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XIV. Empirical Methods for Characterizing Soil Redox

XIV. Empirical Methods for Characterizing Soil Redox

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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.

2 00


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.







20 1



1. Tetramethylbenzidine

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

positive test.

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




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).



Mn (IV)

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

Section XIV,I,b.




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.


20 3

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

exceeds reduction.


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

soil sample.



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).



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.



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.


OF H,02

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,).




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.


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N. S. Pasricha’ and R. L. Fox’

Department of Soils,

Punjab Agricultural University,

Ludhiana, India

Department of Agronomy and Soil Science,

University of Hawaii at Manoa,

Honolulu, Hawaii 96822

1. Introduction

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


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XIV. Empirical Methods for Characterizing Soil Redox

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