures as high as 10- 12°C can be tolerated in metastable moist soils.
Well-mixed subsamples may be weighed for analyses after determining
moisture on separate samples. For many determinations, a volume measurement, to be later corrected for dry weight, is more convenient, although somewhat less accurate. A packed and leveled teaspoon of soil is 5
cm3, approximately 5 g dry weight for many topsoils.
The empirical approach frequently involves laboratory incubations of
small amounts of treated soil in polyethylene bags at about field capacity
moisture. Thin polyethylene has the advantage of being quite permeable to
0, and especially CO, but not to ions and solutions (Bartlett, 1965). Water
vapor will pass through slowly, however, and if you wish to prevent drying
for long periods, double bagging with a moist paper towel between the
layers will help.
R I C H M O N D J. B A R T L E T T A N D BRUCE R. JAMES
For monitoring CO, ,the soil can be left in a closed bag and placed inside
a 4-liter pickle jar with a tight lid, along with a 100-ml polyethylene beaker
containing 1 M NaOH. The base can be titrated with HCl in the beaker
after adding 2 M BaCl, and phenolphthalein.
Also useful are incubations in flooded beakers for somewhat restricted
0, availability and to study interface processes. These should be kept in the
dark, unless you plan to monitor and control the light as part of the
experiment. Samples may be removed from the supernatant solution, from
interface microlayers, or from deep in the sediment.
Redox has become associated with wet soils and platinum electrodes
because electrode potentials seem to be useful only in such soils. Interpretations of redox potential measurements are thoroughly discussed in Section VII; here we simply present a practical method for measuring an
empirical pe (Bartlett, 1981a).
1. Attach a bright platinum electrode (in place of the glass electrode) to
the plus terminal of a pH meter with a millivolt scale and attach a saturated
calomel electrode (SCE) to the negative terminal.
2. Before each reading, rinse the platinum electrode, but not the reference electrode, in a 1/ 1 6 M HCl/liquid detergent solution followed by 10%
H,02 and then thoroughly rinse with distilled water. Clean in aqua regia
after a few hours of use.
3. Adjust the potentiometer to read +219 mV when the electrodes are
in a pH 4 suspension of quinhydrone in 0.1 M potassium acid phthalate.
4. Add 30 ml of 10 m M CaC1, or 30 m M of NaNO, to 10 g of soil (dry
weight basis) in a polyethylene beaker, stir until soil and solution are well
mixed, and let stand for 20 to 30 min with occasional swirling.
5. Insert electrodes so that the calomel reference electrode is in the
upper half of the supernatant solution and the platinum electrode is near
the bottom of the suspension. Swirl for a few seconds, let stand for at least 5
min, and without jiggling or touching the cup, read 8, in millivolts.
Measure pH of the same suspension.
6. EMpe = $
244 for SCE)/59.
Since we do not know how to interpret Pt electrode measurements made
in aerobic soils, their chief value seems to be in telling us whether a soil is
anaerobic. A single sniff may be as useful as an EMpe measurement. It
could be worth our time to develop a method, analogous to the Munsell
color chip book, for quantifymg odors. Meanwhile, common scents, aided
by common sense, will have to suffice.
REDOX CHEMISTRY OF SOILS
AND Fe OXIDES
A safe substitute (Liem ef a/., 1979) for carcinogenic benzidine is made
(TMB) in 28.6 ml
by dissolving 250 mg of 3,3’,5,5’-tetramethylbenzidine
glacial acetic acid and quickly diluting it to 500 ml with distilled water.
When a few drops of this reagent are added, on a spot plate, to a pinch of
moist soil, containing Mn oxides, dark blue points and zones begin showing up after a few seconds, and then slowly or quickly, depending on the
Mn oxide form, the surrounding solution will tend to turn intensely blue.
In very acid soil solutions, ionic Fe(II1) will sometimes give a similar
The TMB indicator produces a positive blue color rather slowly with
NO? and still more slowly with H 2 0 2 . In each case, it appears that the
TMB is acting as a weak reducing agent and forming, respectively, various
free radical species, most likely Mn3+, possibly the fenyl radical, Few?
(Cohen, 1985), probably NO, or NO free radicals, and the OH radical. If
the supermanganese Mn3+ free radical is present to begin with, it will of
course give an immediate reaction with the TMB. However, until time has
been allowed for free radical formation by partial reduction of MnO, by
TMB, the full blue color will not develop. Nitrite, Fe(I11), and H,02each
will produce full color immediately on TMB addition if first partially
reduced by hydroquinone, dipyridyl or o-phenanthroline, or peroxidase,
respectively. An immediate positive TMB test results with Cr( VI).
Thus, although TMB will readily form blue color in the presence of
oxidizing free radicals, especially the hydroxyl free radical or superoxide,
TMB is most useful as an indicator of oxidized species that have in
common the proclivity for being very easily reduced to oxidizing free
radicals. The first increment of TMB added sensitizes the substrate (causes
it to become a free radical) by being oxidized by it so that, when more
TMB is added, it will change color quickly and intensely.
An intense blue color forming instantly on addition of TMB to a spot
plate sample of field soil indicates the presence of available Mn( 111). If the
blue color intensity builds up slowly, this means that reactive Mn( IV)
and/or Fe( 111) is present. Addition of three or four drops of 0.1 A4 citric
acid before addition of the TMB will prevent reaction of Fe with the TMB
and will enhance the Mn color development by driving the reverse dismutation of Mn(I1) and Mn( IV) toward Mn( 111). Many soils contain Fe( 111)
minerals that will not react with TMB even without the citrate, but most
Mn oxide minerals found in soils will give positive TMB tests. Thus TMB
is a specific test for soil Mn oxides and can also be used to indicate the
R I C H M O N D J. B A R T L E T A N D BRUCE R. JAMES
presence of highly reactive Fe( 111). It also measures relative proportions of
reactive Mn( 111) and Mn( IV) in a soil sample.
2 . Other Tests
Gum guaiac, a dye soluble in ethyl alcohol, reacts similarly to TMB as
an indicator, forming a bright red color with strong oxidants. Methylene
blue and tetrazolium blue are dyes that change color in alkaline solutions,
to colorless or red, respectively, with strong reducing agents (e.g., ascorbic
acid and phenolic compounds).
Another spot plate test for reactive Fe( 111), for example, recently formed
Fe(OH), or temporary Fe(II1) in a wetland soil, consists of 10 m M 2,2’-dipyridyl in pH 4.8 NH,OAc, 1.25 M acetate (Vermont buffer). A pink or
red color that develops immediately is a test for Fe( 11). Color that develops
after about a 1.5 hr, or in a few minutes in the sun, is indicative of Fe( 111)
that has been reduced to Fe( 11) by the dipyridyl. Used at 8 X dilution with
unknown or standard solutions, this reagent is useful for quantitative
determination of Fe(I1) in the laboratory (at 522 nm).
Dissolve 40 mmol of KMnO, in about 40 ml of distilled water heated to
approximately 60°C and transfer with mixing into 30 ml of 2 M MnSO,.
Add 80 mmol KOH dissolved in 10-20 ml of water. Mix and adjust the
pH to 7.5 with KOH or sulfuric acid and let stand overnight with occasional stimng, and then adjust the pH to 6.0 with additional sulfuric acid.
There should be no permanganate color remaining. Transfer into dialysis
tubing and dialyze against fresh distilled water until the outside solution is
close to salt free, as checked by barium precipitation or conductivity.
Dilute the suspension to 500 ml, or any desired volume. The procedure
may be camed out quantitatively so that there is exactly 100 mmol of
Mn02,or a diluted suspension can be standardized by iodine titration as in
1. Shake 2.5 g of soil (dry weight basis or 2.5 cm3packed volume) for 15
min with 25 ml of 1 m M CrCl,.
2. Add 0.25 ml of 1 M pH 7.2 KH2P0,.K2HP0,, shake 15 sec longer,
and then filter or centrifuge.
REDOX CHEMISTRY OF SOILS
3. Determine Cr( VI) by adding 1 ml diphenylcarbazide (DPC) reagent
to 8 ml of extract or water, mix, and let stand 20 min, and then compare
the color with that in the standards (0.5-50 p M ) at 540 nm. (Prepare the
DPC reagent by adding 120 ml of 85% phosphoric acid, diluted with
280 ml distilled water, to 0.38 g of s-diphenylcarbazide dissolved in 100 ml
of 95% ethanol.)
4. If cloudiness from precipitated organic or mineral matter is present, it
is easily removed by filtering, following color development, using a 0.2-pm
filter and syringe. The colored complex remains in solution, and filtration
of standards and unknowns improves sensitivity.
A portion of the Cr oxidized during the course of this test is not measured as Cr(V1) because it is reduced almost as fast as it is oxidized.
Depending on the availability of easily oxidizable organic matter, some or
even most of the Cr(V1) formed is reduced during the 15-min period.
Leaching a sample, especially a dry one, will remove some of the low-molecular-weight reducing organics and thereby will increase the Cr( VI)
quantity measured (Bartlett, 1981a). With dried and stored “lab dirt”
samples, the reduction frequently equals the oxidation and no net Cr(V1)
is measured. This test characterizes oxidation only to the extent that it
1. Shake intermittently 2.5 cm3 of moist soil I8 hr with 20 ml of pH 4.0
NH40Ac, 0.6 M with respect to ammonium, containing 0.1, 0.5, or
2.5 m M K,Cr,O,.
2. Filter or centrifuge, and determine concentration of Cr(V1) remaining (Section XI1,F). If all of the Cr(V1) is reduced, repeat with increased
concentrations until the Cr(V1) remaining is measurable; 1 mol of Cr is
equivalent to 6 mol of manganese plus charge.
3. To measure the tendency of a particular oxidizable organic substance
to reduce Cr( VI), repeat this test after adding the organic substance to the
1. Shake intermittently 2.5 g soil, dry weight basis (or 2.5 cm3 of moist
soil), for 18 hr, with 25 ml of 0.1, 0.5, 2.5, or 10 m M as K2Cr20, in
10 m M H3PO4, filter or centrifuge, and determine Cr(V1) not reduced in
the extract (Section XI1,F).
R I C H M O N D J. BARTLETT AND BRUCE R. JAMES
2. Begin with the lowest concentration of Cr(VI). If all of the Cr( VI) is
reduced, repeat with increased concentrations until the Cr( VI) remaining
is measurable but below 0.1 mM.
BY IODIDE OXIDATION
1. As a direct titration of the reducible Mn oxides in a soil sample, the
manganese electron demand (MED) determination is a direct way of
estimating the oxidizing capability by the Mn oxides in a soil, without
consideration of the reduction that might take place under the conditions
of a less direct oxidation measurement, such as Cr( VI) (Bartlett, 1988).
2. Shake intermittently 2 g soil, dry weight basis (or 2 cm3 moist soil),
for 18 hr, with 12 ml of pH 4.0 NH,OAc, 0.6 M with respect to ammonium, and 4 ml of 0.2 M KI. Add 4 drops of starch solution (0.3 g potato
starch boiled with 50 ml water) and titrate to a colorless endpoint with
2 m M Na,S20, . Millimoles of thiosulfate per unit of soil are equivalent to
millimoles of e- or plus charge of Mn.
3. “Total” electron demand (TED) is modified from MED as follows:
12 ml of 0.1 M HCI is added instead of pH 4.0 acetate, and the equilibration time, instead of an approximate 18 hr, is a rigid 15 min with centrifuging and titration required immediately afterwards. The time is critical in
soils that contain high amounts of recently oxidized Fe, because the
amount of easily reducible Fe that will react may be an ambiguous valley
rather than an easily identifiable peak. A more serious problem is that
iodide is slowly oxidized by 0, at low pH, and inflated TED values can
result if time is not strictly limited. Thus, TED is very much an empirical
measurement, although, with strict control of time, the test is amenable to
calibration for measuring a particular species or fraction of reducible Fe.
Manganese oxides, at pH >6, and catalase enzymes both destroy H,O,
by catalyzing its dismutation [Eq. (34)]. The rate of dismutation is a better
indication of quantities and activities of catalytic substances present than is
amount of H,02 dismutated, because a small amount of catalyst will act on
a large amount of substrate.
The rate of dismutation can be evaluated by adding 5 ml of 0.5 M H,O,
solution to 2.5 g of moist soil, on a dry weight basis, and clocking the time
required for the soil to evolve enough bubbles to displace 24 ml of H 2 0 (1
mmol of 0,).
REDOX CHEMISTRY OF SOILS
Most colorimetric tests useful in studying redox processes in soils also
involve redox reactions, and because we are trying to measure colorimetrically quantities of particular redox species in the presence of other redox
species, we have many opportunities for color-interfering interactions
among the various reactions. Manganese oxides produce positive interference in measuring nitrate by the hydrazine reduction method (Prochazkova, 1959) and also with brucine or diphenylamine, positive interference
in nitrite by the diazonium salt method, but no interference with the
diphenyl carbazide color for Cr( VI). Nitrite, citrate, and hydroxylamine
negatively interfere with the Cr(VI) test, and nitrite and Cr( VI) positively
interfere with determination of nitrate by brucine. Highly oxidized hypochlorite and peroxides and highly reduced substances, such as thiosulfate, sulfides, amines, and ascorbic acid, interfere with everything imaginable. Nitrate is too inert to interfere with most tests, which tells us
something about the reactivity of nitrate. Wariness, ingenuity, and flexibility are required for redox titrations.
Afanas’ev, I. B. ( 1 989). “Superoxide Ion: Chemistry and Biological Implications,” Vol. I., pp.
10-260 CRC Press, B o a Raton, Florida.
Amacher, M. C., and Baker, D. E. (1982). Redox reactions involving chromium, plutonium,
and manganese in soils. Final Rep. US.Dep. Energy Div. Energy Technol. (DOE/DP/
041 5- I).
Arndt, D. ( I98 I ). “Manganese Compounds as Oxidizing Agents in Organic Chemistry.”
Open Court, La Salle, Illinois.
Bartlett, R. J. (1965). A biological method for studying aeration status of soil in sifu. Soil Sci.
Bartlett, R. J. (1981a). Oxidation-reduction status of aerobic soils. In “Chemistry in Soil
Environments” (D. Baker, ed.),pp. 77 - 102. Soil Sci. Soc. Am., Madison, Wisconsin.
Bartlett, R. J. (1981b). Nonmicrobial nitrite-to-nitrate transformation in soils. Soil Sci. Soc.
Am. J. 45, 1054- 1058.
Bartlett, R. J. (1986). Soil redox behavior. In “Soil Physical Chemistry” (D. L. Sparks, ed.),
pp. 179- 207. CRC Press,Boca Raton, Florida.
Bartlett, R. J. (1988). Manganese redox reactions and organic interactions in soils. Manganese Soils Plants Proc. Int. Symp., 59-13.
Bartlett, R. J. (1990a). An A or an E: Which will it be? In “Proceedings of the Fifth
International Soil Correlation Meeting: Characterization, Classification, and Utilization
of Spodosols” (J. M. Kimble and R. D. Yeck, eds.), pp. 7-18. US. Dep. Agric., Soil
Conserv. Serv., Lincoln, Nebraska.
Bartlett, R. J. (1990b). Role of manganese in formation of organic/mineral A horizons in
place of albic E horizons in podzolized soils. Abstr. Inl. Soil Sci. Soc., 18- 19, Kyoto,
RICHMOND J. BARTLETT AND BRUCE R. JAMES
Bartlett, R., and James, B. (1979). Behavior of chromium in soils. 111. Oxidation. J. Environ.
QWl. 8, 31 -35.
Bartlett, R. J., and James, B. R. (1980). Studying airdried, stored soil samples-Some
pitfalls. Soil Sci. Soc. Am. J. 44, 72 1 -724.
Bartlett, R. J., and Ross. D. S. (1988). Colorimetric determination of oxidizable carbon in
acid soil solutions. Soil Sci. Soc. Am. J. 52, I 191 - 1 192.
Berner, R. A. (1981). A new geochemical classification of sedimentary environments. J.
Sediment. Petrol. 51,359-365.
Blaylock, M. J., and James, B. R. (1992). Oxidation-reduction behavior of selenite in soils.
Soil Sci. Soc. Am. J. (submitted).
Bouldin, D. R. (1968). Models for describing the diffusion of oxygen and other mobile
constituents across the mud-water interface. J. Ecol. 56,77-8 1.
Bricker, 0. P. (1982). Redox measurement: Its measurement and importance in water
systems. Warer Anal. 1.
Brudvig, G. W., and Crabtree, R. H. (1989). Bioinorganic chemistry of manganese related to
photosynthetic oxygen evolution. Prog. Inorg. Chem. 37,99- 142.
Castellan, G. W. (1983). “Physical Chemistry,” 3rd Ed. Addison-Wesley, Reading, Massachusetts.
Cohen, G. I. (1985). The Fenton reaction. In “Handbook of Methods for Oxygen Radical
Research’’ (R. A. Greenwald, ed.), pp. 55 - 69. CRC Press, Boca Raton, Florida.
Cotton, F. A., and Wilkinson, G. (1980). “Advanced Inorganic Chemistry.” Wiley, New
Diem, D., and Stumm, W. (1984). Is dissolved Mn2+being oxidized by 0, in absence of
Mn-bacteria or surface catalysts? Geochim. Cosmochim. Acta 48, 157 I - 1573.
Dion, H. G., and Mann, P. J. G. (1946). Three-valent Mn in soils. J. Agric. Sci. 36, 239245.
Ehrlich, H. L. (1976). Manganese as an energy source for bacteria. Environ. Biogeochem.
Proc. Int. Symp. 2nd, 2.
Farrell, R. E., Swerhone, G. D. W., and van Kessel, C. (199 I). Construction and evaluation of
a reference electrode assembly for use in monitoring in situ soil redox potentials. Commun. SoilSci. Plant Anal. 22, 1059- 1068.
Fridovich, 1. (1975). Superoxide dismutases. Annu. Rev.Biochem. 44, 147- 159.
Fridovich, 1. (1978). The biology of oxygen radicals. Science (Washinglon. D.C.) 201, 875880.
Fruton, J. S., and Simmonds, S. (1961). “General Biochemistry.” Wiley, New York.
Carrels, R. M., and Christ, C. L. (1965). “Solutions, Minerals, and Equilibria.” Freeman, San
Halliwell, B. (1974). Manganese ions, oxidation reactions and the superoxide radical. Neure
tOXiCOlOgY 5, 1 13 - 1 18.
Harter, R. D., and Smith, G. (1981). Langmuir equation and alternate methods for studying
“adsorption” reactions in soils. In “Chemistry in Soil Environments” (D. Baker, ed.),pp.
167- 182. Soil Sci. Soc. Am., Madison, Wisconsin.
Hines, M. E., Knollmeyer, S. L., and Tugel, J. B. (1989). Sulfate reduction and other
biogeochemistry in a northern New England salt marsh. Limnol. Uceanogr. 34, 578590.
James, B. R. (1989). Electron activity in soils: A key master variable. Agron. Abstr., 20 1.
James, B. R., and Bartlett, R. J. (1983). Behavior of chromium in soils. VI. Interactions
between oxidation-reduction and organic complexation. J. Environ. Qual. 12, 173176.
Jenny, H. (1980). “The Soil Resource,” pp. 1 1 3- 195. Springer-Verlag, New York.
Kittrick, J. A,, Fanning, D. S., and Homer, L. R. (1982). Acid sulfate weathering. SSSA
Spec. Publ.. No. 10.
REDOX CHEMISTRY OF SOILS
Liem, H. H., Cardenas, F., Tavassoli, M., Poh-Fitzpatrick, M. B.,and Muller-Eberhard, U.
(1979).Quantitative determination of hemoglobin and cytochemical staining for peroxdihydrochloride, a safe substitute for benzidine.
ide using 3,3’,5,5’-tetramethylbenzidine
Ann. Biochem. 98,388-390.
Lindsay, W. L. (1979).Chemical Equilibria in Soils,” pp. 386-412. Wiley-Interscience, New
Liu, C. W., and Narasimhan, T. N. (1989).Redox-controlled multiple-species reactive chemical transport. I . Model development. Waier Resour. Res. 25,869-882.
Loach, P. A. (1976).Oxidation-reduction potentials, absorbance bands, and molar absorbance of compounds used in biochemical studies. Handb. Biochem. Mol. Biol. 3rd Ed.
Loganathan, P., Burau, R. G., and Fuerstenau, D. W.(1977).Influence of pH on the sorption
of Co2+,Zn2+and Ca2+by a hydrous manganese oxide. Soil Sci. Soc. Am. J. 41,57-62.
Lutz, H. J., and Chandler, R. F. (1946).“Forest Soils,” pp. 140-480. Wiley, New York.
McBride, M. B. (1982).Electron spin resonance investigation of Mn2+ complexation in
natural and synthetic organics. Soil Sci. Soc. Am. J. 46, 1137- 1142.
Magdoff, F. R., and Bouldin, D. R. (1970).Nitrogen fixation in submerged soil-sand-energy
material media and the aerobic-anaerobic interface. Plant Soil 33,49- 53.
Marschner, H. (1990).“Mineral Nutrition of Higher Plants,” pp. 336-337. Academic Press,
San Diego, California.
Masscheleyn, P. H., Delaune, R. D., and Patrick, W. H. (1991). Arsenic and selenium
chemistry as affected by sediment redox potential and pH. J. Environ. Qual. 20, 522527.
Matia, L., Rauret, G., and Rubio, R. (1991).Redox potential measurement in natural waters.
Fresenius Z. Anal. Chem. 339,455-462.
Moore, J. N., Walker, J. R.. and Hayes, T. H. (1990).Reaction scheme for the oxidation of
As(lI1) to As(V) by birnessite. Clays Clay Miner. 38, 549-555.
Mueller, S. C., Stolzy, L. H., and Fick, G. W. (1985).Constructing and screening platinum
microelectrodes for measuring soil redox potential. Soil Sci. 139, 558-560.
Pohlman, A. A,, and McColl, J. G. (1988).Organic oxidation and metal dissolution in forest
soils. Soil Sci. Soc. Am. J. 52, 265-27 I.
Ponnamperuma, F. N. ( I 972).The chemistry of submerged soils. Adv. Agron. 24,29-96.
Prochazkova, L. (1959).Bestimmung der Nitrate in Wasser. Frescnius 2. Anal. Chem. 167,
Rabenhorst, M. C., and James, B. R. (1992).Iron sulfidization in tidal marsh soils. In
“Biomineralization processes of iron and manganese” (H. C. W.Skinner and R. W.
Fitzpatrick, ed.). (Catena Suppl. 21.)Catena Verlag, CremCngen-Destedt, Germany.
Rabenhorst, M. C., James, B. R., and Shaw, J. N. (1992).Evaluation of potential wetland
substrates for optimizing sulfate reduction. Proc. Nail. Meet. Am. Soc. Surf Min. Reclam.. Duluth, Minnesota (in press).
Reddy, K. R., and Patrick, W. H. (1980).Evaluation of selected processes controlling nitrogen loss in a flooded soil. Soil Sci. Soc. Am. J. 44, 1241 - 1243.
Ross, D. S..and Bartlett, R.J. (1981).Evidence for nonmicrobial oxidation of manganese in
soil. SoilSci. 132, 153-160.
Rowell, D.L.( I98I ). Oxidation and reduction. Chem. Soil Processes, 40I -463.
Russell, E. W. (1973).“Soil Conditions and Plant Growth,” 10th Ed., pp. 670-695.Longman, London, England.
Schnitzer, M., and Khan, S. V. (1972).“Humic Substances in the Environment,” p. 300
Dekker, New York.
Senesi. N., and Schnitzer, M. (1978).Free radicals in humic substances. Environ. B i o g w
chem. Geomicrobiol., Proc. Inr Symp., 3rd, 2,467-480.
Shindo, H. (1990).Catalytic synthesis of humic acids from phenolic compounds by Mn(1V)
oxide. Soil Sci. Plant Nutr. (Tokyo) 36,679-682.
RICHMOND J. BARTLETT AND BRUCE R. JAMES
Shindo, H., and Huang, P. M. (1982). Role of Mn(1V) oxide in abiotic formation of humic
substances in the environment. Nuture (London) 298, 363-365.
Shindo, H., and Huang, P. M. (1984). Significance of Mn(IV) oxide in abiotic formation of
organic nitrogen complexes in natural environments. Nature (London)308, 57- 58.
Sillen, L. G. (1967). Master variables and activity scales. Adv. Chem. Ser.
Silver, M., Erlich, H. L., and Ivarson, K. C. (1986). Soil mineral transformation mediated by
soil microbes. In “Interactions of Soil Minerals with Natural Organicsand Microbes” (P.
M. Huang and M. Schnitzer, eds.).pp. 497-5 19. Soil Sci. Soc. America, Madison, WI.
Sparks, D. L. (1985). Kinetics of ionic reactions in clay minerals and soils. Adv. Agron. 38,
23 I -265.
Sparrow, L. A., and Uren, N. C. ( I 987). Oxidation and reduction of Mn in acidic soils: Effect
of temperature and soil pH. Soil Biol.Biochem. 19, 143- 148.
Sposito, G. (198 I). “The Thermodynamics of Soil Solutions.” Oxford, New York.
Sposito, G. (1989). “The Chemistry of Soils.” Oxford, New York.
Stone, A. T., and Morgan, J. J. (1984). Reduction and dissolution of manganese(ll1) and
manganese(1V) oxides by organics: 2. Survey of the reactivity of organics. Environ. Sci.
Stumm, W., and Morgan, J. J . (1981). “Aquatic Chemistry,” 2nd Ed.,pp. 418-504. WileyInterscience, New York.
Sullivan, J. C., Gordan, S., Cohen, D., Mulac, W., and Schmidt, K. H. (1976). Pulse
radiolysis studies of uranium (VI), neptunium (VI), neptunium (V), and plutonium (VI)
in aqueous perchlorate media. J. Phys. Chem. 8, 1684- 1686.
Sunita, J. M., Loll, M. J., Snipes, W. C., and Bollag, J. M. (1981). Electron spin resonance
study of free radicals generated by a soil extract. Soil Sci.131, I45 - 150.
Thompson, 1. J. (1923). “The Electron in Chemistry.” Franklin Institute, Philadelphia,
Thorp, H. H., and Brudvig, G. W. (1991). The physical inorganic chemistry of manganese
relevant to photosynthetic oxygen evolution. New J. Chem. 15,479-490.
Vincent, A. (1985). “Oxidation and Reduction in Inorganic and Analytical Chemistry.”
Wiley, Chichester, England.
Wang, T. S. C., Huang, P. M., Chou, C.-H., and Chen, J.-H. (1986). The role of soil minerals
in the abiotic polymerization of phenolic compounds and formation of humic s u b
stances. Interact. Soil Miner. Nat. Org. Microbes Proc. Symp., 1983, 25 I - 28 1.
Weaver, J. H. (1987). “The World of Physics.” Simon and Schuster, New York.
Westcott, C. C. ( I 978). “pH Measurements.” Academic Press, New York.
Zumdahl, S. S. (1986). “Chemistry,” pp. 931 -935. Heath, Lexington, Massachusetts.
SULFURIN THE I~OPICS
N. S. Pasricha’ and R. L. Fox’
Department of Soils,
Punjab Agricultural University,
Department of Agronomy and Soil Science,
University of Hawaii at Manoa,
Honolulu, Hawaii 96822
11. Extent of Sulfur Deficiency
111. Forms of Sulfur in Soil
A. Sulfur Transformation Products
B. Sulfate Sulfur
IV. Sulfur Cycling in the ‘Tropics
A. Sulfur Supplies of Atmospheric Origin
B. Sulfur Accession through Precipitation
V. Effects of Acid Rain
A. Effect on Crop Plants
B. Effect on Forest Vegetation
C. Effect on Soil Acidification
VI. Sulfur in Irrigation Waters
A. Sulfur in Streams
B. Sulfur in Groundwater
VII. Sulfate Retention in Soil
A. Sulfate Adsorption and Desorption
B. Sulfate Adsorption Curves
C. Mechanism of Sulfate Adsorption
VIII. Diagnosis of Sulfur Needs
A. Soil Tests
B. Plant Analysis
IX. Critical Soil Solution Concentration
X. Crop Responses
XI. Sulfur Fertilization and Crop Qualiry
A. Effect on Protein Oualiry
B. Effect on Oil Content
Adwnrti m Abmnary. C‘ol 10
Copyright 0 1993 by Academic Press, Inc. All rights of reproduction in any form reserved