Plant and Soil 36, 251-260 (1972) Ms. 1566

A Q U E O U S I R O N - S U L F U R S Y S T E M S I N

R I C E F I E L D S O I L S O F L O U I S I A N A

by GARY PITTS*, A. I. ALLAM, and JOHN P. HOLLIS

Dept. Plant Pathology, Louisiana Stare University, Baron Rouge, Louisiana, U.S.A.

SUMMARY

A phase diagram of an aclueous irort-sulfur system constructed from thermo- dynamic data for initial concentrations of 100 ppm H2S and 100 ppm Fe permitted us to predict that the principal precipitate found in the Eh - pH range of flooded Louisiana rice paddys would be FeS2. Analysis of precipitates formed in the laboratory at Eh and pH levels simulating paddy conditions showed tha t both FeS2 and FeS were present; however more FeS than pre- dicted was actually preeipitated in the FeS~ field of dominance.

A mult iparameter diagram for all aqueous system was constructed re- lating equilibrium iron concentration, Eh, pH and the equilibrium HeS concentration (that H2S which is in equilibrium with precipitated sulIides). I t was demonstrated from the diagram and associated thermodynamic ecIuations tha t the H2S ,~ FeS2 ecluilibrium predicts theoritical values of H~S in agreement with experimental values of H2S from rice paddy soil samples s.

INTRODUCTION

Ferrous sulfide (FeS) has been reported frequently from rice paddy soils 4 5 s 9 11 12) whereas FeSe (marcasite or pyrite) has not yet been identified from rice soil or root samples. This fact, plus the predominant occurrence of FeS2 in marine sediments 10, its low concentration in newly-formed sediment, and its slow formation in a thermodynamic sense relative to FeS, all indicate, that FeS is probably the only iron sulfide species in submerged rice fields.

There are many interesting observatio~as and facts which suggest however that the equilibrium between insoluble and soluble sulfides in submerged solls is more complex than the simple relation FeS

* Present address: Institute of Dental Research, University of Alabama in Birming- harn, Birmingham, Alabarna 35233.

252 GARY PITTS, A. I. ALLAM, AND JOHN P. HOLLIS

H2S. Many reports of FeS as the black precipitate occurring on soil and root surfaces are simple noncritical assumptions based on pre- vious reports. Theoretically-impossible levels of H2S, in terms of the FeS ~ H2S equilibrium, have been reported in the presence of a black precipitate on rice roots and excess ferrous iron in the soff solution s 9. B l o o m f i e l d 2 3 reported rapid loss of H2S in laboratory experiments in the presence of soluble ferrous iron compounds. It has not yet been possible to identiIy FeS2 in mixture with FeS2 has been made by reacting FeS and S in the presence of organic marter 3, and by reacting H2S with goethite 1

This investigation was initiated to study the H2S ~ FeS equili- brium in the laboratory under simulated rice paddy conditions. The preliminary results indicated that FeS2 is the theoretically-predicted form of iron sulfide in rice paddies, and we focused our attention therefore on the FeS2 ~ H2S equilibrium. A combined theoretical and experimental approach to this reaction was made at Eh, pH, sulfur and iron concentrations known to exist in Louisiana rice pad- dies.

METHODS

I n all aqueous iron-sulfur system several compounds and ionic species may be iormed as the redox potential and hydrogen ton concentration v a r y . A phase diagram (Fig. 1) showing fields oI dominance of these species, was con- s t r u c t e d Irom thermodynamic d a t a (Table 1). Standard temperature and pressure, an iron concentration of 100 ppm ([Fe~ = 1.3 × 10-aM) , a n d a sulfur concentration of 100 p p m ([H2S? = 3.125 × 10-3M) were stipulated, as t h e s e v a l u e s are in the range found in rice Iield soils 4 14).

Lines on the phase diagram represent conditions at which adjacent species are present in equal concen~ration. These lilleS are generated by linear e q u a - t i o n s derived from balanced redox equations (Table 2). For example:

in E q u a t i o n 7,

3Fe++ + 8H~O ~ Fes (OH)8 -]- 8 H + + 2 e -

substituting thermodynamic data from T a b l e 1,

3(-20.3) + 8(-56.7 ) = -451.2 + 8(0) + 2(0)

A F ° r : A F°oxiaants - - A F ° reduetants : - - 4 5 1 . 2 - - ( - -514 .5 ) : + 6 3 . 3 .

The equation

AF°r E o - -

n F

RICE FIELD IRON-SULFUR SYSTEMS 253

re la tes t h e r e d o x p o t e n t i a l of t h e r eac t i on a t s t a n d a r d cond i t ions a n d u n i t r e a c t a n t c o n c e n t r a t i o n s (E0) to t h e s t a n d a r d free ene rgy of t h e r eac t ion (AF°r) b y t h e n u m b e r of e lec t rons t r a n s f e r r e d (n) a n d t h e F a r a d a y c o n s t a n t (F). B y s u b s t i t u t i o n :

63.3 E 0 - - - - - - "bl .373.

2(23.06) The N e r n s t e q u a t i o n

R T [oxidants ] E h = E0 ºb n ~ In [ r educ tan t s ]

m a y be s impli f ied b y s u b s t i t u t i n g c o n s t a n t va lues to

.0592 [oxidants ] E h = E0 . b - - l o g

n [ r educ t an t s ] B y s u b s t i t u t i o n :

ù0592 [Fe3(OH) s] [H+] s E h = Eo + - - l o g

n [Fe++] 8 [HsO] s

I n d i lu te so lu t ion ac t iv i t i e s of t h e r e a c t a n t s a p p r o x i m a t e c o n c e n t r a t i o n s wh ich were used in al l ca lcula t ions . T he ac t iv i t i e s of w a t e r a n d t h e p r ec ip i t a t e Fe3 (OH)s can be cons idered un i ty . S u b s t i t u t i n g :

[H+]S E h = 1.373 .b .0592 log - -

[Fe++]a o r

E h = 1.373 - .089 log [Fe ++] - .236 p H

Since we desire to descr ibe cond i t ions where [oxidant ] = [ reduc tan t ] , we speciIy t h a t [Fe ++] = [Fes(OH)s] .

[Fe++] = [Fe] i n i t i a l - [Fea(OH)s],

100 p p m [Fe++] = 1 0 0 p p m - - - - 5 0 p p m = 9 × 10-4M.

2 Subs t i t u t i ng ,

E h = 1 . 3 7 3 - - . 0 8 9 ( 9 × 10 .4 ) - - . 2 3 6 p H

E h = 1.634 -- .236 pH,

w h i c h is a l inea r ecluat ion descr ib ing t h e b o u n d a r y on one side of which [~'e ++] > [Fe3(OH)s] a n d on t h e o t h e r side of w h i c h [Fes(OH)s] > [Fe++]. E q u a t i o n s such as No. 10, Tab le 2 requ i re a s l igh t ly d i f fe ren t d e r i v a t i o n :

Fe ++ "b 2 H 2 0 ,~-Fe(OH)2 -b 2H +

AF°r = 18.1 as descr ibed above .

F r o m t h e equ i l ib r ium c o n s t a n t express ion [products] [Fe(OH)2] [H+] 2

K e q - - [ r eac tan t s ] [Fe ++] [H20] "

Since [H20] ~~- 1 a n d [Fe(OH)2] = [Fe++]

2 5 4 GARY PITTS, A. I. ALLAM, AND JOHN P. HOLLIS

EH+j2 Keq -- , log Keq = 2 log [H+]

1

The equa t ion AF°r ~ -- 1.364 log Keq allows

18.1 = 1.364 (2 log [H+]) which reduces to

pI-I --~ 6.63

which generates a line separa t ing fields of dominance of Fe (OH)2 and Fe ++. The f inal s tab i l i ty d iagram (Fig. 1) was evolved by e l iminat ion of redun-

dancy in the s imul taneous ly p lo t t ed equat ions of Table 2. The va l id i ty of t he phase d iagram was tes ted in an a tmosphere-contro l led

chamber wi th E h and p H sensors. The source of Fe ++ was Fe(OH)2 prepared by reac t ing FeC 12 wi th N a O H in a hydrogen a tmosphere . Aqueous solutions of H~S were prepared f rom the gaseous p roduc t of FeS and HC 1 and s tandard- ized by de te rmina t ion wi th methy lene blue 15 af ter di lut ing wi th H2 sa tu ra ted wate t . B o t h Fe ++ and H~S were added a t 100 p p m ; H2S the r eby being in excess. The chamber was main ta ined under one a tmosphere hydrogen or oxygen-free n i t rogen and s t i r red constant ly .

Prec ip i ta tes formed were collected by v a c u u m f i l t ra t ion through a fine s intered-glass disc in oxygen-free n i t rogen and dried over CaCO8 in vacuo

af ter which t h e y were re la t ive ly s table in air. Character iza t ion of precipi ta tes and of au then t i c samples of FeS and FeS~ was a t t e m p t e d by chemical and physical analyses.

In order to relate free dissolved H~S to the eoncent ra t ion of Fe ++ a t various E h and p H values in the H~S field, E q u a t i o n 26, Table 2 was rear ranged by the subs t i tu t ion of

[FeS2] = [I-I?] (from balanced redox equat ion) z ~

to give

where Eh = -- .077 -- .148 p H + .0295 p Fe ++ + .059 p H2S

px = -1og[XL

From this equation a diagram was constructed (Fig. 2) which graphically

relates the four variables continuously from pH = 4 to pH = 7, Eh = 0 to Eh= --.4V, pFe ++= 3topFe ++= 6, andpH2S ~ 2topH2S = 10. When

any three coordinates corresponding to parameters of the equation are select-

ed, a li~e drawn through the origin and these coordinates determines the fourth parameter. For example, at pH = 4.5, Eh ~ --. 150 v, and p Fe ++ = 4, p H2S is indicated to be 8 (by caleulations 8.05 is determined).

RICE FIELD IRON-SULFUR SYSTEMS 255

RESULTS

Under the conditions specified for the phase diagram (Fig. 1) FeS2 will, theoretically, be the dominant iron sulfide formed at Eh-pH values prevalent in rice field soils. Samples from the FeS2 field of dominance exhibited a heat-absorption peak at 450 C during differential thermal analysis (DTA) although H2S was liberated by treatment with mineral acids. FeS2 (marcasite) is converted to FeS2 (pyrite) at 450 C; FeS2 is not decomposed by non-oxidizing mineral acids. Samples from the FeS field reacted with HC1 to form H2S and showed no typical absorption peaks during DTA. This in- dicates that although FeS2 is predieted by thermodynamic calcula- tions, a mixture of FeS and FeS2 is produeed in the FeS2 field under the conditions maintained.

The lower oxides and hydroxides of iron (produced experimentally in the absence of sulfide), which are similar in appearanee to the iron sulfides, were found to oxidize rapidly to characteristic reddish compounds when exposed to the air, even after thorough drying.

Fig. 1.

m-O00

~100 0

" 2 0 0 x 0 3 0 0 O tt

4 0 0 > " ~ 5 0 0 M

;~600

ù"700 0 >

::800

4.0

Total EFe~ = lOOppm Total EH2S~ = 10 Oppm

STABILITY DIAGRAM

Fe$ 2

FeS

Fa °

s.o d o 7'.0 .... 8.0 p H

Stab i l i ty d iagram of precipi ta tes in the aqueous i ron-sul fur sys tem at lOOppm iron and lOOppm H2S.

2 5 6 G A R Y P I T T S . A. I . A L L A M , A N D J O H N P. H O L L I S

T A B L E 1

AF ° of formation for various species (condensed from G a r r e l s and C h r i s t 7)

Species F o

Fe °, 0

Fe ++ -- 20.3

Fe(OH)~ -- 115.6

Fe3 (0H) s - - 451.2

Fe(OH)3 -- 166.0

FeS -- 23.3

FeS~ - -36 .0

H2S (aq) - -6 .5

H S - +3.0 S o 0

S = +22.0 $2= +19.8

H20 0 H+ 0

e- 0

The absence of such reddish appearance in the FeS2 and FeS samples confirmed the absence of iron oxides and hydroxides as predicted by the phase diagram. The presence of erratic, slight heat absorp- tion peaks in the range 100 to 130°C and partial melting in this range of some FeS2 samples may indicate the presence of Fe2Sa which readily decomposes to sulfur and FeS.

The multiparameterdiagram predicts that significant concentra- tions of H2S can exist in equilibrium with FeS2 under conditions simulating the Louisiana rice paddy. For example: at pH = 5.0, free disolved Fe++ = 10-4M (5.6 ppm) and Eh = - -350 mV the H2S concentration in the aqueous system -- 10-5"9M or. 07 ppm. Applying the equation:

Eh = , . 1 3 5 - - .0295 (pFeS2 + 2pH -- pFeS -- pH2S),

we find that EFeS2]

l o g - - - - 8.6 [FeS]

verifying that this Eh -- pH location lies in the FeS2 field.

RICE FIELD IRON-SULFUR SYSTEMS 257

DISCUSSION

Thermodynamic calculations indicate that FeS2 should be the principal sulfide under simulated rice-field conditions, but experi- ments carried out to test this idea indicate that more FeS than pre- dicted is formed in the FeS2 field of dominance. The calculations assume that physico-chemical equilibrium is instantaneously achiev- ed in a thoroughly mixed system. The relative time required for precipitation of the iron sulfides is apparently an important factor in the composition of the precipitate at any Eh -- pH condition. The time required for the reaction:

H2S + Fe + + ~ FeS q- 2H +

is short in both directions whereas the reaction:

2H2S + Fe ++ H FeS2 + 4H + -~- 2e-

is very slow 6 and FeS, once formed, is converted with difficulty tò FeS21 a notwithstanding the thermodynamic tendency toward FeS2

(Ha S @ FeS ~ FeS2 + 2H + + 2e-, F°r = --6.2 kcal/mole)

at equilibrium. * The calculation of a meaningful value of H2S existing in equili-

brium with ferrous precipitates and thereby being available as a toxicant to rice plants is made more difficult by the uncertainty as to which of the ferrous sulfides to consider in the equilibrium equation. II the near-universal assumption that FeS is the b l a c k precipitate which collects on rice-root surfaces at low redox poten- tials is valid, then Equation 24, Table 2 predicts that toxic levels of H2S are extremely unlikely in the paddy. For example at pH = 6.0, total iron = 10-3M, and total sulfide = 10-aM (34 ppm as H2S) only 6 × 10-TM HgS (.02 ppm) is dissolved assuming that

[FeS~ : ~ H 2 S ] i n i t i a l - I H 2 S ] i n equi l ib r ium

and that

[Fe ++] = ~Fe~init ial - - ~H2S~init ial + [H2S~in equi l ibr ium

and solving for [H2S~ in equilibrium by the quadratie formula

- - b ~ %/b 2 -- 4ac [H2SJ = x =

2a

258 GARY PITTS, A. I. ALLAM, AND JOHN P. HOLLIS

TABLE 2

E q u a t i o n s c h a r a c t e r i z i n g the a q u e o u s i r o n - s u l f u r s y s t e m

1. F e ° ~ Fe ++ -5 2 e - E h = - - .441

2. 3 F e ° -5 8 i l s O H F e s ( O H ) s -5 8 H + -5 8e - E h = .013 - - .059 p H

3. Fe ° -5 2 H 2 0 ~ Fe (OH)2 -5 2 H + -5 2 e - Eh = --.048 -- .059 p H

4. Fe ° -5 3H20 ~~- Fe(OH)~ -5 3H + -5 3e-

Eh = .060 -- .059 pH

5. Fe ++ -5 3H20 H Fe(OH)3 -5 3H + -5 e-

Eh = 1.060 --.059 log [Fe ++] --.177 pH Eh = 1.240--.177pil

6. Fe3 (OH)s -5 H s O .~ 3 F e ( O H ) 3 -5 H + -5 e - Eh = .430 --.059 pH

7. 3Fe ++ -5 8HsO H Fez(OH)s -5 8H + -5 2e-

Eh = 1.373 --,089 log [Fe ++] --.236 pH

Eh = 1.656 --,236 pH

8. Fe(OH)2 -5 H~O ~-~ Fe(OH)s -5 H + -5 e-

Eh = .273 --.059 pH

9. 3Fe(OH)s -5 2H20 H Fes(OH)s -]- 2H+-52e -

Eh = .195 --.059 p H

I0. Fe ++ -5 2ii20 H Fe(OH)2 -5 2H +

p H = 6.63

I1. H 2 S ~ H + - 5 H S - p H = 7.0

12. I l S - H H + -5 S = p H = 14.0

13. I l2S ~~- S o -5 2 H + -5 2 e - E h = - - . 141 - - . 0 5 9 p H

14. H S - ~.~- S° -5 H + -5 2e - E h = - - . 0 6 5 - - . 0 3 0 p H

15. 2 t t 2 8 H S 2 = -5 4 H + -5 2 e - E h ~ .712 - - . 0 3 0 l o g IHRS] - - . 1 1 8 p H E h = .819 - - . 1 1 8 p H

16. 21-18- H S~ = -5 2 H + -5 2 e - E h = .298 - - . 0 3 0 log [ H S - ] - - . 0 5 9 p H E h = .405 - - . 0 5 9 p H

17. Ss= H2S°+2e -

Eh = --.429

18. H~S 4- FeS ~~- FeS2 -5 2I-I + + 2e-

Eh = --. 135 --.030~1og [H2S] --.059 pH

Eh ~ --.028 --.059 pH

19. HS- 3c FeS H FeS -5 1-1 + -5 2e-

Eh = --.365 ~.03021og [HS-] --.030 pH

Eh = --.258 --.030 pH

20. Fe ° -~- HsS H FeS -5 2H + -5 2e-

Eh = -- .365 -- .030 log IHRS] --.059 pH

Eh = --.258 --.059 pH

21. Fe ° q- HS- ~-- FeS -5 H + -5 2e-

Eh = --.572 -- .030 log [I-IS-] -- ,030 pH

Eh = --.465 --.030 pH

22. Fe o -5 2H~S H FeS2 -5 4 H + + 4 e -

E h = - - . 5 0 0 - - . 0 3 0 log [H2S] - - . 0 5 9 p H E h = - - . 3 9 3 - - . 0 5 9 p H

23. F e ° -5 2 H S - H FeS~ -5 2 H + -5 4 e - E h = - - . 9 1 3 - - . 0 3 0 log [ H S - ] - - . 0 3 0 p H E h = - - . 8 0 6 - - . 0 3 0 p H

24. Fe ++ -5 H~S H FeS q- 2 H + p H = 3.08

25. Fe ++ + H S - H FeS + H + p H = - - . 1 5

26. Fe ++ + 2 H 2 8 H FeS~ + 4 H + + 2 e - E h = - - . 0 5 9 - - . 0 5 9 log [H2S] - - . 1 1 8 p H E h = + . 1 5 5 - - . I 1 8 p i l

27. Fe ++ + 2HS- ~~- FeS2 + 2H + + 2e-

Eh = --.472 --.059 [HS-] --.059 pH

E h = - - . 2 5 8 --.059 p H

28. FeS + 2 H 2 0 ~ H2S + Fe (OH)2 n o A [ H +] o r A e -

29. FeS + 2H20 H H S - -5 H + -5 Fe(OH)~ p H = 14.1

30. Fe (OH)~ -5 2 H 2 8 H FeS2 -5 2 H 2 0 -5 2 H + -5 2 e - E h = - - . 4 5 2 - - . 0 5 9 log [H2S] - - . 0 5 9 p H E h = --.239 --.059 p H

31. F e ( O H ) s -5 2 H S - ~- FeS~ -5 2H~O -5 2 e - E h = - - . 8 4 3 - - . 0 5 9 log [ H S - ]

E h ~ - - . 6 3 0 32. 3FeS -5 8H~O ~ 3 H s S -5 Fe3 (OH)s -5 2 H + -5 2e -

E h = 1.147 - - . 0 8 9 log IHRS] - - . 0 5 9 p H

E h = .846 - - . 0 5 9 p H 33. 3FeS -5 8H~O H 3 H S - -5 Fe3 (OH)s -}- 5 H + -5 2 e -

Eh = 1.745 -5 .089 log E}IS -] --. 148 p H E h = 1.424 - - . 148 p H

34. Fe3 (OH)s -5 6 H S - -5 2 H + ~ 3FeS2 -5 8H~O -5 4 e - E h = - - 1.395 - - . 0 8 9 log [ H S - ] -5 .030 p H

Eh = -- 1.074 -5.030 pH 35. F e s ( O H ) s -5 6 H 2 8 H 3FeS2 -5 8 H 2 0 -5 4 H + -5 4 e -

E h = - - . 9 8 6 - - . 0 8 9 log [H2S] - - . 0 5 9 p H

E h = --.665 --.059 p H

36. FeS -5 3HsO H 1-12S -5 Fe(OH)8 + H + -5 e-

E h = 1.343 -5 .059 log IHRS] - - . 0 5 9 p H H E h ~ 1 . 1 3 0 - .059 p H

37. FeS -5 3H~O ~ H S - -5 Fe (OH)3 -5 2 H + -5 e - E h = 1.756 - 5 . 0 5 9 log [ H S - ] - - . 118 p H E h = 1 . 5 4 3 - . i i 8 p H

38. F e ( O H ) s -5 2H~S ~ FeS~ -5 3H~O -5 H + -5 e - E h = - - 1.613 - - .118 log [H~S] - - . 0 5 9 p H Eh = --I.187--.059pH

39. Fe(OH)s -5 2HS- + I-1 + ~ FeS~ -5 3ii20 -5 e-

Eh = --2.435 -- . 118 log [ilS-] -5.059 pH

Eh = --2,009 -5.059 pH

RICE H E L D I R O N - S U L F U R SYSTEMS 259

DISSOCIATION of FeS2 p Fe ++

ó 4.5 3 ]0 Eh volts

_.4 _.3 _.2 _.1 0

! 6 pH 5

p H2S 4

Fig. 2. Multiparameter diagram describing the system Fe ++ + 2I-IsS ~ FeS~ + 4H + + 2e-

under Louisiana rice paddy conditions.

Origen I

I

where ax 2 + bx + c ---- 0.

With higher iron concentrations the equilibrium H2S value is even lower; only when the molarity of H2S exceeds that of total iron will sufficient H2S be available to produce toxicity.

H o l l i s s has deteeted 0.3 to 11.7 ppm H2S in samples of paddy soils. If the thermodynam]cally predicted FeS2 ~~- H2S system is accepted, values in this fange would be predicted at iron and H2S levels occurring in Louisiana paddy soils. For example at pH = 4.5, Eh ~- --.350V, and free dissolved Fe ++ ~-- 10-SM (0.56 ppm) the multiparameter diagram (Fig. 2) prediets an H2S concentration of 10-4'2M (2.9 ppm). This value should be near the maximum as these conditions would be considered extreme in Louisiana rice paddys.

Reconciliation of the theoretical with the elnpirical is a diffieult task in the study of the physieo-chemical react]ons in the soff. Theoretical construets such as the one presented hefe are limited in seope and must assume homogeneity; the composition of the soil is highly complex and heterogeneous, although P o n n a m p e r u m a la has demonstrated that the soil solution is amenable to thermodyna- mic treatments. The findings of A l l a m et al. (unpublished data)

2 6 0 RICE FIELD IRON-SULFUR SYSTEMS

suggest that clay sorption of soluble sulfides from the soil solution can be treated in a straight forward manner unless complications are caused by the mediation of iron in the sorption and desorption process.

ACKNOWLEDGEMENT

I n v e s t i g a t i o n of t h i s p r o b l e m w a s s u p p o r t e d in p a r t b y N a t i o n a l Science

F o u n d a t i o n (U.S.A.) g r a n t GB-8653.

Reeeived August 12, 1970

LITERATURE CITED

1 B erl ler , R. A., Iron sulfides formed from aqueous solutions at low temperatures and atmospherie pressure. J. Geol. 72, 293-306 (1964).

2 B l o o m f i e l d , C., Mobilisatiou phenomeuain solls. Rothamsted Expt. Sta. Rep. p. 226-239 (1963).

3 B l o o m f i e l d , C., Sulfate reduetion in water-logged solls. Soil Sci. 20, 207-221 (1969). 4 Conne l l , W. E., The reduetion of sulfate to sulfide under anaerobie soff conditions.

M.S. thesis. Louisiana State University (1966). 5 Conne l l , W. E. and P a t r i e k , W. H. Jr., Sulfate reduetion in soil: Effects of redox

potential and pH. Seienee 159, 86-87 (1968). 6 F r a z i e r , K e n d r i e k , Computing the link between sea and air. Science News. 97:

533-535 (1970). 7 G a r r e l s , R. M. and C hr i s t , C. L., Solutions, Minerals, and Equilibria. Harper and

Row, New York (1965). 8 Hol l i s , J o h n P., Some new faets about soll diseases of rieB. Louisiana Agrieultnre.

10, 4 (1967). 9 Hol l i s , J o h n P., Toxieant Diseases of Riee. Louisiana State University Agr.

Expt. Sta. Bull. No. 614. 24 p. (1967). 10 K a p l a n , I. R., E m e r g , K. O. and R i t t e n b e r g , S. C., The distribution andisotopie

abundanee of sulfur in reeent marine sediments off Southern California. Geochim. et Cosmoehim. Aeta 27, 197-331 (1963).

11 Pa rk , Y. D. and T a n a k a , A., Studies of the riee plant on an "Akioehi" soll in Korea. Soil Sei. Plant Nutr. (Japan) 14, 27-34 (1968).

12 P a t r i e k , W. I-I. Jr., Extraetable iron and phosphorus in a submerged soil at controll- ed redox potentials. 8th Intern. Congress of soff Sei. Bueharest, Romania 4:605-609 (1964).

13 P o n n e m p e r u m a , F. N., Dynamic aspeets o1 flooded soils and the nutrition of the rieb plant. In: ProB. Symposium Mineral Nutrition of the riee plant 1964, 295-328. The Johns Itopkins Press, Baltimore (1965).

14 R o d r i g u e z - K a b a n a , R o d r i g o , Chemieal antibiosis to nematodes in rice fields. PhD. dissertation. Louisiana State University (1964).

15 Sands , A . E . , G r a f i u s , M.A., W a i n w r i g h t , H. W. and W i l son , M. W., The determination of low eoneentrations of hydrogen sulfide in gas by the methylene blue method. Report of Investigation 4547, U. S. Dept. Interior - Bureau of Mines (1949).