The sulfur fertility status of Florida soils


Material Information

The sulfur fertility status of Florida soils
Physical Description:
xiv, 178 leaves : ill. ; 28 cm.
Mitchell, Charles Clifford, 1948-
Publication Date:


Subjects / Keywords:
Soils -- Florida   ( lcsh )
Soils -- Sulfur content   ( lcsh )
Plants -- Effect of sulfur on   ( lcsh )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )


Thesis (Ph. D.)--University of Florida, 1980.
Includes bibliographical references (leaves 153-166).
Statement of Responsibility:
by Charles Clifford Mitchell, Jr.
General Note:
General Note:

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University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
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aleph - 000014493
notis - AAB7720
oclc - 07158375
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Full Text







The author wishes to express his sincere appreciation to the pro-

fessors and staff of the Soil Science Department who have offered so

much time, guidance, encouragement, and help during the progress of

this investigation. Dr. W. G. Blue, chairman of the supervisory com-

mittee, deserves special recognition for his guidance and help in this

project. In spite of his many commitments and busy schedule, Dr. Blue

always is available to discuss problems which inevitably arise in

research and to offer guidance and encouragement.

Dr. R. D. Rhue and all of the personnel of the Soil Testing and

Analytical Research Laboratories where most of the analytical work was

conducted deserve special recognition for their help, friendship,

cooperation, patience, and tolerance throughout this program.

The contributions of Dr. T. L. Yuan, Dr. J. B. Sartain, Dr. R. N.

Gallaher, and Dr. R. D. Rhue as members of the Supervisory Committee

are gratefully acknowledged.

The author is also indebted to Dr. H. L. Breland, Professor

Emeritus, and Dr. C. F. Eno, Chairman of the Soil Science Department,

for securing the graduate assistantship through the Extension Soil

Testing Laboratory. Their support and encouragement of this project

and others are appreciated.

Special appreciation is extended to Pegg:., his wife, for her

support, perseverance, and understanding and for the many hours she

has spent assisting in the laboratory and in the field. The author

would especially like to thank his father for instilling in him a sin-

cere appreciation and practical understanding of the soil and the crops

it produces and for encouraging its conservation. The support, confi-

dence, and encouragement of his parents are especially appreciated.




ACKNOWLEDGEMENTS. . ... .. .. ii

LIST OF TABLES. . . . vii


ABSTRACT. . . . .. xii




1.1 SOIL SOURCES. . . 4
1.1.1 Sulfur-Containing Minerals. . 4
1.1.2 Soil Organic Matter . . 5 C:N:S relationships . ... 6 Organic sulfur fractionation. . 8
1.1.3 Soil Sulfur Reactions . .. 13 Oxidation-reduction . ... 13 Precipitation reactions . 16 Adsorption reactions. . ... 20 Factors Affecting Adsorption. ... 20 Mechanisms of Sulfate Adsorption. ... 22 Leaching. . . 26
1.2.1 Sulfur as a Pollutant . ... 28 Damage to plants. . .. 28
1.2.1..2 Damage to animals . ... 29
1.2.2 Sources of Atmospheric Sulfur .. 29 Anthropogenic sources . 30 Biogenic sources. . . 32
1.2.3 Local and Regional Studies of Rainfall
and Atmospheric S . ... 32

2.2.1 Root Uptake . . 41
2.2.2 Absorption from the Atmosphere. . .. 42


2.3.1 Critical Concentrations . .. 43
2.3.2 N:S Ratios . . .. 43
2.3.3 Sulfate Sulfur. ... . 48
2.4.1 Extraction Techniques ... ... .. 50
2.4.1 Biological Techniques . ... 55




5.1 NITROGEN. .... ........... 72
5.1.1 Total Nitrogen in Plant Tissue. . ... 72
5.1.2 Total Nitrogen in Soils . .. 73
5.2 SULFUR. . . ... 73
5.2.1 Tissue Digestion for Total Sulfur .. 73
5.2.2 Soil Extraction for Sulfate Sulfur. ... 73
5.2.3 A Comparison of Two Extraction Procedures
for Soil Sulfur . . 74
5.2.4 Estimation of Total Sulfur in Soils .. 76

6.1 TURBIDIMETRY. . . .. 84
6.2 INDIRECT METHODS. . . .. 84


7.1 SPODOSOLS . . .. 86
7.2 ENTISOLS. . . ... 91
7.3 ULTISOLS. . . 97
7.4 C:N:S RELATIONSHIPS . .. 106

SOILS STUDIES . . ... 109
8.2 YIELDS .. . . .112
8.2.1 Harvest 1 . . ... .. 112
8.2.2 Harvests 2,3, and 4 . ... 117 Myakka. . . ... 117 Lakeland. . . ... .119 Orangeburg and Norfolk. .. . .. 119


THE FIELD. . . .. .124



BIBLIOGRAPHY. . . .. 153

APPENDICES. . .. . 167

IN PLANT TISSUE. . . .. 168
IN SOILS . . 169
AGRONOMY FARM . . .. 175
GAINESVILLE, FLORIDA, FOR 1978 AND 1979. ... 176

VITA. . .. . 177













1. Mean sulfur fractions and C:N:S ratios for different
soils . . .

2. The oxidation states of sulfur in soils . .

3. Sulfur oxidation reactions by certain species of
thiobacilli . . .......

4. Sources of biogenic S from wetlands in Florida .

5. The use of selected sulfur-containing fertilizers in
Florida . . .

6. Critical levels of total sulfur, sulfate sulfur, and N:S
ratios for selected crops . .....

7. Selected methods used to determine sulfate and
extractable sulfur in soils . .

8. Nutrients applied to surface soils used in a
greenhouse evaluation of subsoil sulfur . .

9. Sulfate sulfur removed by four extraction methods
and percent recovery of added sulfur ...

10. Sulfur content of finely ground soil and screened soil
using the MgNO3/HNO3 digestion procedure . .

11. A comparison of three rapid methods of estimating
total sulfur in soils . ........ *

12. Selected chemical and mineralogical properties of
some Florida Spodosols . ......

13. Correlation coefficients of some soil properties
with extractable and total sulfur in Florida
Spodosols . . .

14. Selected chemical and mineralogical properties of
some Florida Entisols . ...... .




15. Correlation coefficients of some soil properties with
extractable and total sulfur in Florida Entisols .... 98

16. Selected chemical and mineralogical properties of some
Florida Ultisols . . ... ... 99

17. Correlation coefficients of some soil properties with
extractable and total sulfur in Florida Ultisols .... .104

18. Carbon, nitrogen, and sulfur relationships in Florida
soils . .. . 107

19. Some chemical and mineralogical properties of soils used
in a greenhouse evaluation of subsoil sulfur .. 110

20. Nutrient levels in soils used in a greenhouse evaluation
of subsoil sulfur before treatment . .. 111

21. Sulfur and nitrogen nutrition and yields of 28-day old
sorghum-sudangrass in a Myakka fine sand in the
greenhouse . . ... ...... 113

22. Sulfur and nitrogen nutrition and yields of 28-day old
sorghum-sudangrass in an Orangeburg fine sand
in the greenhouse. . .. 114

23. Sulfur and nitrogen nutrition and yields of 28-day old
sorghum-sudangrass in a Norfolk fine sand in the
greenhouse . . ... ...... 115

24. Sulfur and nitrogen nutrition and yields of 28-day old
sorghum-sudangrass in a Lakeland fine sand in the
greenhouse . . 116

25. Extractable sulfate sulfur in treated soils used in a
greenhouse evaluation of subsoil sulfur
availability . . ... ..... 118

26. The effect of nitrogen and sulfur fertilization on
bahiagrass in a Myakka fine sand--1978 ... 127

27. Sulfur removed in herbage. . ... 130

28. The effect of nitrogen and sulfur fertilization on
bahiagrass in a Myakka fine sand--1979 .. 131

29. Extractable and total sulfur in bahiagrass plots
from the Beef Research Unit . .. .133




Table Page

30. Stolon-root mass from bahiagrass plots at the Beef
Research Unit. .. . . .. 135

31. The effect of nitrogen and sulfur fertilization on
bermudagrass in a Kendrick fine sand--1978 ... 136

32. The effect of nitrogen and sulfur fertilization on
bermudagrass in a Kendrick fine sand--1979 ... 138

33. Extractable and total sulfur in bermudagrass plots
from Green Acres Agronomy Farm . .. 142


Figure Page

1. Rajan's (1978) proposed mechanism of sulfate
adsorption .......... . .. 25

2. Sources and sinks of sulfur compounds . ... .31

3. Annual deposition of total sulfur, 1952 to 1955 and 1978
to 1979, and the mean pH of rainfall in Florida,
1978 to 1979. . ... .. . 35

4. Location of soil pedons where samples were collected
for a survey of sulfur in Florida soils ... .63

5. Soil pH buffer curves for four Florida soils. ... 66

6. Schematic of one experimental unit in the greenhouse
evaluation of subsoil S in Florida soils. ... 69

7. Mean distribution of total and extractable sulfate
sulfur in nine Florida Spodosols. ... 90

8. Mean distribution of total and extractable sulfate
sulfur in ten Florida Entisols. . ... 96

9. Mean distribution of total and extractable sulfate
sulfur in ten Florida Ultisols. .... 102

10. Total yields of four harvests of sorghum-sudangrass as
affected by sulfur and subsurface soil horizons .. 120

11. The effect of herbage sulfur concentration on relative
yield of 4-week old sorghum-sudangrass tops ... 123

12. The effect of herbage N:S ratio on relative yield of
4-week old sorghum-sudangrass tops ... .. .... 125

13. The effect of sulfur and nitrogen rates on the annual
yield of bahiagrass in a Myakka find sand . 129

14. The effect of sulfur and nitrogen rates on the annual
yield of bermudagrass on a Kendrick fine sand ...... 141



15. A contour plot of the depth the the argillic horizon (cm)
in the experimental area on a Kendrick fine sand at
Green Acres Agronomy Farm. . ... 144

16. The effect of S concentration in bahiagrass and bermuda-
grass on relative yields at high N rates ..... 145

17. Annual sources and cycling of sulfur (kg/ha/yr) in the
surface (0-15 cm) of a north Florida Spodosol. .. 147

Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



Charles Clifford Mitchell, Jr.

December 1980

Chairman: W. G. Blue

Major Department: Soil Science

Extractable and total sulfur (S) in 29 soil profiles from central

and north Florida were measured and compared to selected soil proper-

ties. The surface horizons of most Florida Ultisols and the entire

profile to 200 cm of Spodosols and Entisols contained less than 6 ppm

of 0.01 M Ca( HPO )2-2H20 extractable sulfate S. Total S was highly
2- 4 2 2
correlated with organic C (r = 0.89) and total N (r = 0.95) in the sur-

face horizons of all soils and with Na-pyrophosphate soluble Al

(r = 0.86) in the spodic horizons of Spodosols. Both extractable and

total S were significantly correlated with citrate-dithionite soluble

Al and Fe in the surface horizons of Ultisols. The argillic horizons

of Ultisols contained an average of 33 ppm extractable sulfate S which

represented over -C', of the total S. Extractable sulfate S represented

less than 7% of the total S in the entire profiles of Entisols and

Spodosols and the surface horizons of Ultisols. The mean C:N:S ratios

for the surface soils of the three soil orders were 112:7.1:1 for

Ultisols, 150:6.4:1 for Spodosols, and 166:5.9:1 for Entisols.

The relative contribution of subsurface soil S to plant nutrition

was studied in a greenhouse experiment with a sorghum-sudangrass hybrid

(Sorghum sudanese (Piper) stapf 'Dekalb SX16A') in four soils with and

without the subsurface soil included in a-simulated horizon sequence.

The surface soils were treated with 0 or 20 ppm S as CaSO 42H20.

Plants were harvested each 4 weeks over a 16-week period and'analyzed

for S and N. Sulfur increased yields at all harvests in Myakka fine

sand (Aeric Haplaquod, sandy siliceous, hyperthermic). The presence of

soil from the spodic horizon did not improve the S status of the

plants; the presence of soil from the C horizon of Lakeland fine sand

(Typic Quartzipsamment, thermic, coated) increased S uptake slightly

over the check but not enough to prevent severe S deficiencies and

decreased dry matter yield where S was omitted. Applied S did not

increase yields in the surface soils of Orangeburg and Norfolk fine

sands (Typic Paleudults, fine-loamy, siliceous, thermic) until the

third harvest. The argillic horizon of the Norfolk provided adequate

S to maintain optimum dry matter production throughout the 16-week

experiment. The presence of the argillic horizon of the Orangeburg

improved the S status of the plants and increased S uptake but not

enough to prevent S deficiencies by the third harvest. In all soils,

S increased yields before deficiency symptoms appeared in the plants.

Critical S concentration in the 4-week old plants was 0.12%, and the

critical N:S ratio was 16.


Sulfur application with two rates of N (200 and 400 kg/ha) did not

increase yields of bahiagrass (Paspalum notatum Flugge 'Pensacola')

and bermudagrass (Cynodon dactylon L. 'Coastcross I') in the field un-

til the second year and only at the highest rate of N. All rates of

applied S increased S uptake by the plants during the growing season.

There seemed to be no advantage to applying S in split applications

during the season-over a single application in early spring.

Atmospheric and soil sources of S account for an estimated minimum

of 21 kg/ha of plant-available S during a growing season. With an

average of 15 to 39 kg of S per fertilized hectare applied in fertil-

izers in Florida, S deficiencies among field crops are unlikely except

where high yields are maintained and little or no S is applied in




In 1971, Beaton et al. made the following statement concerning the

S status of Florida soils:

Sulphur fertilizers are needed on most of the soils of Florida.
Except for the Everglades and associated areas, most of the
soils are coarse textured. Such soils account for about 28
million acres (11.3 million ha) or about 80% of the land area
of the state (Beaton et al., 1971, p. 6).

Research during the 1940's and early 1950's showed conclusively

that many crops growing on Florida soils would respond to S fertiliza-

tion. However, the conscious application of S as a fertilizer nutrient

has never received wide acceptance. Sulfate is applied in many fertil-

izer materials as an associate anion or as a by-product of the fertil-

izer manufacturing process. Ammonium sulfate, ordinary superphosphate,

sulfate of potash-magnesia, and gypsum may supply adequate quantities

of S to growing crops. Micronutrients applied in the soluble sulfate

form will also contribute to the S nutrition of plants. However,

intensively grown, biih-producing crops are often fertilized with large

quantities of N as urea, anhydrous ammonia, or ammonium nitrate, P as

concentrated superphosphate, diammonium phosphate, or ammonium poly-

phosphates, and K as muriate of potash. If no S is included in the

fertilizer, this practice could lead to soil-S depletion and reduced


Soils serve as a source and a sink for plant available S, buc

attempts to measure plant-available and labile S have been only


marginally successful. Organic matter is the largest source of soil S

in most Florida soils, but the availability and dependability of this S

are difficult to determine. Soils which contain an argillic horizon

such as the Ultisols of northern Florida contain large quantities of

adsorbed S associated with the clay and minerals in this subsurface

horizon. This S may be an important source for crops, but the roots of

young seedlings must germinate and grow through 20 to 60 cm of leached,

sandy soil to reach it. Spodic horizons of flatwoods soils (Spodosols)

and subsurface horizons of Florida's sandy Entisols supply an unknown

amount of S to crops and are incapable of retaining much applied S.

The atmosphere may be an important contributor to the S nutrition

of Florida crops. Sulfate-S in rainfall and the direct absorption of

SO2 through plant leaves may supply all of a plant's S needs in some

locations, but this source has not been evaluated in Florida.

Because S is an essential component of certain amino acids, it is

an important consideration in assessing forage quality. Forages such

as bahiagrass (Paspalum notatum Flugge) and bermudagrass (Cynodon

dactylon L.) require high rates of N for optimum yields, but their

protein contents and quality remain quite low when compared to some

other grasses and legumes. Sulfur fertilization has been shown to

improve crop quality, and where large quantities of S have been

removed in successive cropping of these forages, yields may be in-

creased with S fertilization.

Little research has been conducted with S as a plant nutrient in

Florida soils during the past 25 years. This dissertation will review

pertinent S research as it relates to soil fertility and crop nutri-

tion, and evaluate the current S fertility status of representative

Florida soils. The overall objectives of this study were (1) co

identify the sources of soil S in the profiles of some Florida soils,

(2) to evaluate the effects of subsoil S on plant nutrition, and

(3) to evaluate the relative response to S by crops growing on differ-

ent soils and under different S fertility levels.




All of a growing plant's S requirement can be supplied from the

soil. Ensminger and Jordan (1958) reported that the amounts of S ab-

sorbed by crops at moderate yield levels ranged from about 9 to

39 kg/ha. Terman (1978) compiled values which averaged 26 kg/ha for

grain crops, 35 kg/ha for hay crops, cotton, and tobacco, and 48 kg/ha

for fruit, sugar and vegetable crops. This S is absorbed as sulfate

from the soil solution primarily due to mass flow of ions (Barber and

Olsen, 1968). The sources of this available soil sulfate ma' be pri-

mary and secondary S-containing minerals, soil organic matter, adsorbed

sulfate, atmospheric S, and fertilizers containing S.

1.1.1 Sulfur-Containing Minerals

The original source of most soil S was probably the sulfides of

metals contained in plutonic rocks. As life evolved and these minerals

were Jeconnosed, sulfate was taken up by living organisms and incorpor-

ated into organic matter. Under anaerobic conditions, S was reduced to

inorganic sulfides or elemental S. Sulfate was also precipitated as

soluble or insoluble salts of Ca, Mg, Ba, Sr, K, Fe, Cu, Zn, etc. in

arid climates; in humid climates, much of this sulfate was washed into

the sea (Tisdale and Nelson, 1975).


Although plants take up S in the inorganic, sulfate form, only 0

to 14% of the total soil S in the surface horizons of humid-region

soils is present as the sulfate ion (Ensminger, 1954; Williams and

Steinbergs, 1959; Neller, 1959; Freney, 1961; Tabatabai and Bremner,

1972a; Bonner, 1973). The inorganic sulfate and sulfide minerals are

associated with alkaline or acid-sulfate soils. Sulfate minerals that

have been identified in alkaline soils include gypsum (CaSO4 2H20),
hemihydrate (CaSO 1/2H 0), mirabilite (Na2SO 10H20), thenardite

(Na2S 4), epsomite (MgSO4 7H20), hexahydrite (MgSO4 6H20), and

bloedite (Na2Mg(S 4)2 4H20) (Doner and Lynn, 1977). These are all

very soluble minerals and are quickly removed from soils where precipi-

tation is high enough to cause deep leaching. These minerals may be

used as soil amendments in humid regions but do not persist in the soil.

Acid-sulfate soils or "cat-clays" develop when tidal flats or

interdistributary basins of river deltas are drained. Reduced forms of

S such as amorphous mackinawite (FeS), pyrite (FeS2), or greigite

(Fe3S4) are oxidized by Thiobacillus sp. The soil becomes extremely

acidic (pH 3.0 to 3.5), and crystalline jarosite [KFe3(OH)6(SO4)2] can

develop within 1 to 2 years after draining. If carbonates are present

in the sediments, the acidity is neutralized and gypsum forms. Few of

these soils are arable and where they are used for farming, S as a

plant nutrient is certainly not limiting. The presence of 0.05% water

soluble sulfates has been suggested as an indicator of an acid sulfate

soil (Doner and Lynn, 1977).

1.1.2 Soil Organic Matter

Most of the total S in the surface horizons of humid regions is

organic (Eaton, 1922; Evans and Rost, 1945; Williams and Steinbergs,

1959; Freney, 1961; Freney et al., 1962; Nelson, 1964a; Tabatabai and

Bremner, 1972a). This fraction may account for 50 to 75% of the total

soil S (Alexander, 1977). In a study with 22 soils from eastern

Australia, Freney (1961) found an average of 93% of the total S was in

organic form. Bonner (1973) found this value to be 63% for Louisiana

soils, but he was unable to account for a large percentage of the total

soil S.

The availability of this organic S to growing plants is dependent

upon the rate of decomposition of the soil organic matter and the

nature of the organic matter itself. Environmental factors which favor

the proliferation of microorganisms such as moisture, temperature,

energy sources, aeration, pH, etc. will affect the mineralization of

soil S (Parker and Prisk, 1953; Nelson, 1964a;Burns, 1967).

Soil organic matter can serve as either a source or a sink for

plant-available S (Burns, 1967). The decomposition of plant residues

low in S can induce S deficiencies of crops growing on a soil. Avail-

able S can become assimilated by a growing population of microorgan-

isms just as available N can become tied-up when low-N energy sources

are added to a soil. Since S is a component of the amino acids,

cystine and methionine, it is an essential nutrient of microorganisms

as well as of plants and animals. Starkey (1966) stated that the

cells of microorganisms may contain 0.1 to 1% S. C:N:S relationships

During the 1960's, much research emphasis was devoted to studying

the C:N:S relationships in soils. Williams et al. (1960) found that

total soil S was very highly correlated with both C and N in Scottish

soils. They found a mean C:N:S ratio of 100:7.1:1. These values were

similar to C:N:S relationships reported earlier for soils in Australia

and New Zealand (Williams and Donald, 1957; Walker and Adams, 1958;

Williams and Steinbergs, 1958). Nelson (1964a) found an average

organic C:N:S ratio of 114:9.1:1 in the surface horizons of 12 Missis-

sippi soils which had a wide range of organic matter contents. Organic

S was highly correlated with total S (r = 0.920) and organic C (r =

0.947) but not with soluble sulfate-S. The C:N:S ratio of Louisiana

soils was found to be 88:8.3:1 for surface horizons and 94:10:1 for

subsurface horizons (Bonner, 1973). Tabatabai and Bremner (1972b) also

found significant correlations between total S and organic C (r = 0.86)

and between total S and total N (r = 0.87) in Iowa soils. Total S

decreased with depth in the soil profile but remained significantly

correlated to organic C (r = 0.86) and total N (r = 0.92).

The organic C:S ratio, like the N:S ratio, can be used to predict

S mineralization or immobilization by soil microorganisms. Massoumi

and Cornfield (1965) showed that the addition of cellulose to a soil

resulted in a decline in available sulfate levels as the microorganism

populations growing on the polysaccharide assimilate inorganic S. If

the organic matter contains less S than required for microbial pro-

liferation, immobilization will be dominant; if the element is in

excess, mineralization of S will result. Barrow (1960a) found that

mineralization of S from soil organic matter did not occur until

catabolism had lowered the C:S ratio to about 50:1. He also found

that immobilization of S occurred in soils during a 12-week decomposi-

tion period when the C:S ratio of the original organic material was

greater than 200:1 (Barrow, 1960b). Tabatabai and Bremner (1972b)

found that most Iowa soils immobilized S when incubated for 2 weeks,

and none released more than 6 ppm of sulfate-S when incubated for 10

weeks under aerobic conditions. They found no significant correlation

between mineralized S and total S, sulfate S, organic C, total N, or

mineralized N.

Bettany et al. (1980) found C:N:total S ratios of 79:7.9:1 in a

Udic Haploboroll under pasture and ratios of 71:6.6:1 for the same soil

that had been cultivated for 65 years in Saskatchewan. The relatively

narrower C:S and N:S ratios of the cultivated soil, along with a rela-

tively smaller decrease in S than both C and N in the cultivated soil,

suggested that S is more resistant to mineralization than C and N.

Earlier, they had found the largest amount of S mineralization occurred

in soils with the lowest C:N:S ratios (Bettany et al., 1974).

Kowalenko and Lowe (1975) concluded from their research that while

the interrelationships of C, N, and S would be important in a consid-

eration of the S supplying power of a soil, more complex interrelation-

ships than simple C:N:S ratios may exist. Organic sulfur fractionation

At the same time investigators were studying C:N:S relationships

in soils, many developed techniques to fractionate organic S to find

the form or forms of organic S that might influence S mineralization.

Williams and Steinbergs (1959) separated soil S into alkali-soluble S,

soluble sulfate after ignition, heat-soluble S, reducible S, and sul-

fate released by hydrogen peroxide oxidation. Each method removed part

of the organically-bound S, but only the heat-soluble fraction was

highly correlated with the amount of S taken up by plants. Heating

the soil on a water bath, followed by heating for 1 hour at 1020C in a

hot-air oven, released a maximum amount of S from the soil. Most of

this S was assumed to be organic sulfates that could easily be split

from the organic matter. Barrow (1961) deducted from his observations

that sulfate released by heat treatment was inorganic because it could

be removed with a 0.15% CaC12 solution. This extractant does not

decompose organic sulfate, and plant uptake of S was closely related

to sulfate extracted by this procedure. The fact that organically

bound sulfates form a considerable fraction of the total soil S was

based on these early reports where NaOH was used to extract most of the

total soil S (Williams and Steinbergs, 1959; Freney, 1961).

Organic soil S is generally separated into the C-bonded fraction

and the HI-reducible or ester-sulfate fraction. The C-bonded fraction

is believed to consist largely of S in the form of S-containing amino

acids such as methionine and cystine. The HI-reducible S is the frac-

tion which is not bonded directly to C and is reducible to H2S by HI.

This fraction is believed to consist of S in the form of ester sul-

fates, e.g., organic sulfates which contain C-O-S linkages such as

choline sulfate, phenolic sulfates, sulfated polysaccharides, etc.

Prior to the 1960's, only small amounts of cystine and methionine

could be detected in soil organic matter (Bremner, 1950; Sowden, 1955,

1956; Stevenson, 1956). Due to uncertainties in extraction and analy-

sis, the highest values reported for these organic S constituents were

only around 10% of the total S content (Freney et al., 1970).

DeLong and Lowe (1962) were able to separate this C-bonded fraction by

reducing the S to inorganic sulfide with Raney nickel. Carbon-bonded S

occupied 47 to 58% of the total S in organic soils and 12 to 35% in

mineral soils from Quebec (Lowe and DeLong, 1963). Data suggested that

much of the C-bonded S may have been associated with humic acid

materials and was more stable than the HI-reducible fractions. How-

ever, Lowe and DeLong were unable to account for all of the organic S

in their soils, and made no conclusions as to the significance of this

organic-S fraction on plant S nutrition under field conditions.

Freney, Melville, and Williams (1970) found that Fe and Mn could

interfere with the determination of C-bonded S by the method of Lowe

and DeLong. An average of 23% of the organic S in 15 Australian sur-

face soils could not be accounted for by this method even when it was

modified to reduce interference. They calculated C-bonded S by the

difference of total S and HI-reducible S. Each form accounted for

about 50% of the total soil S in Australian soils. Similar values

were obtained for organic S fractions in 37 Iowa soils (Tabatabai and

Bremner, 1972a). Bonner (1973) reported that in 23 Louisiana soils,

44% of the total S was HI-reducible and 25% was C-bonded. This was

slightly more than the 63% organic S present in the soil. He also

found 14% was calcium phosphate-extractable sulfate S but did not

account for all of the total S. No significant relationship was found

among total S, organic S, and the various organic-S fractions with

yield, S uptake, and S concentration in the plant tissue of a sorchum-

sudangrass hybrid.

Analyses of S fractions in some surface soils from Brazil and sur-

face soils from Iowa in the United States indicated that the Brazilian

soils contain much more adsorbed inorganic S than lowan soils (Neptune

et al., 1975). The average percentage of total S as ester surface S

and as C-bonded S was 50 and 7%, respectively, for the Brazilian soil

and 50 and 11% for the lowan soils. However, they were unable to

identify 42% of the organic S in the Brazilian soils and 34% in the

Iowan soils. Values for C:N:S relationships in soils and soil-S frac-

tions reported in the literature are compiled in Table 1.

Bettany et al. (1973, 1979) found a gradual increase in both the

C:S and N:S ratios of soils along an environmental gradient in Sas-

katchewan, Canada. Brown chernozemic soils (Aridic Haploborolls) had

a C:N:S ratio of 61:6.5:1 with 52% of the total S as HI-reducible while

grey luvisolic soils (Typic Cryoboralfs) had ratios of 112:9.7:1 with

32% HI-reducible S. These values for the C:N:S ratios agree with

earlier values for similar soils in the northern United States and

Canada (Evans and Rost, 1945; Lowe, 1965). Intermediate soils were the

dark brown and black chernozemic soils (Typic Haploborolls and Udic

Haploborolls) with a mean C:N:S ratio of 80:7.8:1 and 50% HI-reducible

S. However, differences in C:N:S ratios of humic acid (HAA) fractions

of these soils were greater than those of the total soil organic mat-

ter. Differences in the comparable ratios of conventional fulvic acid

(FAA), clay-associated humic acid (HAB), and less than 2-um humin were

smaller than for HAA along the environmental gradient. In 1973,

Bettany proposed that C-bonded S is more likely to be incorporated

into the strongly aromatic humus whereas the HI-reducible S fraction

would be associated with active side-chain components.

A more recent, refined proposal suggests FAA, HAB, and less than

2-um humin as the sources of potentially labile organic S (Bettany et

al., 1979). However, a study of cultivated versus uncultivated soils

showed that the HAA, HAB, and humin fraction accounted for 80% of the

total S lost upon cultivation. The FAA fraction had narrower C:N:S

ratios and contained more HI-reducible S but contributed only 14% of

the total S loss (Bettany et al., 1980). The small loss by the FAA


0) a0) i0
a o O

4-j r- 4 F- o a 0)

W) -4 1 -i u C p
z z E-'


0 a0

O4- 1-

r-4 ) r-i

1- *

I I I I I I I ll o
J ,





I -

-l4 i-


co0 -

-n so
*-3- C

en\ n

00 r-4

\- -0 CM
s0 -l (O
I-l4 n C

Cu C




I I I I I I 1 I I

00- C4

3 I I 0 n r I
SIII QI-nini I

C' IL(II '


Inen C r- C4N
CMUl nV CM 1

(n -4 Lfoo


C CC 4 C

S *.:3: *
S a0 0 )

^* Z
avi a4 i a I B I ca )

a c cc M cn rn wto
L^4 ^-- *v *s^ '- ^^
pa 35 o


Ir- 0 o0 I
m 0 C o

ClJ-4-4 10'3\O0
I I~-4C-Ju

0o' o

C Cu N- o u
a w N 1ci a
C 1-o C

Cu U
U u


4J *O
a i T(4-I2
> "- 0 -H
-H u (n 0 a

= 4.5:3: >M
Su-4 40 ao




(4 Z

1 0)
1-4 4-

4J 4-4
U) r -4

(U a

(0 i-l
M 3



4-j CJ




fraction was explained as probably due to the accumulation of low

molecular weight S compounds released by the decomposition of some

resistant fraction. The FAA fraction might not lose a large proportion

of its S because of the constant addition and faster turnover rate.

Bettany et al. (1980) suggested that it might be fractions such as the

FAB which show little net change in total S content that are important

in supplying available S in the short term.

1.1.3 Soil Sulfur Reactions Oxidation-reduction

No other element is known to occur in as many different forms as

S. Its valence ranges from -2 to +6 with sulfides and sulfates repre-

senting the two extremes (Table 2). As previously mentioned, the re-

duced forms of S occur primarily in soil organic matter, amino acids,

and the B vitamins including thiamine, biotin, and lipoic acid.

Reduced forms of S may also be found in the soil as sulfides of pri-

mary minerals such as FeS or as intermediate reaction products such as

H2S, thiosulfates, thiocyanates, polythionates, or elemental S

(Burns, 1967).

Sulfide is the principal, stable form of S under anaerobic condi-

tions, but elemental S and organic compounds of S may persist in some

natural anaerobic environments such as in sedimentary rocks associated

with S domes and in peat, coal, and oil (Starkey, 1966).

Reduction occurs through microbial assimilation of sulfate under

aerobic conditions and by obligate anaerobes that use sulfate as the

H-ion acceptor and organic matter as an energy source under anaerobic

conditions. All of the natural organic and inorganic compounds of S

are suscentable to microbial attack. Even the S that occurs in

Table 2. The oxidation states of sulfur in soils.


Sulfate (most oxidized)


(tetra-, tri-, penta-)



Disulfur monoxide

Elemental sulfur


Sulfide (most reduced)

Oxidation state



+2 to +4








H2SO4, SO22-, SO3

H2SO3, SO32 SO2

2- 2- 2-
S406 S3062, S506

Na2S203, S202-



Na2S2 S2-

H2S, S2-

natural deposits as sulfate, elemental S, and sulfide can be oxidized

or reduced by microorganisms in suitable environments. The types of

transformations are affected by the state of the S and the environ-

mental conditions, particularly the availability of 0.

The predominant microorganisms concerned with the reduction of

sulfates are bacteria of the genera Desulfovibrio and Desulfotomaculum.

A number of carbohydrates, organic acids, and alcohols may serve as

energy sources or electron donors while sulfates are the electron



(Alexander, 1977)

Microorganisms that reduce S are extremely important to soil fertility

because they reduce the availability of S in the soil, and the primary

product of their metabolism, H S, can be toxic to plants such as rice,

citrus, and other crops and trees of economic importance (Alexander,


In most well-drained, arable soils, S oxidation is of much more

agricultural importance than S reduction. Several reviews have been

made of the S-oxidation processes that occur in soils (Gleen and

Quastel, 1953; Vishniac and Santer, 1957; Starkey, 1966; Burns, 1967;

Kelley, 1968; Aleem, 1975; Alexander, 1977). Although not all soil S

oxidation reactions are enzymatic, and chemical oxidation of sulfides,

elemental S, and thiosulfates can occur, microbiological oxidation is

much more rapid under favorable conditions.

Organisms capable of oxidizing reduced forms of S may be either

autotrophs or heterotrophs. The most significant microorganisms are

members of the genus, Thiobacillus. Most of these bacteria are

chemoautotrophic, obligate aerobes. The ideal, overall, oxidation

reactions of the thiobacilli bacteria are:

0 2- 2- 2-
SH S S 20- 3 S 406 SO

sulfide sulfur thiosulfate tetrathionate sulfate

Some of the common species of thiobacilli found in soils and the oxida-

tion reactions associated with each species are shown in Table 3

(Burns, 1967).

The acid produced in the oxidation process is of significant

agricultural interest. As previously mentioned, the draining of wet-

lands may result in acid-sulfate soils or "cat clays." Elemental S,

polysulfides, or sulfuric acid may be used to amend alkaline soils or

to lower the soil pH for acidophilic plants:

2S + 302 + H20 -T. thiooxidans H2SO4 + (CaCO3 + H20) +

CaSO4 2H20 + 2CO2

The H2SO4 produced by microbial oxidation reacts with free lime in the
2+ +
soil to form gypsum. The Ca from gypsum replaces adsorbed Na to

reclaim sodic soils (Reuss, 1975). Precipitation reactions

Reducing conditions can result in the precipitation of FeS and

other metal sulfides in soil systems. However, in well-drained soils,

metallic sulfides are not a sink for soil S. As previously mentioned,

inorganic sulfates are associated with alkaline soils in arid or semi-

arid climates. Harward and Reisenauer (1966) noted that sulfate

retention in arid soils is merely a consequence of gypsum solubility.

Most sulfate salts are quite soluble in water and leach rapidly from

soils in humid regions.

Table 3. Sulfur oxidation reactions by certain
species of thiobacilli (from Burns,

Thiobacillus thioparus
S232- + H20 + 402 5S02- + H2SO4 + 4S

S 062- + CO32- + 02 2

2S + 302 + 2H20 -

2KSCN + 40 4H20 -
S2022- + 202 + H20 -

S4062- + 702 + 6H20

2SO4 + CO2 + 2S


(NH)2SO4 + K2SO4 + 2C02
SO2- + H2
2S04 + 6H2S04
4 2 4

T. thiooxidans

S2032- + 202 + H20 SO 2- + H2S04

2S + 302 + H20 2H2S04

2S 062- + 70 + 6H20

+ 302


+ 20.

+ 702

+ 402


- 2S04 + 6H2S04
4 2 4

T. denitrificans
+ 2H20 2H2S04
+ 2H20 3SO 2- + 2H2S0 + 3N2
S+ H20 SO4 -+ H2SO4

+ 6H20 + 2SO 42- + 6H2SO4

+ 4H20 (NH)2 SO + SO42- + 2C02
+ H20 4 9S04 + H2SO4 + 4N2

T. novellus

S2032- + 202 + H20 SO 2- + H2SO4

2S4062- + 702 + 6H20 2SO 2- + 6H2S04


5S +

S 2-
2S 4062-




Table 3. (Continued)

T. ferrooxidans
2S + 302 + 2H 0 2H2SO

S2032 + 202 + H20 O SO 2- + H2SO4

12FeSO + 302 + 6H20 4Fe2 (SO)3 + 4Fe(OH)3

No specific inorganic mineral has been identified as a sink for

soluble sulfates in the well-drained, acid soils of the southeastern

United States. Adams and Rawajfih (1977) proposed that sulfate reten-

tion by acid soils may be a consequence of the solubility of basalumi-

nite, A14(OH)10SO*45H20, and/or alunite, (Na/K)Al3(OH)6(SO4)2, as well

as adsorption reactions. Based on earlier work by Singh and Brydon

(1967, 1969), Adams and Rawajfih were able to show through precipita-

tion experiments and calculated soil solution ionic activities that

soil solution sulfate could be removed by precipitation as basaluminite

and alunite. An increase in soil pH could be expected to be accom-
panied by an increase in solid-phase Al(OH)3and solution SO because

of the following reactions:

A14(OH)10SO4 + 20H- 4Al(OH)3 + SO2-

K+ 2O-
KA13(OH)6(SO )2 + 30H ---3Al(OH) + + 2SO2
3 6 4 2 3 4

These minerals have not been identified in soils, but Adams and

Hajek (1978) further demonstrated that formation of these compounds in

sulfate-containing acid soils was thermodynamically feasible. Precipi-

tation occurred with as little as 10 mM sulfate in the reacting


Using basaluminite and alunite as sources of sulfate for cotton

seedlings (Gossypium hirsutum L.) grown in a greenhouse in a sandy loam

soil from a Typic Hapludult at a pH of 6.5, Wolt and Adams (1979)

demonstrated that S in basaluminite was more available while that in

alunite was almost unavailable. Their data supported the previous

thesis of Adams and Rawajfih that sulfate "adsorption" and "desorption"

in acid soils could be partially due to solubility behavior of Al and

Fe hydroxy-sulfates. Adsorption reactions Factors Affecting Adsorption

In well-drained soils of humid regions, almost all of the inor-

ganic S occurs in the sulfate form. Because of its anionic nature and

the solubility of its common salts, leaching losses of sulfate can be

rather large. Yet plants depend on the availability of this form for

the S they absorb from the soil.

Ensminger (1954) demonstrated the possibility of sulfate adsorp-

tion in soils from Alabama. Water or 0.1 N HC1 extracted very little

sulfate from these soils, whereas extractants containing a replace-

able anion such as phosphate (H2PO4) or acetate (CH3COO ) extracted

considerable sulfate. A 500 ppm P solution of KH2PO4 was the most

efficient extractant used. He also showed that increasing amounts of

superphosphate and lime applied to a Cecil sandy clay loam (Typic

Hapludult) resulted in decreasing amounts of soluble sulfate. Hydrated

Al203 was found to adsorb more sulfate than a number of other soil

minerals and clays. Kaolinite also adsorbed significant amounts of

sulfate. Kamprath et al. (1956) found that 1:1 type clay minerals

adsorbed more sulfate than 2:1 types, and adsorption was related to pH

and the amounts of sulfate and phosphate in solution.

Subsequent research by others has shown that sulfate retention in

soils is closely related to 1:1 type clay minerals, the presence of

Al and Fe hydroxides and oxyhydroxides, soil pH, and the presence of

competing anions, particularly phosphates (Neller, 1959; Chao, Harward,

and Fang, 1962a, 1962b, 1962c; Chang and Thomas, 1963; Elkins and

Ensminger, 1971). Neller (1959) examined acetate-extractable sulfate

S in 10 soil series at different profile depths from various locations


in Florida. The sulfate-S contents of the surface horizons of most of

these soils ranged from 0 to 4.5 ppm. Ultisols contained considerable

amounts of sulfate-S. There was a highly significant increase (r =

0.82) in sulfate content with an increase in the clay-size fraction of

the soil.

In contrast to cation adsorption, data from Chao et al. (1962b)

indicated that sulfate retentive soils did not possess adsorption

maxima or definite anion exchange capacities. They suggested that

perhaps some other mechanism of sulfate retention is functional. No

other mechanism has been identified except the proposed possibility of

the precipitation of sparingly soluble aluminum sulfate minerals

(Adams and Rawajfih, 1977; Adams and Hajek, 1978). Chao et al.

(1962c) also found that the removal of organic matter, free aluminum

oxides, and free iron oxides, considerably reduced sulfate adsorption

in soils from Oregon. They also showed that Al-saturated clays

adsorbed much more sulfate than H-saturated clays. The amounts of

sulfate retained by reference clays were in the order:

kaolinite > illite > bentonite.

These results are consistent with the observations of Berg and Thomas


Three possible mechanisms of sulfate adsorption were proposed:

1. Anion exchange due to positive charges developed on hydrous
Fe and/or Al oxide or on the crystal edges of clays, espe-
cially kaolinite, at low pH values.

2. Sulfate retention by hydroxy-Al complexes through coordina-

3. "Salt Adsorption" resulting from attraction between the
surface of soil colloids and the salt.

4. Amphoteric properties of soil organic matter which develop
positive charges under certain specific conditions.;. Mechanisms of Sulfate Adsorption

Sulfate, as are anions such as phosphate and silicate, may be ad-

sorbed by nonspecific or specific adsorption mechanisms. Nonspecific

adsorption involves the coulombic attraction of negatively charged

species to positively charged sites on soil colloids or metal oxide

surfaces where they are held in the Stern layer or as counterions in

the diffuse part of the electrical double layer (Gast, 1977; Bohn et

al., 1979). This type of adsorption involves a hydrolysis mechanism

for the development of surface sites for anion adsorption. The number

of sites formed depends on the pH and the type of surface but not on

the type of anion (Hingston et al., 1967).

Some anions may be specifically adsorbed on mineral surfaces.

The mechanism involved is frequently referred to as ligand exchange

(Gast, 1977). Specific adsorption coordinates the adsorbing anion with

the metallic cation such that the anion is not easily replaced. This

involves the displacement of a coordinated OH or H20 on the surface

of metal oxides or hydroxides and forms partly covalent bonds with the

structural cations (Hingston et al., 1967). Mineral surfaces contain-

ing partially coordinated oxygen atoms and broken edges of layer sili-

cates are the sites for specific adsorption in soils.

The exact mechanism of sulfate adsorption is not fully understood.

Chang and Thomas (1963) proposed the following mechanism to explain

their observation that the quantity of anions held by a Cecil subsoil

increased with time.
2- 2-
cla-.-Al-(OH) + SO -) clay-Al[(OH) (SO ) ] + 20H
Y 4 y-z 4 z

Divalent sulfate ions replace OH from Al(OH) or Fe(OH) coatings

on the clay surface and substitute for them. The replaced hydroxyl

ions react with H+ in solution which may have resulted from cation

exchange involving a similar mechanism. They explain that sulfate

adsorption is increased as the pH is lowered because the replaced

hydroxyl ions are more effectively neutralized. As the pH increases,

cation affinity increases also, and this results in the replacement of

more Al and more hydrolysis. They used this model to explain the pH

and time-dependent processes which had been observed in sulfate adsorp-


Hingston et al. (1972) studied specific anion adsorption by

goethite and gibbsite and found that where the acid (in this case,

H2SO4) was fully dissociated, specific adsorption occurred only to the

extent of the positive charge of the surface. Little specific adsorp-

tion was found at pH values greater than the zero point of charge (ZPC)

of the mineral. Maximum adsorption would occur when the pH = pKa. At

this pH both the amounts of anion (dissociated acid) available for

ligand exchange and the amounts of proton donor (undissociated acid)

capable of neutralizing liberated OH are greatest. Only the mono-

valent species, HSO could be specifically adsorbed without creating

additional negative charge at the surface. Their mechanism involves
SO accepting a proton from the surface at pH values near the pK

of the acid:

Al-OH2 + SO 4 -- Al-OH + HSO -- Al-HSO + OH .

One mole of SO4 adsorbed as HSO neutralizes one equivalent of sur-

face charge. Gebbardt and Coleman (1974) agreed with the mechanism of

Hingston et al. by concluding that sulfate was adsorbed as HSO .

Rajan (1978) studied sulfate adsorption on hydrous alumina and

offered a refined mechanism for specific adsorption. He discounted

sulfate adsorption as HSO4 because, under experimental conditions (and

conditions in most soils), sulfate exists as SO He proposed that
sulfate is adsorbed as SO across two Al atoms, forming a six-membered

ring. At low sulfate concentrations, water is displaced from the

positive sites; as the concentration increases, increasing proportions

of OH groups are displaced from neutral sites (Fig. 1). His proposed

mechanism is in agreement with experimental findings that the final

surface, after adsorption, carries close to a zero charge, that the

relationship between sulfate adsorbed and charge neutralized is curvi-

linear, and that the process is time dependent.

Allophanic and highly weathered tropical soils possess extremely

high sulfate adsorbing capacities when compared to mineral soils of

temperate regions (Chao et al., 1962b; Bornemisza and Llanos, 1967;

Hanson et al., 1970). Andepts from Mexico, Colombia, and Hawaii were

found to adsorb 10 to 20 meq of sulfate per 100 g in surface soils and

15 to 60 meq sulfate per 100 g in subsoils (Gebhardt and Colemen, 1974).

Minerals associated with these soils are primarily amorphous silicates

and hydroxides and oxyhydroxides of Fe and Al. Experimental ZPC's for

some pure minerals that may be associated with high sulfate adsorbing

soils are listed below (Yoon et al., 1979):





+ 02-



0 OH,




Fig. 1. Rajan's (1978) proposed mechanism of sulfate adsorption.

+ OH2

+ OH-


Mineral ZPC

A1(OH)3 5.1

a-AlO(OH) 7.7

y-A1O(OH) 7.5

a-Fe203 9.04

y-FeO(OH) 7.4

a-FeO(OH) 6.7

Soils containing large quantities of these oxides and hydroxides

and a pH below the ZPC of the dominant minerals would be expected to

retain large amounts of sulfate by nonspecific adsorption. Gillman

(1974) found that phosphate-extractable sulfate increased with pro-

file depth in an Australian rain-forest soil (Rodic Hapludult). He

showed that the ZPC also increased with depth from a ZPC of 4.5 at the

10 to 20-cm depth to 5.8 at the 210 to 240-cm depth. Leaching

Many humid-region soils have argillic horizons which contain

large amounts of hydrated oxides of Al and Fe and 1:1 type clay miner-

als. These materials may accumulate sulfate because of adsorption as

explained in the previous section. In coarse-textured, sandy soils

such as the Entisols, Spodosols, and the surface horizons of Ultisols

in Florida, very little sulfate adsorption takes place (Ensminger,

1954; Neller, 1959; Jordan, 1964). Mineralized sulfate or sulfate

applied in fertilizer may be readily lost by leaching under the high

rainfall conditions which exist during most of the growing season in



Chao et al. (1962a) studied the movement of 35S as gypsum through
columns of 15 Oregon soils. The depth of 35S movement was dependent

on the amount of water moving through the columns and the sulfate

adsorbing properties of the soils. When 20 cm of water was applied to

a sandy loam soil, 15.2% of the S appeared in the leachate at 51 cm.

Less than 5% remained in the upper 10 cm. On the other hand, those

soils with a high clay content (greater than 30%) containing 1:1 type

minerals and greater than 1 meq/100g exchangeable Al retained all of

the applied sulfate above 15 cm when 20 cm of water was applied. Lime

and phosphate applications increased sulfate leaching as would be

expected from knowledge of the adsorption process. Similar leaching

losses of sulfates were observed in undisturbed cores of some Austra-

lian soils (Peverill et al., 1977) and in two Caribbean soils (Haque

and Walmsley, 1974).

Rhue and Kamprath (1973) studied the effects of different sources

of S on leaching of sulfate in two North Carolina soils during the

winter. They found that all of the applied S had leached out of the

surface 45 cm of a Wagram loamy sand (Arenic Paleudult) 180 days after

treatment. Elemental S, 325-mesh and prilled, was oxidized during the

winter months, and after 180 days, extractable sulfate-S was at the

original level in the upper 45 cm of surface soil. Oxidation and

leaching rates were much slower in a Georgeville silty clay loam

(Typic Hapludult).

The Wagram loamy sand from the coastal plain of North Carolina

would be similar in properties to many of the Ultisols of northern

Florida. Winter rainfall in north Florida would be comparable to that

in North Carolina. However, the milder temperatures of Florida would

encourage more rapid oxidation of elemental S, and leaching losses of

mineralized or fertilizer-applied S would be high.


1.2.1 Sulfur as a Pollutant

Most studies of atmospheric S were done by those concerned with S

as an environmental pollutant. Atmospheric S is the cause of acid-

rainfall and direct SO2 damage to plants in areas of high atmospheric

concentrations. Areas of heavy industrial development have been con-

cerned with this problem for decades, but rural areas of the southeast-

ern United States have not been as concerned with atmospheric S as a

pollution source. A few researchers have discussed the beneficial

aspects of atmospheric S as a nutrient source for crops (Fried, 1948;

Hoeft et al., 1972; Jones et al., 1979). This subject will be dis-

cussed in a later section. Damage to plants

The Copper Basin of southeastern Tennessee is an extreme example

of an area where 9,300 ha were denuded of vegetation by a combination

of SO2 from Cu smelting, forest removal, and over-grazing. Acute

damage can occur on certain forest trees at concentrations less than

0.25 ppm SO2 for 8 hours. Chronic injury or long-term effects were

noted in a Canadian forest when the trees were exposed to an average

SO2 concentration of 0.017 ppm for 5 months (Linzon, 1975). Acute and

chronic damage to agronomic plants by atmospheric SO2 has been

reported, but critical atmospheric levels of SO2 have been difficult

to define (Heagle, 1972; Taniyama and Sawanaka, 1973; Tingey et al.,


Once S is in the atmosphere, it can travel many kilometers from

the source and affect forests and cropland. A study of soil and vege-

tation around a high-S, Fe-sintering plant in Ontario showed that
2 2
birch trees were totally killed on 108 km2, heavily killed on 191 km

and damaged on 589 km2 downwind from the plant. Damage occurred up to

48 km from the plant in the direction of the prevailing air currents

(Rao and Leblanc, 1967; McGovern and Balsillie, 1974). Damage to animals

The levels of SO2 in the atmosphere which may be harmful to higher

animals and humans are greater than that which may be harmful to

plants. Levels greater than 1 ppm were necessary before significant

airway resistance occurred in healthy male adults exposed for 10 to 30

minutes (Frank et al., 1962). Other cases of the effects of SO2 and

H2SO4 in the atmosphere on human health are reviewed by Schlenker and

Jaeger (1980).

1.2.2 Sources of Atmospheric Sulfur

Sulfur can exist in the atmosphere as SO2, H2S04, particulate sul-

fates, H2S, and methylmercaptan (CH3SH) (Terman, 1978; Urone and

Kenny, 1980). Sulfur dioxide constitutes about 95% of the S compounds

produced by the combustion of S-containing fossil fuels (Kellogg et

al., 1972). Hydrogen sulfide, the primary S-containing product of

anaerobic decomposition of organic matter, is rapidly oxidized to SO2

and SO3 in the atmosphere.

When SO2 dissolves in a cloud or fog droplets or is adsorbed on

particle surfaces, it reacts with water to form H2SO3 which is rapidly

oxidized by dissolved oxygen or ozone to H2SO4. The detailed mechanism

of these reactions is not well known, and even the nature of the spe-

cies undergoing oxidation has not been clearly established (Cadle,

1975; Gaspar, 1975).

On a global scale, natural sources of S emissions are estimated

to be greater than anthropogenic sources (Junge and Werby, 1958;

Robinson and Robbins, 1972; Terman, 1978). Kellogg et al. (1972) esti-

mated that man is contributing about half as much atmospheric S now as

is nature. However, recent studies in Florida indicate that man-made

sources of atmospheric S account for more than 85% of the total S in

the air over Florida (E. S. Edgerton, personal communication). The

burning of fossil fuels accounts for most of this S. World-wide,

about 70% of this S is from the burning of coal, 16% from petroleum

combustion, 4% from petroleum refining, and 10% from the smelting of

ores (Gaspar, 1975; Terman, 1978). A quantification of the sources

and sinks of S compounds present in the biosphere is shown in Fig. 2

(Cadle, 1975). Anthropogenic sources

In Florida, most of the anthropogenic S comes from 36 electric

power plants (most of which are oil burning), eight pulp and paper

mills in the northern portion of the state, the phosphate industry in

west-central Florida, sugar refineries around Lake Okeechobee, various

oil companies, cement plants, and chemical companies, and automobile

emissions (Urone, 1975). Many of the power plants which are presently

burning low-S oil may have to convert to high-S coal because of the

cost of imported oil and the availability of coal. This conversion

would increase the output of SO2 into the environment. Burning of wood




0 40
o= -.i
o M
IM ow
& g egl

SO2, SO-
/ particles



0 w
0 4
1.4 0
o 12
0r e.


> M

M w
H 0o
0 U




River runc1ff



* Total terrestrial, aquatic, and marine biological emission =
metric tons/year.



262 x 106

Fig. 2. Sources and sinks of sulfur compounds. Units are 10 metric tons
per year calculated as sulfate (from Cadle, 1975).

SO2 SO2 S H2 S, SO2

150 45 30 "






I N m m m


for domestic heat and the controlled burning of forests and rangeland

also contribute a small amount of S to the atmosphere. Biogenic sources

Natural sources of atmospheric S, unrelated to human activity,

include the decomposition and combustion of organic matter, spray from

the oceans, and volcanic and other geothermal activity. Seawater is

one of our greatest natural sinks for S and also one of our greatest

sources. The average concentration of sulfate in the oceans is 2.65

ppm. The concentration of H2S cannot be detected in seawater; the

oceans are supersaturated with dimethyl sulfide (Cadle, 1975), but it

is doubtful if any of this S escapes into the atmosphere in the sulfide

form. The sulfate released annually from the surface of oceans has

been estimated to be around 40 to 130 million metric tons, but only

10% of this passes over land (Eriksson, 1959, 1960; Friend, 1973).

Vegetative decay under anaerobic conditions releases large quanti-

ties of H2S to the atmosphere where it is rapidly oxidized. Freshwater

swamps and salt water marshes are abundant sources of biologically-

produced atmospheric S. These sources are difficult to quantitate,

and most estimates seem to be based on the theoretical need to balance

the S cycle. Gaspar (1975) reviewed literature which indicated 30 to

202 million metric tons of H2S released by biological decay on land.

Edgerton et al. (1980) compiled estimates of biogenic S emitted from

wetlands in Florida (Table 4).

1.2.3 Local and Regional Studies of
Rainfall and Atmospheric S

Recent studies by Edgerton et al. (1980) and Brezonik et al.

(1980) have helped to delineate the areas of acid precipitation in

Table 4. Sources of biogenic S from wetlands in

Potential S
emission Total S
Source Area rate emission
-ha- -kg/ha/yr- -mton/yr-

Poorly drained 2,800,000 0.1-1 280-2,800
organic soils
and freshwater

Tidal marshes 490,000 1-100 490-49,000
and mangrove

Total 3,290,000 -- 770-52,000

Florida, to estimate the amounts of S added to Florida soils through

precipitation, and to evaluate the importance of oceanic aerosols and

biogenically-produced S to Florida rainfall chemistry. While the

deposition of H and excess sulfate (i.e., total sulfate minus sea-salt

sulfate) in Florida rainfall is 30 to 90% of the deposition rates in

the northeastern United States, the acidity of rainfall in Florida has

increased markedly in the past 25 years; the average sulfate concen-

trations have increased 450%. The relative annual deposition of S on

Florida soils in bulk precipitation at five sites from 1952 to 1955

and from 1978 to 1979 and the relative acidity of rainfall are shown

in Fig. 3 (Brezonik et al., 1980).

Northern Florida receives more than 1.5 times as much S as sulfate

annually from bulk precipitation as southern Florida (8.4 and 5.5 kg/ha,

respectively). Northern Florida is the most industrialized area of the

state, and accounts for 85% of the total anthropogenic S emissions in

the state. Coastal areas receive only around 4 kg/ha in rainfall.

These data were based on samples collected from 24 sites throughout the

state. The bulk of this S in north Florida (69%) fell during the

summer months when it could be of most benefit as a nutrient to growing

plants. This period also corresponds to the season of maximum rainfall

in Florida. Sea sulfate was estimated to constitute only a small por-

tion of the total sulfate in Florida precipitation. The percentage of

sea sulfate in precipitation decreased rapidly from coastal to inland


The occurrence of acid rainfall, which is directly related to

atmospheric S, is more frequent in northern counties than in southern

and coastal regions of the state. The most acidic rainfall in 1978

T 15


Skg S/ha/yr



Fig. 3. Annual depositon of total sulfur, 1952 to 1955 and 1978
to 1979, and the mean pH of rainfall, 1978 to 1979
(from Brezonik et al., 1980).

occurred at Jay (pH 3.76) and at Gainesville (pH 3.93) in northern

Florida. Sites north of Lake Okeechobee have annual (volume-weighted)

pH values in the range of 4.6 to 4.8 while those south of the lake

have pH values approaching geochemical neutrality (pH 5.6).

These data for Florida are similar to those reported by Jones

(1976) and Jones et al. (1979) for South Carolina. For the period 1973

to 1975, the mean annual amount of S deposited in South Carolina soil

through precipitation was 11.3 kg/ha. This compared to 6.3 kg/ha for

1953 to 1955. In addition, Jones et al. (1979) used a factor reported

by Alway et al. (1937) to estimate gaseous S adsorbed by the soil from

S adsorbed by PbO candles at 15 locations. This value increased from

2.8 kg/ha in 1973 to 13 kg/ha in 1977. Total S added annually to South

Carolina soils from atmospheric sources ranged from 11.2 kg/ha in 1973

to 20.3 kg/ha in 1976.


The sources of plant-available S that have been previously dis-

cussed are largely beyond the control of the crop producer. The amount

of fertilizer S applied to a crop or soil is completely dependent upon

management and may be the most significant source of S for modern crops

growing on sandy, low-S soils.

Sulfur has traditionally been applied in fertilizers containing

ordinary superphosphate (12% S), ammonium sulfate (24% S), gypsum

(18% S), and other S-containing materials. When sufficient amounts of

the macronutrients were applied, available S was abundant. With the

introduction of high-analysis N and P materials, the S content of

fertilizer materials and mixes has decreased. Beaton et al.

reported that ". while the consumption of N, P205, and K20

increased from about 4 million tons in 1950 to just under 18 million

tons in 1976, the total amount of sulfur in fertilizers decreased from

1.8 to 1.1 million tons for the same period." (Beaton et al., 1974, p.4).

This trend does not appear to be as dramatic for Florida. Table 5

indicates that while ordinary superphosphate consumption is down 40%

from 1950, the use of other S-containing materials has risen in Florida.

Most references do not list S-containing fertilizers separately nor do

they summarize S applied as is the custom for N, P, and K. Therefore,

the total amount of S applied to Florida soils over the years is

difficult to estimate. Mixed fertilizer consumption has also increased

considerably since 1945.

Using data from Beaton et al. and data for fertilizer consumption

in Florida in 1978 (Crop Reporting Board, 1978), one can estimate that

mixed fertilizers and fertilizer materials used in Florida contain an

average of 2.2% S. If this S was distributed evenly on the major crop-

land in Florida (1,089,300 ha of field crops, hay, citrus, and vege-

tables), then approximately 39 kg of S/ha was applied. If major crop-

land and fertilized grasslands are included (2,725,300 ha), then this

value is only 15 kg/ha.

Table 5 shows that the total S as a percent of the N + P205 +

K20 + S has decreased from 32.5% in 1950 to 7.2% in 1973 for the south

Atlantic states (Beaton et al., 1974). However, concentrated super-

phosphate (0-1% S), ammonium phosphates (0-2% S), ammonium polyphos-

phates (0% S), etc. are used more as fertilizer materials or in mixes,

and these contain very little incidental S.

The total harvested area of field crops in Florida has increased

from 518,000 ha in 1965 to 623,000 ha in 1979 (21% increase) while the

Table 5. The use of selected sulfur-containing fertilizers in
Florida (from Fertilizer Statistic Division, 1945-
1979 and Beaton et al., 1974).

Fertilizer Year
material 1945 1950 1960 1970 1979
----------------metric tons-------------------

Ammonium sulfate &
ammonium nitrate

Ordinary superphos-

Sulfate of potash

Sulfate of potash

Elemental sulfur

Calcium sulfate

Mixed fertilizers

Total fertilizers
used on farms

Total S as a
percent of

N:S ratio in

P20 :S ratio in

1,263 1,149

12,600 15,676




643,682 712,843 1,255,669

--- 32.5%

tData for south Atlantic states
South Carolina, and Virginia).

(Florida, Georgia, North Carolina,



















--- 1,914,400











total amounts of plant nutrients (N, P205, and K20) applied per ha have

only increased from 720 to 775 kg/ha (7.6% increase) (Statistical

Reporting Service, 1965-1979; Hargett, 1976). These data are surpris-

ing when considering the increased yields and intensive management of

modern crop production, but point out the trend toward less fertilizer

S applied to Florida farmlands.

Most fertilizer-applied S is in the soluble, sulfate form in

ordinary superphosphate, gypsum, potassium sulfate, sulfate of potash-

magnesia, etc. All of these soluble sulfates as well as thiosulfates

and polysulfides are about equally effective and immediately available

to growing plants (Beaton et al., 1974). Finely-ground, elemental S

is rapidly oxidized in most arable soils (Burns, 1967). Rhue and

Kamprath (1973) showed that finely-ground elemental S was completely

oxidized during the winter months in a North Carolina soil. Prilled S

(325-mesh) offered some resistance to oxidation and remained in the

surface soil longer. Beaton et al. (1974) recommended that 25% of the

total S should be in a soluble sulfate form if the soil is extremely

low in S and if the elemental form cannot be applied 4 to 6 weeks

prior to planting a crop.



Sulfur is considered a secondary nutrient along with Ca and Mg,

but it is needed by many plants in about the same quantity as P

(Tisdale, 1977). The Technical Affairs Committee of the Canadian

Fertilizer Institute recommended in 1978 that S should be .

classified in governmental regulations and by the fertilizer industry

as a 'macronutrient,' and not as a 'lesser nutrient' as it is cur-

rently described (The Sulphur Institute, 1978, p. 21).

Sulfur is found in three general forms in the plant: (1) protein,

(2) sulfate-S, and (3) organic S compounds of low molecular weight such

as free S-containing amino acids or volatile organic S compounds such

as glycosides, mercaptans, and sulfides. The primary role of S in

plant nutrition is as a constituent of the S-containing amino acids,

cystine, cysteine, and methionine, and for protein synthesis. Cysteine

and methionine may be responsible for more than 90% of the organic S

in plants (Thompson et al., 1970). Plants adequately supplied with S

will contain more true protein than plants without adequate S, and the

quality of this protein may be higher (Sheldon et al., 1951). Within

limits, the amount of methionine present in a protein is a measure of

the quality of that protein. Beaton et al. (1971) reviewed literature

showing where increasing amounts of added S improved the quality of

plant protein as indicated by the increase in methionine content.

If true protein is not synthesized in plants due to inadequate S,

non-protein N such as amides and amino acids or even inorganic nitrates

may accumulate and reduce crop quality (Tisdale et al., 1950; Rendig

and McComb, 1956; Coleman, 1957; Dijkshoorn and Van Wijk, 1967;

Stewart and Porter, 1969; Cowling and Jones, 1971). When available S

is abundant and N is limited in non-legumes, sulfate-S may accumulate

while dry-matter and protein yields are restricted. Metson (1973)

pointed out that in practice, the first situation (excess N in relation

to S) is more frequently encountered, particularly in crops regularly

fertilized with N. All of the available S is utilized in the

formation of protein and little remains to accumulate in non-protein,

organic compounds or as inorganic sulfate.

Many of the volatile organic S compounds found in plants are

characteristic of particular plant families such as the Cruciferae,

which include the Brassica (cabbage), the Amaryllidaceae, which include

the Alliae (onion) tribe, and the Tropaeolaceae, which include the

nasturtium (Metson, 1973). These plants have a high S requirement, and

the volatile S compounds impart a characteristic odor to many of them.

A relatively high proportion of the organic S is present as glycosides

which yield organic iso-thiocyanates on hydrolysis (Dijkshoorn and

Van Wijk, 1976).

Sulfur has also been found to be important to plants in other ways.

Disulfide linkages (-S-S-) have been associated with the structure of

protoplasm. Sulfur is required for N fixation by legumes since it is

part of the nitrogenase enzyme associated with this reaction (Anderson

and Spencer, 1950). Sulfhydryl groups (-SH) in plants have been

related to increased cold resistance in some plants (Levitt et al.,

1961). Other quality factors improved by S fertilization are chloro-

phyll and vitamin A content of forages. Alfalfa fertilized with P and

S contained almost twice as much carotene and vitamin A as alfalfa

fertilized with P alone (Tisdale, 1977).


2.2.1 Root Uptake

Sulfur may be absorbed by plant roots exclusively as the sulfate
(SO ) ion or absorbed as SO2 directly through plant leaves. Sulfur

uptake has been shown to be a function of the sulfate concentration of

the soil solution (Spencer, 1959; Barrow, 1967) or of nutrient

solutions (Olsen, 1957). Barrow (1967) was able to relate 0.01 M

monocalcium phosphate extractable sulfate to the decrease in S uptake

by plants grown for 183 days in a greenhouse. The uptake of S was

also greatly influenced by the nature of the soil; these differences

were also indicated in the total extractable sulfate-S. Both plant

uptake and extractable S were significantly modified by the nature of

the soil, time of sampling, and previous fertilization.

2.2.2 Absorption from the Atmosphere

Crops growing on soils low in extractable sulfates may not always

respond to direct applications of S in the field (Bremer, 1976; Jones

et al., 1979). A possible explanation for these observations is

direct absorption of SO2 from the atmosphere. Fried (1948) first

demonstrated that alfalfa plants could take in SO2 through the leaves

and convert it into organic S compounds. Olsen (1957) found that the

amount of SO2 absorbed by healthy cotton plants was a function of the

effective leaf surface. Healthy plants obtained about 30% of their S

from the atmosphere whereas S-deficient plants absorbed over 50% of

their S from the atmosphere. He concluded that while the SO2 concen-

tration of the atmosphere is inadequate as the sole source of S for

plants, the atmosphere could provide an important supplementary source

of S to growing plants.

Sulfur-deficient alfalfa plants exposed to a greenhouse atmosphere

during the winter months in Wisconsin were capable of absorbing as much

as 73% of their total S from the air (Hoeft et al., 1972). Total S

collected by PbO candles during the period the plants were grown was

1.05 mg S/100 cm2 for a candle located in the greenhouse and 10.57 mg

S/100 cm2 for a candle located outside the greenhouse. Air movement

around plants would be expected to increase SO2 absorption. Hill

(1971) found that absorption of SO2 by vegetation increased with wind

velocity above the plants, height of the canopy, and temperature.


2.3.1 Critical Concentrations

The S requirement of a plant has been defined as the ". .. mini-

mum uptake of this nutrient associated with maximum yield of dry matter"

(Stanford and Jordan, 1966, p. 258). The concentration of the nutrient

present in a plant when this condition is met is usually referred to as

the critical concentration or the critical percentage (Ulrich, 1952;

Thompson et al., 1970). As with any nutrient, the critical concentra-

tion of S in plant tissue can vary with species, the type of tissue

(leaves, stems, roots, grain, etc.), and the age of the tissue. Some

values for critical S concentrations in different crops are reported

in Table 6.

Since the protein contents of plants vary widely with species,

age, environment, and nutrition, the wide variability in S content of

plants is not surprising; the S requirement will depend on the amount

of S associated with plant protein. The protein content of the vege-

tative part of a plant decreases with age as a result of growth dilu-

tion of the protein by carbohydrates, hemicellulose, lignin, etc.

2.3.2 N:S Ratios

The relationship of S to plant protein and the corresponding

relationship of N to protein has led to the realization that the ratio

of N to S in the plant may be a more reliable measure of the S require-

ment than is the absolute level of S. Genetics determine the sequence

and number of amino acids in the polypeptide chains of a specific pro-

tein, but nutritional and environmental factors may influence the


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amounts and relative proportions of proteins formed (Thompson et al.,


The N:S ratio is relatively constant in plant protein. Dijkshoorn

et al. (1960) found an atomic ratio of S to N in the protein [(N:S)p]

of perennial ryegrass foliage (Lolium perenne L.) of 0.027. This is

equivalent to a (N:S)p of 16.2:1 on a percentage basis. Dijkshoorn and

Van Wijk (1967) reviewed the literature on the N and S relationships in

plants and found that the organic N:organic S ratio [(N:S)o] in plants

ranged from 13.7 for grasses to 17.5 for legumes. These values were

similar to those for the (N:S)p ratio, since proteins constitute about

80% of the organic N and S present. Jones et al. (1971) found that

the (N:S)p in the dried tops of the legume, Stylosanthes humilis L.,

decreased as the S supply increased, but most researchers found this

ratio in legumes to be fairly constant and within the ranges defined

by Dijkshoorn and Van Wijk (Stewart and Whitfield, 1965; Stewart, 1969;

Stewart and Porter, 1969). The (N:S)o in plants of the Brassica tribe

is lower because of the presence of other forms of organic S.

With adequate S fertilization, grasses tend to accumulate sulfate

in luxury amounts resulting in a very narrow total N:total S ratio

[(N:S)t]. If S is limiting, non-protein forms of N tend to accumulate,

and the (N:S)t ratio exceeds the (N:S)p ratio of about 14 normally

found in gramineous plants. Consequently, the (N:S)t ratio would

appear to be an effective index of the S status of many grasses

(Metson, 1973). Roberts and Koehler (1965) found a (N:S)t ratio of 25

in wheat that had not been fertilized with S and ratios of 11 to 15

where S was applied. Woodhouse (1969) regarded high (N:S)t ratios in

bermudagrass (Cynodon dactylon L.) as one of the factors resulting in

poor animal performance when this forage was fed. He considered values

of 12 to 17 to be normal. Bremer (1976) also found that the (N:S)t ra-

tios widened greatly in bermudagrass as S became deficient. Cowling and

Jones (1971) considered 20 the critical ratio for perennial ryegrass.

Terman et al. (1973) used the (N:S)t ratio at the minimum concen-

trations of N and S necessary for continued growth of corn to compute

a critical ratio of 16 for this crop. However, they concluded that

". .. the ratios appear to be meaningful only to indicate the relative

amounts of labile N and S present for assimilation by the crop" (Terman

et al., 1973, p. 636). The ratios were not closely related to the

corresponding dry matter yields. Daigger and Fox (1971) and Kang and

Osiname (1976) also failed to relate (N:S)t ratios to yields and S

responses in corn. Their poor correlations were associated with

late sampling of the ear leaf, low (N:S)t ratios, and variations

in the (N:S)t ratio with location.

2.3.3 Sulfate Sulfur

Sulfur in excess of that required for protein synthesis will

accumulate in the plant as inorganic sulfate (Rendig, 1956; Walker and

3entley, 1961; Jones, 1962, 1963; O'Connor and Vartha, 1969). Using

data from Walker and Bentley (1961), Dijkshoorn and Van Wijk (1967)

calculated that a yield response by legumes to applied sulfate could

be expected when the sulfate-S content of the dry matter was below

0.006 g-atoms per kg (0.0192% S as sulfate). A slow rate of redistri-

bution and the localization of metabolic consumption require that this

small amount of sulfate must be present within the plant and available

for metabolism. This sulfate concentration was proposed as a guide

for diagnosing S deficiency since the sulfate concentration in a plant

depends on the supply of mineral ions and varies to a much greater

extent than that of organic S.

The ability of the species to accumulate sulfate in excess of

metabolic requirements must also be considered. Grasses are vigorous

sulfate accumulators, whereas the legumes rarely accumulate high con-

centrations of sulfate (Metson, 1973). Dijkshoorn et al. (1960)

reported a critical level of sulfate in ryegrass herbage of 0.01

g-atoms S per kg dry weight (0.032%). Some critical sulfate-S levels

in selected crops from the literature are given in Table 6.

Metson (1973) compiled an excellent review of the literature

relating to total S, sulfate-S, and N:S ratios and the S fertility of

grasses and legumes. He found that sulfate-S varied in a similar

fashion to total S and concluded that the preferred diagnostic tech-

nique might be whichever property is the more conveniently determined.

Freney et al. (1978) studied total S, sulfate-S, (N:S)t, amide N,

and sulfate as a percentage of total S as indices for diagnosing the

S status of wheat in Australia. They found that each of the indices

was strongly related to dry matter yield. Critical, total S values

varied considerably with type of the tissue and age (Table 6).

Sulfate-S changed only slightly with age and N supply but failed to

provide any discrimination between plants suffering from different

degrees of S deficiency; only "deficient" and "adequate" categories

could be identified. The (N:S)t was most appropriate for young wheat

plants only. Amide N provided the greatest relative change in values

between S-deficient and S-adequate plants, but could be influenced by

the Zn and Fe supply to the plant. They concluded that sulfate-S ex-

pressed as a percentage of the total S provided the best correlation with

yield; this index was unaffected by plant age, N level, or plant part.

Their data indicated that wheat plants with more than 10% of their S

in sulfate form were adequately supplied with S.

Recently, Spencer and Freney (1980) reported a critical value for

sulfate S (as a percent of total S) of 13% for field-grown wheat. This

value was least affected by the age of the plant or N supply and was

recommended as a convenient index of the S fertility status of wheat.


2.4.1 Extraction Techniques

Soil testing for S to evaluate S available to growing crops is not

as developed nor as precise as it is for P, K, Ca, Mg, and even some of

the micronutrients. The complicated and relatively unknown nature of

soil S and the multiplicity of sources of S to plants makes the rapid

assessment of plant-available S even more difficult. We know that soil-

solution sulfate and adsorbed sulfate are readily available sources of

S. Other sources include soil organic matter, precipitation and irri-

gation water, atmospheric S, and fertilizers and pesticides. Therefore,

crop response to S is frequently as dependent upon management practices

and location as it is upon available soil S.

Excellent reviews of available techniques to assess the S fer-

tility status of soils have been written by Reisenauer et al. (1973)

and Beaton et al. (1968). Few new developments have been made in

recent years to improve analytical techniques for S determination or

the soil test calibration necessary to apply these techniques to crop

response in the field. Some soil S extractants that have been used are

listed in Table 7. Reisenauer et al. (1973) pointed out that no one

Table 7. Selected methods used to determine sulfate and extract-
able sulfur in soils (from Reisenauer et al., 1973).

Correlations of measured S critical level
Extractant with plant response for most crops

water (cold)




Ca(H2PO4 )2H20


NaOAc (pH 4.8)

Soluble Sulfates
0.98** with S uptake (Barrow,
0.94** with S uptake (Fox et al.,
0.73** with yield (Spencer &
Freney, 1960)
0.99** with S uptake (Barrow,
0.78** with S uptake (Williams &
Steinbergs, 1959)
0.36* with S uptake & 0.28* with
yield (Hoeft et al., 1973)
0.86 with S uptake (Roberts &
Koehler, 1968)

0.89 with S uptake (Roberts &
Koehler, 1968)

Soluble + Adsorbed Sulfates
0.93** with S uptake (Fox et al.,
0.83** with S uptake and 0.76**
with yield (Spencer & Freney,
0.95** with S uptake (Fox et al.,
0.37* with S uptake & 0.32* with
yield (Hoeft et al., 1973)
0.92 with S uptake (Barrow,
0.69** with S uptake & 0.60**
with yield (Spencer & Freney,
0.55** with yield (Bardsley &
Kilmer, 1963)
(Ensminger & Freney, 1966)

3.3-5.8 ppm S
(Bettany et
al., 1974)

6-10 ppm S

6-7 ppm S

Table 7. (Continued)

Correlations of measured S critical level
Extractant with plant response for most crops

Soluble + Adsorbed Sulfates + Portions of Organic S

Ca(H2PO4)2 H20
in 2N HOAc

NaH PO4 in 2N

water (hot)

"A" values

"L" values

0.39** with yield & 0.58** with
S uptake (Hoeft et al., 1973)
0.59** with yield (Bardsley &
Kilmer, 1963)
Suggested by Kilmer & Nearpass
(1960) & recommended by
Ensminger & Freney (1966) &
Bardsley & Lancaster (1965)
0.22 with yield and 0.42** with
S uptake (Hoeft et al., 1973)

Soluble + Labile Organic S
0.91 with S uptake & 0.93 with
yield (Spencer & Freney, 1960)
0.84** with S uptake (Fox et al.,
0.90** with S uptake (Williams &
Steinbergs, 1959)

Biological Methods
0.91 with S concentration & 0.96
with S uptake (Nearpass et
al., 1961)
Availability index increased
with successive harvests due
to net S mineralization
(Bettany et al., 1974)

*Significant at the 95% probability level.
**Significant at the 99% probability level.

6-10 ppm S

10 ppm S

15 ppm S

procedure has been consistently superior in predicting response to

applied S.

Most surface soils contain so little water-soluble S that fre-

quently poor correlations with S uptake are obtained with water

extracts, dilute, neutral-salt extracts, and weak-acid extracts which

contain no replacing anion. Water, especially hot water, may extract

a portion of the soil organic matter. This imparts a color to the

solution and interferes with the precipitation of sulfate. The

organic matter must be digested before sulfates can be determined tur-

bidimetrically, and digestion could increase the measured sulfate con-

centration of the extract. If only reducible S is determined (Johnson

and Nishita, 1952), then color and organic matter are no problem in

the extract. Neutral salts such as CaC12, MgCl2, and LiCI extract

less organic matter than water but are not effective in removing

adsorbed sulfate (Ensminger and Freney, 1966; Harward and Reisenauer,

1966; Roberts and Koehler, 1968).

Extractants which have gained confidence among researchers as the

most satisfactory indicesof plant S uptake and yield have been those

extractants which remove readily-soluble sulfates, portions of the

adsorbed sulfates, and possibly some organic S. Beaton et al. (1968)

pointed out that adsorbed sulfate is in kinetic equilibrium with the

sulfate in solution, and it may be replaced by other anions of greater

coordinating ability according to the lyotropic series:

hydroxyl > phosphate > sulfate = acetate > nitrate = chloride

Ensminger (1954) showed that adsorbed sulfate could be extracted

with a KH2PO4 solution containing 500 ppm P. Fox et al. (1964) used

Ca(H2PO4)2 because it gave similar values to KH2PO4 and also eliminated

the problem of turbid extracts. The Ca ions suppress organic matter

extraction by promoting flocculation, and the phosphate anions dis-

place adsorbed sulfate. Hoeft et al. (1973) also obtained significant

correlations with Ca(H2PO4)2 extracts and S uptake and yield of alfalfa.

A Ca(H2PO4)2 solution of 500 ppm P in 2 N acetic acid solution ex-

tracted an average of 4 ppm more S per sample and gave slightly higher

correlations than Ca(H2PO4)2 solutions alone; this was probably due to

the extraction of some plant-available, organic sulfates. Other

researchers have used Ca(H2PO4)2 extractions as effective indices of

available S (Barrow, 1967; Rehm and Caldwell, 1968). Sodium acetate

and ammonium acetate solutions have been effectively used as sulfate

extractants (Ensminger, 1954; Bardsley and Lancaster, 1960; Bartlett

and Neller, 1960; Bardsley and Kilmer, 1963; Jordan, 1964; Nelson,

1964a, 1964b; Rehm and Caldwell, 1968). However, acetate is not a

strong replacer of sulfate in neutral solutions (Beaton et al., 1968).

Acidic solutions would not be expected to increase sulfate extrac-

tion because the adsorption of sulfate by soils increases with decreas-

ing pH (Ensminger and Freney, 1966). Spencer and Freney (1960) com-

pared a number of extraction methods and found that the amount of

sulfate extracted from soils increased in the following order:

acetate < cold-water < phosphate < hot-water

Bardsley and Lancaster (1965) reported that the alkaline extrac-

tion of soils with NaHCO3 removed more S than is obtained with acetate

extractants. Sulfur extracted from 30 soils with :aHCO3 atpH 8.5

correlated well (r = 0.89) with S "A" values. Plants with less than

10 ppm extractable S responded to applications of S. This extractant

was effective in solubilizing and replacing anions as well as some

organic fractions. The Johnson and Nishita (1952) reduction technique

and S determination as methylene blue must be used with alkaline

extracts. Color due to organic matter and cations does not interfere

with this method as it may with BaSO4precipitation in the turbidimetric

determination of sulfate (Beaton et al., 1968).

2.4.2 Biological Techniques

Several biological methods have been used for measuring available
S in soils. These include "A" and "L" values where radioactive 3S is

used to determine availability of soil sulfate (Nearpass et al., 1961;

Bettany et al., 1974). "A" values were developed for measuring avail-

able macronutrients, particularly P (Fried and Dean, 1952), but have
been adapted for S measurements. A known amount of 3S is applied to a
soil and the subsequent proportion of 35S in the plant allows calcula-

tion of an "A" (availability) value. Nearpass et al. (1961) found "A"

values from 9.8 to 42 ppm in 30 soils from the southeastern United

States by growing cotton plants at six levels of applied S. These

values were significantly correlated with S concentration in the

plants (r = 0.91) and S uptake (r = 0.96).

Bettany et al. (1974) used a modification of the "A" value which

they called the "L" (labile) value (Fried, 1964; Larsen, 1967). The

following equation was used to calculate the availability index:

35 35 32 32
L = Sadded to soil ( Splant Sseed) Sadded to soil

They found that "L" values increased with subsequent harvests of

alfalfa and concluded that this increase in the availability index was

a direct result of isotopic dilution of the added S due to mineraliza-

tion of native soil S. Labile S apparently changes with time as a

result of mineralization of organic matter; this change could explain

some of the variable correlations obtained with chemical extractants.

Robertson and Yuan (1973) studied S availability on two Florida

soils using the "relative specific activity" (RSA) ratios of 35S in

plant tissue at two rates of S fertilization. Their calculations were

similar to S "A" values. Increased S availability would be indicated

by a narrower ratio of RSA at two rates of S. They found a ratio of

2.6 in soybean tissue for an Orangeburg fine sandy loam (Typic

Paleudult) and a ratio of 2.9 for a Lakeland fine sand (Typic

Quartzipsamment). A significant yield increase with soybeans was ob-

served in the greenhouse on the Lakeland soil when 45 ppm S was

applied. No response was observed at the low S rate (15 ppm S).

Plants did not respond to applied S on the Orangeburg soil.

Beaton et al. (1968) list several other "biological" methods

which have been used to evaluate available soil S. Since all of these

involve either a bioassay or incubation procedure, they are time-

consuming and are not conveniently adapted to most soil chemistry

laboratories. These techniques are listed below:

"a" value obtained by extrapolation of yield of nutrient
curves, and is closely related to "A" values.

algae growth of algae indicates soil S status.

Aspergillus -rowth of Aspergillus niger indicates soil S

incubation soil incubated to measure biological conver-
sion of organic S to inorganic sulfate.

Neubauer seedlings of spring barley used to extract
soil S.

respirometer degree of S deficiency assessed by comparing
respiration curves with and without applied S.

short-term uptake intense, short-term extraction by root pods
of turnips or wheat.


Most of the research with S as a plant nutrient occurred during

the 1950's and 1960's as is evident by the citations in this paper.

An important contribution to this research was by scientists studying

crops and soils in the southeastern United States (Ensminger, 1954;

Neller, 1959; Kilmer and Nearpass, 1960; Chang and Thomas, 1963;

Jordan, 1964; Nelson, 1964a). By 1971, S deficiencies had been identi-

fied or suspected in every state in the Southeast (Beaton et al.,

1971). Most of the S-deficient crops were on the sandy soils of the

Coastal Plain. The Coastal Plain includes all of Florida, but only

the coarse-textured soils of north and central Florida are expected

to be low in S.

Some of the earliest reports of S deficiency in crops in Florida

were in the 1940's and early 1950's by Harris, Bledsoe, and coworkers

(Harris et al., 1945, 1954; Bledsoe and Blaser, 1947). Harris et al.

(1945) achieved a highly significant response to S by cotton on an

Arredondo loamy fine sand (Grossarenic Paleudult). Two-week-old cotton

seedlings were severely stunted and yellow where no S was applied and

produced 70% of the dry matter of S-fertilized plants. At 6-weeks,

the deficient plants produced less than 20% of the dry matter of S-

fertilized plants. They also observed a response to S on a Norfolk

fine sand (Typic Paleudult) and suggested that S deficiency could occur

in wide areas of Florida.

Bledsoe and Blazer (1947) found that red and black medic clovers

(Medicago sp.) were highly responsive to S fertilization on a virgin

Leon fine sand. Pensacola bahiagrass showed little or no response to S.

Ordinary superphosphate was shown to be an effective source of plant-

available S.

In 1954, Harris et al. found that corn plants in a greenhouse

study produced four times more dry matter when S was applied to an

Arredondo loamy fine sand as compared to treatments without S.

Volk and Bell (1945) conducted a study in lysimeters containing a

Norfolk loamy fine sand and found that sulfate leaching from gypsum in

bands was only 20 to 30% of that from gypsum applied broadcast.

In 1951, Neller et al. (1951a, 1951b) reported significant

responses to added S by clover and clover-grass mixtures on a Rutledge

fine sand (Typic Humaquept) in northern Florida, on an Immokalee fine

sand (Arenic Haplaquod) in south-central Florida, and on Carnegie and

Tifton fine sandy loams (Plinthic Paleudults) in west Florida. Clover

failed to grow at all on the Immokalee fine sand where the fertilizer

did not contain a source of S. Water-soluble sulfate in the surface

soil was less than 1 ppm S where no S was applied and 7 to 9 ppm S

where superphosphate or gypsum had been applied. The Ultisols of west

Florida failed to produce a response to added S as dramatically as the

soils of the Peninsula. Improved growth of the legume during the

cooler months due to S fertilization resulted in higher yields and

higher protein content of bahiagrass (Paspalum notatum Flugge) during

the summer months.

Neller (1956) also studied S as it affected the availability of

other nutrients. Plants on most of the soils of Florida which


responded to S also responded to P; he found that elemental S increased

the availability of rock phosphate and supplied an essential nutrient

to oats and clover on a Leon fine sane (Aeric Haplaquod).

Ozaki conducted an extensive greenhouse experiment with seven

soils from fields in north and west Florida to evaluate the value and

source of S as a nutrient (Ozaki, C. T. 1950. Sulfur fertilization of

Florida soils. Master's thesis. University of Florida. Gainesville,

Florida). He made the following conclusions from his study:

1. Oats and clover responded to S on all soils studied. The
Ultisols (Ruston and Greenville fine sandy loams) did not
produce a definite response to S until the second cutting.

2. Gypsum applications above 16 kg/ha of S as gypsum did not
produce further increases in yield.

3. All S applications increased S uptake by oats and clover.

4. S-deficient oats had a higher concentration of N than those
plants with adequate S while S-deficient clover had a much
lower concentration of N.

5. All sources of applied S were equally effective in improving
the yield of oats and clover (gypsum, elemental S, normal
superphosphate, ammonium sulfate, and potassium sulfate).

6. Peanuts responded to S applications at pegging time but did
not respond to Ca.

In 1955, Bartlett, working with Neller, concluded a more extensive

study of soil S and plant nutrition in Florida (Bartlett, F. D. 1955.

Nature and distribution of sulfur in five soil profiles correlated with

plant responses. Ph.D. Dissertation. University of Florida. Gaines-

ville, Florida). He found about 10% of the total S in Florida soils

was extractable with a buffered acetic acid solution; this S correlated

well with S uptake by six forage and field crops in a greenhouse experi-

ment. Leon (Aeric Haplaquod), Blanton (Grossarenic Paleudult), and

Gainesville (Typic Quartzipsamment) series were the most S-deficient

soils studied and produced increased yields of grasses and legumes an

average of 1100 kg/ha when S was added. A Red Bay fine sandy loam

(Rhodic Paleudult) from west Florida did not produce a S response by

the crops studied, but the addition of S did cause higher S concentra-

tions in the plant tissue. The addition of S to all soils at the rate

of 34 kg/ha increased the S concentration of the plants by 0.10 of a

percent. Critical levels of S were established for several crops.

These are listed in Table 6.

Neller's paper on the increase in extractable sulfate with an

increase in the clay content in the profile of a number of Florida

soils (Neller, 1959), led to the realization that some deep-rooted

crops may not respond to S on low-S soils because adequate S is avail-

able from deeper soil horizons. Researchers working with Ultisols in

other areas of the southeastern United States have made similar con-

clusions (Ensminger, 1958; Jordan, 1964; Anderson and Futral, 1966;

Murdock and Lund, 1979).

Recently, Mitchell and Gallaher (1980) observed S-deficient,

seedling, field corn on an Arredondo fine sand in north Florida. They

were unable to increase yields by foliar and soil S applications

although S concentrations were increased in the plant tissues by the S

treatments. All plants had grown out of the S-deficient conditions by

55 days; they assumed that plant roots were able to reach adsorbed S

associated with the argillic horizon at 60 to 80 cm. Soil in the Bt

horizon contained 5 to 8 times more S than soil in the 0 to 60-cm


Most of the research with S on Florida soils was done during the

1940's and 1950's. Since that time there have been few positive

reports of S deficiencies on crops within the state. Although these

workers clearly demonstrated the need for S on crops on most of

Florida's sandy soils, little attention has been devoted to this nutri-

ent. Less S is applied through fertilizers, but more is reaching the

soils through precipitation and direct SO2 adsorption. Yields of most

field crops have doubled, and in some cases, tripled since the 1950's.

New, higher-yielding varieties and even some new crops have been

introduced to Florida farms. Management practices have changed dra--

matically, and fertilization is more intense. A re-evaluation is

needed of the S fertility status of Florida soils to determine if crop

yields can be improved by S fertilization during the 1980's, and to

identify the sources and availability of soil S in Florida.




Soil series which represented some of the major soils of Florida

and which are important in crop production were selected. These were

grouped into the orders Ultisols, Entisols, and Spodosols. The county

where profile samples were taken and the soil order are shown in

Fig. 4.

Pedon samples were collected by personnel of the Soil Characteri-

zation Laboratory of the University of Florida. Samples were taken

from each horizon or subhorizon and prepared for physical, chemical,

and mineralogical analyses for characterization purposes (Calhoun et

al., 1974; Carlisle et al., 1978). After all characterization data

were collected, the air-dried, screened samples were used for extract-

able sulfate, total S, total C, and total N determinations by the

author. Ten Ultisols, ten Entisols, and nine Spodosols were selected

and a total of 174 profile samples were studied. The parameters were

compared and correlated with chemical and mineralogical data collected

by the Soil Characterization Laboratory on these same samples. Chemi-

cal methods used for analyses will be discussed in sections 5 and 6.


A greenhouse experiment was designed to evaluate the contribution

of subsurface horizons to the S nutrition of plants. The experiment


U = Ultisol
E = Entisol
S = Spodosol
(Each letter represents one soil pedon)

*" fl

Fig. 4. Location of soil pedons where samples were collected for a
survey of sulfur in Florida soils.

involved four soil series, two horizon sequences nested within each

series, and two S rates within each horizon sequence. Each treatment

was replicated four times. Four soils which represented a Spodosol, an

Entisol, and two Ultisols were collected from unfertilized sites near

cultivated fields or improved pastures. The four soils and their loca-

tions are listed below:

Series Pedon location

Myakka fine sand Beef Research Unit,
(Aeric Haplaquod, sandy Alachua Co.
siliceous, hyperthermic)

Lakeland fine sand 16 km SW of Williston,
(Typic Quartzipsamment, Levy Co.
thermic, coated)

Orangeburg fine sand A.R.E.C., Quincy,
(Typic Paleudult, fine-loamy, Gadsden Co.
siliceous, thermic)

Norfolk fine sand A.R.E.C., Quincy,
(Typic Paleudult, fine-loamy, Gadsden Co.
siliceous, thermic)

Bulk samples of soil were collected from the surface 0 to 20 cm

and from the upper 0 to 20 cm of the argillic and spodic horizon of the

Orangeburg, Norfolk, and Myakka soils. Soil was collected from the sur-

face 0 to 20 cm and from the upper 0 to 20 cm of the C horizon in the

Lakeland soil. Soil from each location and profile depth was air dried

and screened twice through a 4-mm stainless steel screen and once

through a 2-mm screen. Samples were collected for mechanical, chemical,

and mineralogical analysis.

Mechanical analysis was by the Bouyoucos (1962) hydrometer method.

The less than 2 um clay-size fraction from each horizon was separated

and prepared for X-ray diffraction for mineral identification by the

method outlined by Jackson (1969). Citrate-dithionite soluble Fe and


Al was determined on samples from every soil and horizon. Sodium pyro-

phosphate extractable Fe and Al were determined only on the Myakka

soil (Jackson, 1969).

A pH-buffer curve was determined on soil from the surface horizon

by adding increasing increments of CaO to given weights of soil. The

limed samples were moistened with demineralized water, mixed well, and

allowed to air dry. This was repeated three times during a two-week

period. At the end of 2 weeks, the soil pH was determined in a 1:1

soil:water ratio. The soil pH-buffer curves of the four soils are

shown in Fig. 5.

A mixture of CaO and MgO (1.4:1 ratio) was added to raise the soil

pH of the surface horizons to near 7.0. Lime was mixed with the bulk

soil in a mechanical, cement mixer. The soil was moistened with dis-

tilled water, allowed to equilibrate in plastic bags for 1 week, air

dried, and screened again. A basic N, P, K, and micronutrient mix was

added to all surface soils (Table 8). Two rates of S, 0 and 20 ppm S,

were applied as reagent-grade CaSO 42H20, and the soil was mixed

again. The S treatment was inadvertently omitted from the Lakeland

soil. The error was noticed after the first harvest of plants in the

greenhouse, and the appropriate rate of S as CaSO 42H20 was added in


Polyvinyl chloride (PVC) pipe with a diameter of 15.2 cm (6 in.)

was cut into 50-cm sections for use as containers for the bioassay.

The pipe was thoroughly cleaned and rinsed with distilled water. A

plastic bag, perforated at the bottom for drainage, was fitted over

the lower end of each pipe section. Each section was placed in a

20.3-cm (8 in.) plastic saucer for support and drainage.

~L r

H B~O z:~I o
oxo D,

00 .


9 -0
* u P





Table 8. Nutrients applied to surface soils used in a
greenhouse evaluation of subsoil sulfur.

Basic rate of nutrient
Myakka & Orangeburg
Nutrient Lakeland & Norfolk
applied Source soils soils

N NH NO3 50.0 50.0

P Ca(H2PO )2*H20 50.0 250.0

K KC1 50.0 50.0

Cu Cu(NO3)2*3H20 2.0 2.0

Fe FeC12 4H20 5.0 5.0

Mn MnC2* 4H20 2.0 2.0

Zn ZnO 5.0 5.0

B H3BO4 0.25 0.25
3 4

The two horizon sequences were established by placing either

washed, quartz sand or the dried, screened, subsurface soil in the

bottom of the pipe. Distilled water was added to settle the soil and

moisten it to near field capacity (1/3 bar suction). The amount of

soil or sand added was predetermined so that a final depth of approxi-

mately 24 cm was achieved. The moistened soil was allowed to

equilibrate for 3 days to ensure uniform moisture distribution.

The treated, surface soil was weighed and placed above the sub-

surface layer so that the final depth, after wetting, would also be

approximately 24 cm. Distilled water was added to moisten the surface

soil slightly beyond field capacity. The soil-filled tubes were

arranged on a center, greenhouse bench and randomized within soil

series. A schematic of one experimental unit is shown in Fig. 6.

After 1 week of equilibration, all tubes were planted on 5 Febru-

ary 1980 to a sorghum x sudangrass hybrid (Sorghum sudanese (Piper)

stapf 'Dekalb SX16A'). After emergence, seedlings were thinned to

allow eight per pot (tube) to grow. Two weeks after emergence, all

pots were fertilized with 25 ppm N and 25 ppm K as NH4NO3 and KC1

based on the weight of the surface soil. Plants were watered as needed

with distilled water in an attempt to maintain a reasonably uniform

moisture level throughout the tube. Greenhouse temperatures were ad-

justed for a maximum of 300C and a minimum of 16 C.

Plants were harvested every 4 weeks for a total of 4 harvests.

Plants were clipped 7.5 cm above the soil line, placed in paper bags,

and dried in a forced-air oven at 700C. Samples were then weighed,

ground to pass a 1-mm screen, ashed, and analyzed for N and S. Details

of tissue analyses will be given in a later section. Relative dry



z -

surface soil

15.2cm(6-inch) PVC pipe

subsurtce soil or washed sand

perforated plastic bag
e- plastic tray

Fig. 6. Schematic of one experimental unit in the greenhouse
evaluation of subsoil S in Florida soils.



matter yields were calculated within each harvest and soil series. The

highest yielding treatment was assigned a relative yield of 100%.

All pots were fertilized with 50 ppm N and 50 ppm K after the

first, second, and third harvests. After the third harvest, 50 ppm P

as Ca(H2PO4)2*2H20 and 1/2 of the original micronutrient rate were

applied to all pots.

An infestation of red spider mites warranted spraying with

Kelthane(TM) after the second harvest. The chemical spray caused

severe "burning" of the foliage of most plants. The plants recovered

rapidly from the damage, and yields from the third harvest were not


After the fourth harvest, soil was washed from the roots with tap

water; the roots were rinsed in distilled water, dried, and weighed.


In the spring of 1978, two experiments were begun to study the

response of bahiagrass (Paspalum notatum Flugge 'Pensacola') and ber-

mudagrass (Cynodon dactylon L. 'Coastcross-l') to S fertilization in

the field. The bahiagrass experiment was located at the Beef Research

Unit, approximately 16 km northeast of Gainesville, on a Myakka fine

sand; the bermudagrass experiment was located at the Green Acres

Agronomy Farm, approximately 16 km west of Gainesville, on a Kendrick

fine sand (loamy, siliceous, hyperthermic, Arenic Paleudult). The

experiments were located in established fields of bahiagrass and

bermudagrass which had not been fertilized for several years.

Dolomitic limestone was applied at the rate of 4,500 kg/ha

1 month before initial fertilizer treatments were begun. Phosphorus

and K were applied uniformly to all plots in four applications during


the season at an annual rate of 49 kg/ha P (112 kg/ha P205) and 186 kg/

ha K (224 kg/ha K20). The source of P and K was an 0-10-20 grade

fertilizer mixture composed of concentrated superphosphate and muriate

of potash. Micronutrients were applied each spring at 34 kg/ha of

Fritted Trace Elements (F.T.E.) no. 503.1

The experimental design at both sites was a randomized block with

nine treatments and four replications. Plot size was 2.44 x 4.87 m

(.00119 ha). Two annual rates of N--200 and 400 kg/ha--and four annual

rates of S--0, 10, 20, and 40 kg/ha--were applied. Nitrogen and S were

applied as agricultural grade NH NO3 and CaSO4 2H20, respectively. The

N was applied after each harvest in four split applications during the

season. All of the S was applied in the spring before growth began

except for the ninth treatment. This treatment included four split

applications of S at an annual rate of 20 kg/ha and 400 kg/ha of N.

Plots were harvested four times during the growing season:

(1) the second week in May, (2) the last week in June, (3) the first

week in August, and (4) the last week in September. A 1-m strip the

length of the plot (4.87 m) was harvested with a Gravely tractor with a

sicklebar mower. The forage was weighed, and a 300 to 500-g subsample

was removed for moisture determination and chemical analysis. This

sample was weighed, dried in a forced-air oven at 700C for several

days, weighed again, and double-ground to pass a 1-mm sieve. Total N

and S were determined on the tissue by methods described in the next


1Frit Industries, Inc., Ozark, Alabama.

Soil samples were taken at the end of each growing season from

each plot. Samples were taken at depths of 0 to 15 cm and 15 to 30 cm.

Samples were also taken at greater profile depths from those plots

receiving 0 and 40 kg/ha S. These samples were air dried and screened

through a 2-mm sieve. Sulfate S was extracted with a 0.01 M

Ca(H2PO4)2'2HD solution and determined either turbidimetrically

(Massoumi and Cornfield, 1963; Chaudry and Cornfield, 1966) or by

indirect barium (Ba) absorption spectroscopy (Hue and Adams, 1979). An

estimate of total S was made on samples from 1978.

Stolon-root samples of bahiagrass were collected in the fall of

1979 from the 0 and 40 kg/ha S plots at the Beef Research Unit. These

were carefully washed, rinsed three times with distilled water, dried,

weighed, and ground. Total N and S were determined on the tissue.



5.1.1 Total Nitrogen in Plant Tissue

Plant tissue for N determination was dried and ground twice to

pass a 1-mm screen. Digestion was in an aluminum digestion block simi-

lar to the one described by Gallaher et al. (1975). The reagents and

procedure were from Nelson and Sommers (1973). A 0.2 g sample of tis-

sue was weighed into pyrex tubes. A 1.l-g scoop of a K SO -CuSO -Se

salt-catalyst mixture and 4 ml of concentrated H2SO4 were added to each

sample. A small glass funnel was placed in the top of tubes to ensure

efficient refluxing of the sample. Snploas were placed on a preheated

digestion block at 3750C and digested for 1.5 hours afcer the solution

cleared. The di-e.3ted sample was cooled and quantitatively transferred

to micro-Kjeldahl flasks with demineralized water. Ammonium in the

digested sample was then determined by a conventional, semi-micro

Kjeldahl distillation procedure.

5.1.2 Total Nitrogen in Soils

Total N in soils was determined by the same method as total N in

plant tissue. Soil samples were finely ground for 5 minutes using a

mechanical mortar and pestle. A 0.2-g sample was weighed and digested

on an aluminum digestion block for 3 hours after the solution cleared.

Ammonium was determined by a semi-micro Kjeldahl procedure.


5.2.1 Tissue Digestion for Total Sulfur

Plant tissue was digested using a Mg(NO3) /HNO3 solution on a hot

plate followed by heating in a muffle furnace at 5000C for 2 hours.

The procedure is a slight modification of the methods of Butters and

Chenery (1959) and Chaudry and Cornfield (1966) and is similar to the

digestion procedure for the gravimetric determination of S as given by

the Association of Official Analytical Chemists (1970). A preliminary

study indicated that this method resulted in more complete digestion

of the tissue and gave comparable recovery of added S when compared to

the HNO 3/HCIO4 wet digestion technique described by Beaton et al.

(1968). Sulfate S was determined on the digested tissue by the

turbidimetric procedure of Massoumi and Cornfield (1963) and Chaudry

and Cornfield (1966) and will be given in a later section. The

digestion procedure is given in Appendix A.

5.2.2 Soil Extraction for Sulfate Sulfur

Extractable sulfate S was determined on air-dried, screened soils

by extracting 10 g of soil with 25 ml of a 0.01 M Ca(H2PO4)2*2H20

solution. Soil and extractant were placed in 100-ml bottles and

shaken for 30 minutes on a reciprocating, mechanical shaker. All ex-

tracts were filtered through Whatman no. 42 filter paper. A 5 or 10-ml

aliquot was used for the turbidimetric determination of sulfate S.

Values reported for the study of S distribution in selected Florida

soils were obtained by the indirect method of Hue and Adams (1979) and

a 10-ml aliquot was used. All other extractable sulfate-S values were

determined turbidimetrically (Massoumi and Cornfield, 1963; Chaudry and

Cornfield, 1966).

Some difficulties were encountered with the turbidimetric deter-

mination of sulfate S in soil extracts. These difficulties, along with

the justification for using the Ca(H2PO4)2.2H20 extractant, and a pre-

liminary study of extractants are discussed in section 5.2.3.

5.2.3 A Comparison of Two Extraction
Procedures for Soil Sulfur

A suitable technique for extracting and determining sulfate S in

soils should be convenient to use with the laboratory facilities avail-

able; the procedure should be rapid and reproducible. The extractant

should remove a minimum of soil organic matter which could interfere

with sulfate determination. Extractable S must also be correlated with

plant uptake and/or yield. A study of the available extractants indi-

cated that a Ca(H2PO4)22H2 0 solution (Fox et al., 19643 Beaton et al.,

1968) or an NH OAc + HOAc solution (Bardsley and Lancaster, 1960, 1965)

would be most suitable for this investigation. However, preliminary

studies showed that both of these solutions alone would extract some

color from a few Florida soils. This color was difficult to remove

with the HNO3/HC104 digestion described by Beaton et al. (1968).

Bardsley and Lancaster (1960, 1965) used activated charcoal to remove

color from their extracts before determining S turbidimetrically.

A 0.01 M Ca(H2PO4)2-2H20 extractant (approximately 500 ppm P) and

the NH4OAc+HOAc extractant of Bardsley and Lancaster (39 g NH OAc in

1 liter of 0.25 N HOAc) were compared. Both extractants were studied

with and without activated charcoal. Since most of Florida soils are

low in extractable S, a narrow soil:solution ratio was used (Bardsley

and Lancaster, 1960). Ten grams of air-dried, screened soil were

shaken for 30 minutes with 25 ml of the extractant in 100-ml extraction

bottles on a reciprocating, mechanical shaker. Where charcoal was

used, approximately 0.25 g of "Darco G-60" activated charcoal was added

after the 30-minute shaking period using a calibrated scoop. The sam-

ples were reshaken for 3 minutes. All extracts were filtered through

Whatman no. 42 filter paper; sulfate-S was determined turbidimetri-

cally. Ten milliliters of the extract were used for sulfate determina-

tion. An equal volume of the extracting solution was added to all

standards and blanks. Charcoal "blanks" were run to determine any

sulfate-S extracted from the charcoal.

Where no charcoal was used and color appeared in the extract,

duplicate samples were read on the spectrophotometer when determining

S by turbidimetry. No BaC12 or BaSO4 seed suspension was added to one

of the samples. Absorbance was read on both samples, and the absor-

bance due to color in the extract was subtracted from the absorbance

of the sample with BaC12 added. From this value, S in the extract was


Air-dried, screened soil from a Myakka fine sand and from a

Kendrick fine sand at 0 to 15 cm and 15 to 30 cm depths was' used for


extraction. Three rates of S--O, 4, and 8 ppm--were added to each soil.

The soil was moistened and allowed to dry and screened before extrac-

tion. The results are given in Table 9. All values are the mean of

two determinations.

None of the methods were comparable in the amounts of S extracted

from the soils. The 0.01 M Ca(H2PO4)2 2H20 without the charcoal

(Method A) produced the most consistent results. Considerable amounts

of S were extracted from the charcoal (Methods B and D). The amount of

S in the charcoal blank was subtracted from that in the sample. The

amount of S was quite variable in the charcoal blanks, and this vari-

ability probably accounted for the differences in the samples extracted

using charcoal. The NH OAc+HOAc extractant (Method C) removed slightly

less sulfate-S than the Ca(H2PO4)2 2H20 extractant since acetate is not

as effective in replacing sulfate as the phosphate anion. The decision

was made to use the 0.01 M Ca(H2PO 4)22H 0 extractant. In those sam-

ples where color remained in the extract, a blank was run when S was

determined turbidimetrically.

The 1979 paper by Hue and Adams on the indirect method of sulfate

determination on soil extracts offered another alternative for sulfate

determination on colored extracts. Sulfate sulfur in the 174 soil

extracts in the study of S distribution in Florida soils was determined

by this method and is discussed in section 6.2.

5.2.4 Estimation of Total Sulfur in Soils

The Mg(JO3)2/HNO3 procedure used for plant tissue digestion was

used for estimating total S in soils. Some modifications were needed

and these are described in Appendix B. Some preliminary studies and

justification for using this technique are described below.

Table 9. Sulfate sulfur removed by four extraction methods
and percent recovery of added sulfur.

Soil Added Extraction methodtt
series Depth S A B C D
--cm- -ppm- ----------------ppm------------------

Myakka 0-15 0 3 5 2 2
S" 4 7(100) 8(75) 2(0) 10(200)
8 10(88) 11(75) 8(75) 12(125)
15-30 0 4 6 2 2
S" 4 7(75) 8(50) 2(0) 8(150)
S" 8 11(88) 13(88) 8(75) 15(162)
Kendrick 0-15 0 3 8 2 2
4 6(75) 9(25) 5(75) 8(150)
S" 8 10(88) 12(50) 8(75) 12(125)
S15-30 0 4 7 2 7
4 9(125) 10(75) 10(200) 15(200)
S" 8 16(150) 16(112) 16(175) 19(150)

A = Ca(H2PO4) 22H2O; B

= Ca(H2PO)2- H20 + charcoal; C =

NH OAc+HOAc; D = NH OAc+HOAc + charcoal.

Figures in parentheses are percent recovery of added S.

In 1968, Beaton et al. listed more than 36 different methods for

determining total S in soils. Most of these methods involved acid

digestion or alkaline fusion of the soil to oxidize all reduced forms

of organic and inorganic S to sulfate, extraction of the residue, and

sulfate determination by BaSO4 precipitation gravimetricc, turbidi-

metric, or titrimetric methods). Of the various acid treatments,

HC104 and a mixture of HNO3+HC104 have been the most popular and con-

venient to use. These digestion procedures are not as tedious as

sodium carbonate-sodium peroxide fusion techniques, but they may not

decompose all the soil minerals in some soils. Since most of the

soil S in the surface horizons of Florida soils is considered to be

associated with soil organic matter, any technique that effectively

digests organic matter and removes sulfate-S should be suitable for

the estimation of total S in soils. The use of HC104 can be dangerous

without proper ventilation. The required ventilation was not avail-

able in the Analytical Research Laboratory where most of these analy-

ses were conducted; therefore, the Mg(NO3)2/HNO3 digestion/oxidation

procedure was adopted.

Soils which were only air dried and screened before digesting may

not completely react with the digesting solution. Incomplete recovery

of total S may result. Nine soil samples were finely ground with an

agate mortar and pestle and digested according to the above procedure.

Sulfate-S was determined turbidimetrically and compared to the results

of soil samples that had been screened through a 2-mm sieve. rnese

data are presented in Table 10. Grinding these samples did not seem

to affect the amount of S extracted after digestion. Therefore, subse-

quent soil samples were only air dried and screened.

Table 10.

Sulfur content of finely ground soil and
screened soil using the MgNO3/HNO3
digestion procedure.

Sulfur content
Soil Finely-ground Screened
identification soil soil

1 60 1 64 1
2 25 1 25 0
3 17 1 17 1
4 46 3 44 3
5 73 7 91 9
6 29 1 30 2
7 26 0 26 2
8 92 8 83 2
9 81 + 7 80 + 6

Values are

the mean of 3 analyses.

The Mg(NO3)2/HNO3 digestion procedure was compared to the diges-

tion procedure of Bardsley and Lancaster (1960) and to total S esti-

mation using a Leco Sulfur Analyzer.2

Bardsley and Lancaster mixed finely ground soil with NaHCO3 in a

porcelain crucible and ignited the mixture at 500 C for 3 hours in a

muffle furnace. The oxidized mixture was then extracted with NaH2PO '
2 4
H 0 in 2 N HOAc. Sulfate-S was determined turbidimetrically.

The Leco Sulfur Analyzer used in this study ignites a soil (or

plant) sample in a high frequency induction furnace. A 0.5-g sample of

finely-ground soil was treated in a ceramic crucible with Fe, Sn, and

Cu accelerators. The crucible was covered with a porous cap and heated

to a very high temperature (ca. 1,6000C) in a stream of oxygen. Sulfur

is converted to gaseous SO2 which is trapped in dilute HC1 containing

KI, starch, and a trace amount of KIO3 solution. The reactions

involved are indicated below (Bremner and Tabatabai, 1971):

KIO3 + 5KI = 6HC1 312 + 6KC1 + 3H20

SO2 + I1 + 2H20 HSO4 + 2HI .
2 2 2 2 4

The results of these comparisons are presented in Table 11. The

soils studied and treatments were the same as those used in the com-

parison of different extraction methods (Table 9).

The Leco method was unsatisfactory for the determination of total

S at the levels present in these soils. Tabatabai and Bremner (1970b)

and Bremner and Tabatabai (1971) also experienced poor recovery of

soil S and low precision using this technique for mineral soils. They

Laboratory Equipment Corp., St. Joseph, Michigan.

Table 11.

A comparison of three rapid methods of estimating total
sulfur in soils.

Total S
NaHCO3 digestion
S Mg(N03)2/HN03 (Bardsley &
Soil added digestion Lancaster, 1960) Leco

Myakka A 0 65 68 90
4 70 68 110
8 70 68 100

Myakka B 0 58 52 90
4 60 53 60
8 66 56 100

Kendrick A 0 68 57 100
4 69 61 110
8 74 61 110

Kendrick B 0 24 <20 30
4 26 <20 <15
8 30 <20 15

reviewed some of the problems with the Leco method and suggested that

the problems may be related to (1) the size of sample and the mesh-

size, (2) the amount and type of combustion accelerator, (3) the Method

of instrument calibration, and (4) the incomplete sample combustion

or the release of SO3 as well as SO2. They concluded that the Leco

method was ". .. unsatisfactory for research requiring accurate and

precise determination of total sulfur in soils" (Tabatabai and Bremner,

1970b, p. 419). However, the Leco method has proven to be of value in

plant tissue analysis (Jones and Issac, 1972; F. Adams, personal com-


The Mg(NO3)2/HNO3 digestion resulted in slightly higher levels of

S than the Bardsley and Lancaster method. This was probably due to

more effective digestion of the soil minerals because of the prediges-

tion step on the hotplate. The better contact of the soil particles

with the Mg(NO3)2/HNO3 may have resulted in less loss of S through

volatilization. Therefore, the Mg(NO3)2/HNO3 digestion procedure was

adopted as a suitable alternative to either HC10 /HNO3 digestion or

NaHCO3 digestion with acid extraction.


Sulfate S in soil extracts, in digested-soil extracts, and in

digested plant tissue was determined by a turbidimetric procedure

(Massoumi and Cornfield, 1963; Chaudry and Cornfield, 1966) or by the

indirect method of Hue and Adams (1979). Because of difficulties

associated with S analyses, these techniques and others are discussed


Most analytical techniques for determining S first involve the

conversion of various forms of S to the sulfate ion and quantitatively

estimating S as sulfate. As previously mentioned, gravimetry, turbi-

dimetry, titrimetry, and colorimetry are the most common methods for

measuring sulfate in digested plant tissue, soil extracts, digested

soil samples, and other aqueous solutions containing sulfate-S. In

almost all of these techniques, the sulfate anion reacts with the Ba

cation in solution to form insoluble BaSO An extensive review of

some of these techniques has been made by Beaton et al. (1968).

Reduction of S to H2S and determination of S as methylene blue

has generally been accepted as the most satisfactory method for the

colorimetric estimation of traces of S in soils, soil extracts, plant

tissue, and tissue extracts (Johnson and Nishita, 1952; Steinbergs et

al., 1962; Tabatabai and Bremner, 1970a, 1970b). This procedure was

developed by Johnson and Nashita (1952) for soil and plant tissue

analysis and has been reviewed by Beaton et al. (1968). This method

is very sensitive but quite tedious. Often very small samples are

necessary which may lead to sampling difficulties and erroneous

results. Aliquots larger than 2 ml cannot be used. A heat source and

S-free N are needed for the digestion-distillation apparatus. Each

sample must be boiled and refluxed for an hour to reduce all S to H S.

The large number of samples handled in this study could not be accomo-

dated using the Johnson and Nashita method because of the time involved

and the lack of necessary equipment. The author recognizes that some

accuracy may have been sacrificed in estimating S by the standard

turbidimetric method rather than using the more accurate but tedious

methylene blue method.


Most of the S analyses reported in this dissertation were done by

turbidimetry. The procedure used in this study is given in Appendix C.

The turbidimetric method is rapid and sensitive, but there are a num-

ber of problems and limitations with this technique which should be

mentioned. Turbidimetry is subject to many interference, and the

formation of reproducible BaSO4 suspensions under uniform precipitating

conditions is extremely difficult. High concentrations of Na, K, Ca,

Mg, NO3, PO4, and SiO2 can all interfere with the turbidimetric sulfate

determination, but only large amounts of Ca are likely to cause sig-

nificant interference in practice (Butters and Chenery, 1959). Col-

loidal organic matter can interfere with the precipitation of BaSO4

by acting as a protective colloid in solutions low in S and by copre-

cipitating with BaSO4 at high concentrations of S.

Uniform precipitating conditions are essential in order to obtain

reproducible results. The use of concentrated acetic and phosphoric

acids buffers the pH and reducesvariability due to acid concentration.

A seed suspension of small and uniformly sized BaSO4 crystals acts as

nuclei or seed crystals and insuremore rapid precipitation and uni-

form results at low concentrations. Uniform tubes, stoppered and

carefully inverted a given number of times, also help to insure uni-

form precipitating conditions. Gum acacia added to the solution acts

as a stabilizer and provides for greater reproducibility of the sus-



Sulfate S in soils analyzed in the study of S distribution in

selected Florida soils was' determined by the indirect method of Hue

and Adams (1979). Several indirect methods of sulfate determination

by measuring the removal of Ba, Pb, or Sr from solution by sulfate have

been proposed (Roe et al., 1966; Gersonde, 1968; Dunk et al., 1969;

Borden and McCormick, 1970; Loeppert and Breland, 1972; Hue and Adams,

1979). These methods are comparable to turbidimetry where accuracy

and reproducibility are of concern. They also involve difficulties in

analysis of the associated cation. The most successful techniques in-

volve the indirect measurement of sulfate by Ba precipitation and

determination of Ba by atomic absorption spectroscopy. Major difficul-

ties associated with this technique are achieving complete precipita-

tion of small quantities of sulfate, Ba ionization interference by

atomic absorption, Si and Al interference with Ba absorption, and

organic matter, K, and Ca interference with BaSO4 precipitation

(R. H. Loeppert, Jr. 1972. Analysis of sulfate in soil extracts by

atomic absorption spectroscopy. Master's thesis. University of

Florida, Gainesville). Hue and Adams (1979) improved the technique by

seeding samples with BaSO4, by precipitating in 50% ethanol solution to

lower BaSO4 solubility, by using C1CH2COOH and KOH to control ioniza-

tion interference, and by determining Ba in a N20/acetylene flame.

Their technique was adopted for use in this study on soil ex-

tracts with very low concentrations of sulfate or where the presence of

color due to Fe or organic matter might present problems with turbidi-

metric techniques. Their method is briefly outlined in Appendix D.




Extractable sulfate S was very low throughout the profile in all

of the Spodosols studied (Table 12). Extractable S in the surface

soils ranged from 1 ppm in a Myakka fine sand from Alachua County to

8 ppm in a Leon fine sand from Duval County. Most of these profiles

were from rural locations. However, the City of Jacksonville encom-

passes Duval County, and the higher SO emission rates from industry in

that area could account for the higher sulfate-S level. Duval County

had one of the highest SO2 emission rates in Florida in 1978. Between

100 and 200 kg/ha of S were emitted on a county-wide basis. The aver-

age for the entire state is 32 kg/ha. Rainfall in the Jacksonville
area was found to contain 43.5 ueq/liter of SO in 1979--also among

the highest in the State (Brezonik et al., 1980; Edgerton et al.,

1980). Spodic horizons contained slightly more sulfate S than the

eluviated A2 horizons but less than the surface horizons.

Extractable sulfate S did not appear to be related to total S in

the soil. Extractable S varied little with horizon depth, whereas con-

siderable differences wera observed in the total S levels. The distri-

butions of the mean total and extractable S level in the nine Spodosol

profiles are given in Fig. 7.