P l a n t a n d Soil X X I I I , no. 1 A u g u s t 1965

R E D O X P O T E N T I A L T R E N D S IN A S U B M E R G E D

R I C E S O I L

by F. S. C. P. KALPAGÉ

Department of Agriculture, University of Ceylon.

INTRODUCTION

Physico-chemical changes in submerged rice soils have been studied by a number of workers, including S h i o r i and T a n a d a 12 and M i t s u i S in Japan, de Gee4, P o n n a m p e r u m a 1 1 , J e f f e - r y a 6 7 and P a t r i c k 9

S h i o r i and T a n a d a 12 and M i t s u i s in their investigations on the chemical changes occurring in submerged rice soils have shown that the furrow slice of a submerged rice soll is differentiated into distinct layers; an oxidized layer, yellowish brown in colour, on the surface, a few millimeters to one centimetre deep, where conditions are aerobic; and a bluish grey, reduced layer below this oxidized zone, containing mainly reduced produets where micro-organisms live anaerobically and which constitutes the main part of the furrow slice. Below the furrow slice are the plow-sole and the subsoil, the latter usually remaining in an oxidized condition even during the summer irrigation season in Japan. This profile differentiation has been observed and studied, under laboratory conditions, with soil submerged in glass cylinders.

This characteristic differentiation in the profile of a submerged rice soil has practical implications. For example, reports from Japan (Shior i and T a n a d a 12; Mi tsu i 8) and also from India and Ceylon ( C h a n d r a r a t n a 2) indicated the superiority of deep placement when compared to surface application of ammonium sulphate, in increasing rice yields. This was attributed to the retention of ammonium in the reduced layer in the case of deep placement whereas surface applieations led to nitrification in the oxidized layer

- - 1 2 9 - -

130 F . S . C . P . KALPAGÉ

followed by loss of nitrate in the surface water and by denitrifi- cation of the nitrate in its downward passage through the reduced layer. More recently, however, C h a n d r a r a t n a et al. a have reported that eren in the relatively well-drained Batalagoda soils of Ceylon, sub-surface placement produced no significant benefit over broad- cast application of ammonium sulphate at transplanting. The Eh values of the surface soil at planting were distinctly less than 350 mV, the value that P e a r s a l l 10 considers critical for nitrification.

The experiments reported in this paper were designed to in- vestigate in the field the redox potential trends in a submerged rice soil and to study the influence on redox potentials of the growing rice crop and added organic matter.

EXPERIMENTAL DETAILS

The field experiments were conducted on terraced rice fields (Fig. 1) located on the University Campus at Peradeniya in the north-east monsoon season 1960. A randomised-block experiment, with four replicates, was laid out with the following treatments, using cropped and uncropped plots with four levels of organic marter (O.M.) in each series:

0"- - ' . . . . " . . . . . . « ~ . . . . . x ¢ k \ \ \ x - . / \

/ _ \ ~ ~ . / ~ q r e v s lLty c L a y

6 - -

5 / J / / / / / _ / / / ~ / m o t t t i ~ g

~0Y///(///////z Fig. 1. The soil profile.

1. Cropped 0% O.M. 5. Uncropped 0% O.M. 2. , 2% O.M. 6. , 2% O.M. 3. , 4% O.M. 7. , 4% O.M. 4. 6% 0.•. 8. , 6% O.M.

The organic mat ter used was the grass weed Isachne australis, common in the riee fields in the low-country wet zone in Ceylon. Quantities of fresh [sachne aus¢ralis were evenly spread out on the plots, about 10 days before the rice seedlings were planted, mamotied in, and then the plots were flooded.

One pound of the variety H4, presoaked for 1 night and sprouted for two

R E D O X P O T E N T I A L I N R I C E S O l L 13 1

days was broadcast in two nurseryplots, 20 it. by 5 it. Nine ounces of a NPK tertilizer mixture (4-4-1) were applied broadcast to each plot. Three-week- old seedlings were transplanted in the experimental plots, two per hill, in hills spaced 6 by 6 inches.

Ammonium sulphate (200 lb/acre), and superphosphate (200 lb/acre) were applied in three equal doses, just beIore transplanting, at tillering and at initiation of inflorescence primordia rëspectively. Muriate of potash (100 lb/acre) was applied just before transplanting and at primordia initiation in two equal doses. The fertilizers were broadcast evenly over each plot after draining and the plots submerged again about 24 hours later.

Redox potentials and pH values of the submerged soil at depths of 0, 3, and 6 inches were recorded at intervals throughout the growing season, commencing before the application of the organic matter. A Cambridge pH meter was used with a saturated calomel electrode as reference. For Eh measurements platinum electrodes were introduced into various depths of soll in the field. A glass electrode was used for measuring pH.

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

The d a t a in Tab le 1 give the m e a n E5 va lues of all p lo ts at each

s ampl ing da t e for the d i f ferent dep ths a nd levels of organic mar te r .

T h e r e d o x po ten t i a l s rise w i th t ime reach a m a x i m u m and then

fall oft. T h e m e a n E5 va lue at t he sur face is h igher over the whole

e x p e r i m e n t t h a n at e i ther depth . The re does no t seem to be a n y

cons i s ten t d i f ference for t he d i f ferent levels of o rganic ma t t e r .

T A B L E 1

Redox po ten t i a l s (Es) a t d i f fe ren t dep th s and Ievels of organie m a t t e r

Levels of organie m a t t e r

S.E. 1 4 S.E.

Dep th Days D a t e

Mean 0 in, 3 in. 6 in.

0 7 -11 -60 198 261 177 158 14 2 1 - l l - 6 0 229 227 232 227 22 29 -11 -60 245 241 248 247 28 5 -12 -60 283 239 286 324 35 12-12-60 273 268 256 295 50 27 -12 -60 332 314 349 334 63 9 - 1-61 267 279 263 258 71 17- 1-61 316 332 304 311 78 24 - 1-61 223 251 215 203

84 3 0 - 1-61 250 294 t 2 3 8 1219 91 6 - 2-61 250 281 1241 1227 98 13- 2-61 245 265 I 237 1234

106 21- 2-61 262 276 I 261 I 250 113 28- 2-61 247 275 / 237 I 230

I ] Mean I 271'6 I 253 .1~ 251.2

B 11.9 B 5.0

6.4 9.7

.0.8 9.4 7.7 7.4 3.8 4.8 6.0 2.9 3.2 3.9

192 221 269 262 286 340 274 343 210 245 245 243 256 241 259.1

2 3

180 204 230 233 243 229 295 276 274 262 323 313 281 255 307 298 218 227 245 249 232 257 239 242 262 262 250 244 255.6 253.6

16. 10. 12. 13. 14. 20, 17. 19.

9. 5.

I0. 5. 6. 7.

132 1~. s, c. P. KAL]?AGÉ

C~

P~ <

0 ~ 0 ~ 0 ~ 0

0 0 0 ~ ~ ~ 0

~ ~ o ~ o ~ ~

©,

~ N ~ ~ ~ ô N ~ õ

«

",0 +--

la

~~i~õo~ • 0 0

• u 6 ,-6 ~ N , - :

g ' d '~ "-; £ ''4 d d '

õ e +-

oi~~~~~~~ , ~ , 5 ~ ~ ~ c S 8 -

~ ~ ~ ~ 0 0 ~

~ ~ 0 0 0 0 0

0 0 0 ~ ~ 0 ~

o O o x o o x ~ X x o

o o ~ ~ x

~ õ o , 5 o A A

A A ; g d d

o õ ~ õ

0 n~

Il II II

REDOX POTENTIAL IN RICE SOIL 133

Table 2 summarizes the variance ratios for the main effects and interactions up to 71 days. At no time (up to 71 days) does the level of organic marter have any effect on Eh, nor is there any interaction between organic matter and cropping (0 × C). Cropping affects the Eh in the same way whether or not organic matter is present.

Large and consistent effects are due to depth of sampling, par- ticularly at the beginning of the experiment and again from 28 to 71 days. There is a slight indication of an interaction between cropping and depth after 14 days and at 50 days, but this may well be non-significant. None of the other first-order or second-order interactions approach significance.

There is no evidence of interaction between depth and organic marter, or between depth, cropping, and organic matter. The presence of organic matter has not produced changes in En at any given depth which are different from the plots without organic marter, nor does cropping affect the Eh diIferently when measured at different depths in the presence or absence or organic matter.

The effect of cropping is shown in Figure 2. Once the crop has been established, the En from 28 to 71 days is consistently much lower in the cropped soils. This corresponds to the period when vegetative growth is at a maximum. After 71 days, however, the E5 values in the cropped and uncropped plots are almost equal.

The depressing effect of a standing crop on En values is, at first glance, unusual. The Eh regime in the root environment is likely to be influenced by a growing crop in two ways; oxygen consumption in root respiration and oxygen diffusion out of the root due to the transport of oxygen from shoot to root. The riet effeet will be. determined by the difference in the amounts of oxygen involved in root respiration and photosynthesis. During the period of vegetative growth when the photosynthetic process is particularly vigorous the net result is likely to be a diffusion of oxygen out of the roots. This is borne out by the observation of S t u r gis 13 that incrustations of Ierric compounds occur around the roots of rice plants, the oxygen diffusing outwards from the roots and oxidizing the ferrous compounds around the roots to insoluble ferric compounds. The extent to which the soil in the root zone is oxidized by this diffusing oxygen has however not been established although observation would seem to indicate that it is confined to the soil in the immediate vicinity of the root.

134 F . S . C . P . KALPAGÉ

350

2 5 0

1 5 0 u)

E 50 Œ

m 350 LU

250

150

g

s00~

,& o / / I ° " , k /; o : / ~

o" uncroppéd p{ots

B cropped plots /~

. ,:~,~.#,~ß... f . o

/ / " c 0 1 . "10 ¢11

o" ~. O E ._~ • I t ) . ~ o (11 L .

uO-O O- O- 0 0 6 E z ~ z z ,-, a. ,-

, 1 L ~ ) , . 1 , ~ , J, , J , , , 20 40 6'0 80 100 120 140

days f r om s o w i n g

F i g / 2 . Eh t r e n d s in u n c r o p p e d (A) a n d c r o p p e d (B) p lo t s a t t h r ee dep th s .

sown = sown in nursery o.m. = organic marter added and turned in main. = plots mamotied and levelled NPK = NPK fertilizer applied transpl. = seedlings transplanted in plots NP topdr. = NP-fertilizer topdressed pan. emerg. = panicles emerging pan. ripe = panictes ripening harv. = crop harvested

If oxygen diffusion out of the roots due to increased photo- synthetic activity is greater than oxygen consumption in respi- ration, the Eh should be greater in the cropped plots. But the results show that the Eh is lower in the plots with a standing crop during the 28 to 71 day period.

A l b e r d a 1 has referred to the reduction of the culture solution, in which rice plants were growing, during the period of stem elongation. During this period, the resistance to oxygen transport through the plant becomes greater as the internodes elongate and

REDOX POTENTIAL IN RICE SOIL 135

the distance between the leaf sheath and the root increases. It is thus possible that oxygen diffusion into the soil during this period is not as great as oxygen consumption, the net result being the withdrawal of oxygen form the soil solution and the consequent lowering of the Eh.

Another observation is of signiIicance. In the cropped plots the Eh is uniform throughout the profile down to 6 inches depth until the heading stage. In the uncropped plots the En at the surface is less than that at the lower depths. This would seem to be the conse- quence of the diffusion of air through the vertical faces of the terrace (Fig. 3). This aeration of the lower layers in a terraced rice soil will act in opposition to the usual tendency for the lower layers to be reduced and the surface layer to be in an oxidized condition. The trend of En values in a terraced rice soil on submersion will therefore be different from that in an unterraced Iield.

Fig. 3. The l a t e ra l d i f fus ion of a i r in to t e r r aced rice soils.

A slight indication of an interaction between cropping and depth (Table 2) has been referred to earlier. E5 values for cropped and uncropped soils at each depth are shown graphically in Figure 2. At 22 and 28 days there is no interaction (the variance ratio is non- significant) and Eh changes in the same way with depth, whether the soil is cropped or not. After 35 days there is a hint, and at 50 days the near certainty, that cropping reduces Eh more at 3" and 6" than in the surface layer. These differences persist slightly at 63 days but disappear at 71 days.

S U M N A R Y

Dif fe ren t levels of added f resh organic m a t t e r h a v e h a d no s ign i f ican t ef iects on El~ va lues in a s u b m e r g e d rice field. T h e effect of c ropp ing d u r i n g

136 REDOX POTENTIAL IN RICE SOIL

t h e p e r i o d of i n t e n s i v e v e g e t a t i v e g r o w t h is t o d e p r e s s t h e r e d o x p o t e n t i a l o f t h e soil. Th i s e f fec t is e v i d e n t l y d u e to o x y g e n c o n s u m p t i o n b y t h e r o o t s in r e s p i r a t i o n b e i n g g r e a t e r t h a n oxyge~ t r a n s p o r t t h r o u g h t h e p l a n t a n d s u b s e q u e l l t d i f fu s ion o u t of t h e roo t .

H i g h e r Eh v a l u e s in t h e b a r e p l o t s a t t h e l ower d e p t h s s u g g e s t l a t e r a l o x y g e n d i f f u s i o n t h r o u g h t h e v e i t i e a l face of t h e t e r r ace . T h e a b s e n c e of a m a r k e d d e p r e s s i o n in r e d o x p o t e n t i a l fo l lowing s u b m e r s i o n of a t e r r a c e d r ice f ie ld is a n i m p o r t a n t obse rva t io l l .

ACKNOWLEDGEI~IENTS

T h e a u t h o r is d e e p l y g r a t e f u l to Dr . M. F. C h a n d r a r a t n a for i n s p i r a t i o n a n d i n v a l u a b l e g u i d a n c e t h r o u g h o u t t h i s work , a n d w i shes t o t h a n k Mr. C. J . E k a n a y a k e for t e c h n i c a l a s s i s t a n e e w i t h t h e m e a s u r e m e l l t S in t h e f ie ld . T h e s t a t i s t i c a l a n a l y s e s were d o n e a t t h e R o t h a m s t e d E x p e r i m e l l t a l S t a t i o n w h e r e Mr. J . H. A. D u n w o o d y a n d Dr . G. E.rG. M a t t i n g 1 y were m o s t he lpfu l .

Received March 16, 1964

REFERENCES

1 A l b e r d a , Th., Growth and root development of lowland rice and its relation to oxygen supply. PIant and Soil 5, 1 (1953).

2 C h a n d r a r a t n a , M. F., Recent rice research in Ceylon. Proc. Ceylon. Assoc. Adv. Sei. (1951).

3 C h a n d r a r a t n a , M. F., F e r n a n d o , L. H., and W e e r a r a t n a , }1., Fertilizer responses of rice in Ceylon: 1. Effect of method and time of nitrogen application. Empire J. Exp. Agr. 30, 16 (1962).

4 Gee, J. C. de, Preliminary oxidation potential determinations in a 'sawah' profile near Bogor (Java). IC'ourth Int. CongL Soil Sci. Trans. 1, 300 {1950).

5 J e f f e r y , J. W. O., Iron and the Eh of waterlogged soils with particular reference to paddy. J. Soll Sci. 11,140 (1960}.

6 J e f f e r y , J. W. O., Defining the state of reductiol~ of a paddy soil. J. Soil Sci 12, 172 (1961).

7 J e f f e r y , J. W. O., Measuring the stare of redu¢tion of a waterlogged soll. J. Soll Sci. 12, 317 (1961).

8 Mi t su i , S., Inorganic Nutrition, Fertilisation and Soll Amelioration for Lowland Rice. Tokyo, Yokendo Ltd. (1954).

9 P a t r i e k , W. H. Jr., Nitrate reduction rates in a sulomerged soll as affected by redox potential. Seventh Int. Congr. Soil Sei. Trans. 2, 494 (1960).

10 P e a r s a l l , W. H., The soil eomplex in relation to plant eommunities. J. Eeol. 26, 180 (1938).

11 P o n n a m p e r u m a , F. N., The Chemistry of Submerged Soil in Relation to the Growth and ¥ield of Riee. Ph. D. Thesis Cornell University (1955).

i2 Sh io r i , M., and T a n a d a , T., The Chemistry of Paddy Soils in Japan. Ministry of Agriculture and Forestry, Tokyo (1954).

13 S t u r g i s , M.B., Changes in the oxidation-reduction equilibrium in solls as related to the physical properties of the soll and the growth of rice. Louisiana Agr. Exp. Sta. Bull. 2"/1, (1936).