P l a n t and Soil X X V I , no. 3 J u n e 1967

E F F E C T O F A S S O C I A T I N G A N I O N S ON

P H O S P H O R U S R E T E N T I O N I N S O I L :

II . U N D E R V A R I A B L E A N I O N C O N C E N T R A T I O N

by D. L. DEB* and N. P. DATTA

Division of Soll Scienee and Agricultural Chemistry, Indian Agrieultural Researeh Institute, New Delhi

In our earlier investigation 4 the effect of associating anions at a particular concentration on the retention of phosphorus by soil under variable phosphorus concentration has been studied. It has been observed that the efficiency of associating anions in counter- acting the phosphorus retention reactions in soil depends on a number of factors such as pH of the reacting medium, phosphorus concentration, the physicochemical nature of the associating anion and the nature and properties of soil. As the efficiency of the anions has been studied only at a particular concentration, it is quite natural to assume that a variation in anion concentration is likely to change the nature of reaction between soil constituents and as- sociating anions. In the present investigation, the effect of the as- sociating anions at variable concentrations on the retention of phos- phorus by soils under a particular phosphorus concentration has been studie&

METHODS

This inves t iga t ion was carried out on the same soils e.g. Alluvial, Black and Red soils of India , t he charac ter i s t ic proper t ies of which have a l ready been descr ibed elsewhere (in our earl ier communica t ions a 4).

The concen t ra t ion of associat ing alliOns used were 2, 5, 10, 15, and 20 p p m and the concen t ra t ion of P was 10 p p m (solution basis) in each case. F r o m a

* Present address: Institut für Bodenkunde, 34 Göttingen, Von-Siebold Strasse 4, West Germany.

- - 4 3 2 -

ASSOCIATING ANIONS AND P-RETENTION IN SOIL. II 43~

s t a n d a r d solut ion of associat ing anion, t he above ment io l led solutiollS were p repa red b y di lut ion in 250-ml vo lumet r ic flask. Before mak ing the final vo lume a sui table a l iquot of s t anda rd d isodium hydrogen p h o s p h a t e solut ion was p ipe t t ed in to t he f lask so as to give 10 p p m P on the f inal -volume basis. The p H of t he solut ions conta in ing P and associat ing anion were ad jus ted before making the final volume, •ollowing the procedure descr ibed earlier.

For c{trate, t a r t r a t e , oxa la te and s.ilicate t r e a tmen t s , P in t he clear ex t r ac t was e s t ima ted by the v a n a d o m o l y b d a t e m e t h o d as descr ibed by K o e n i g and J o h n s o n 7. For f luoride t r e a t m e n t , D i c k m a n and B r a y ' s m e t h o d 5 was used using boric acid 1 and P -es t ima t ion in presence of a rsena te was done by the v a n o d o m o l y b d a t e m e t h o d as proposed by R u b i n s and D e a n o using HBr .

The resul ts have been expressed exac t ly as in our earlier work.

RESULTS

The results will be described separately for each soil. Alluvial soil

The magnitude of phosphorus retention at pH 4.0, 6.0 and 7.0 when equilibrated with 50 ml of 10 ppm P solution in absence of any associating anion is given in Table 1.

TABLE 1

Retention of phosphorus in AIluvial soil at different pH in absenee of any assoeiating anion

P-cone. in Colm. of P in Initial pH of Equilibrium P adsorbed in ppm in equi-

ppm soln. ptI of soln librated soIn. soil in ppm

I0 4.0 7.8 7.50 62.S0

I0 6,0 7.9 8.06 48.60

I0 7.0 8.0 8.20 45.00

The behaviour of the anions is described separately for each pH system.

(i) I n i t i a l pH of the s o l u t i o n a d j u s t e d to 4.0. It can be seen from Table 2 that the presence of associating anions, in general, reduced the retention of phosphorus in soil and in very Iew cases as- sociating anions were found to be ineffective. Citrate, tartrate, oxalate and arsenate showed their maximum efficiency at the 10- ppm level of anion concentration while silicate and fluoride showed their best performance at the 5- and 15-ppm level of anion concen- tration, respectively. Arsenate and oxalate showed two maximum efficiency levels, the second level was found to be at 20 ppm for

434 D . L . D E B A N D N. P. D A T T A

T A B L E 2

Role of associating anions on the retention of phosphorus in A l l u v i a l s o i l :

F luor ide Arsena te / Si l icate

AniOcon~ t ep in Ap in Effi- ev in Ap in Effi- / ev in Av in Effi- / e»~i c ieney e~i~ e ieney %1-1 ciency

i1 p p m I p p m p p m % p p m p p m % p p m p p m %

Init«al pH o/ the mixed solußion is adjusted to 4.0

2 7.5 7.13 71.75 - -14.80 7.7 5 7.6 7.74 56.50 9.60 7.8

10 7.5 7.20 70.00 12.00 7.8 I5 7.7 8.80 30.00 52.00 7.7 20 7.5 8.00 50.00 20.00 7.8

7.87 7174 8.80 7.87 8i 80

Init ial pH o/ the mixed solution is adfusted to 6.0

2 7.6 7.75 56.25 -- 15.97 7.6 8.24 5 7.6 8.03 49.25 -- 1.55 7.7 8.43

10 7.6 8.09 47.75 1.55 7.7 8,98 15 7.7 8.19 45ù25 6.70 7.6 8,98 20 7.6 7.98 50.5'0 - -4 .12 7.5 8,98

Initial

2 5

10 15 20

pH o/ the mixed solution is adjusted to 7.0

7.7 7.79 55.25 - -22.76 7.7 8.33 7.7 8.60 35.00 22.20 7.7 8.33 7.7 9.30 17.50 61.05 7.6 9.54 7.7 8.45 38.75 13.88 7.6 8.72 7.6 9.40 I5.00 66.60 7.6 9.54

53.25 56.50 30.00 53.25 30.00

44.00 39.25 25.50 25.50 25.50

41.75 41.75 11.50 32.00 11.50

14.80 9.60

52.00 14,80 52.00

9.27 19106 47.38 47.38 47.38

7.22 7.22

74.37 28.87 74.37

8.0 8.1 7.6 7,8 7.7

7.69 57.75 8.60 35.00 8.21 44.75 7.77 55.75 7.77 55,75

7.8 8.60 7.9 9.46 7.9 9.25 7.9 8.44 8.0 9.40

7.9 9.24 7.9 9.57 8.0 10.26 8.0 9.49 8.0 9.30

35,00 13.50 18.75 39.00 I5.00

19.00 10.75

--6.50 12.75 17.50

7.60 44.00 28.40 10.80 10.86

27.8I 72.10 61.29 I9.57 69.01

57.72 76.04

100.00 71.60 61.05

Remarks:

e»i~ = p H of the equilibrated supernatant solution after 24 hours, ep = P concen t r a t i on in the equ i l i b r a t ed supernatant solution after 24 hours in p p m (solut ion basis), Av = P adsorbed b y soff in p p m (soiI basis).

arsenate and 15 ppm for oxalate. The fluoride ion was found to be ineffective at the 2- and 10-ppm level while all the other anions showed a gain in efficiency at all levels of anions used. The per- formance of citrate was best amongst the anions at the 10-ppm level, the entire amount of added phosphorus was found in the solution phase at this concentration level.

(ii) I n i t i a l pH of the s o l u t i o n a d j u s t e d to 6.0. Associating anions produced almost similar effect as before i.e. citrate, tartrate, oxalate, and arsenate showed their maximum efficiency at the 10-ppm level of anion concentration while silicate and fluoride showed their best performance at the 5- and 15-ppm level of anion concentration, respectively. Arsenate showed the second maximum at the 20-ppm

ASSOCIATING ANIONS AND P - R E T E N T I O N IN SOIL. I I 4 3 5

c:~~lcentration of phosphorus used is 10 ppm (solution basis) in each case

Citrate Tar t ra te Oxalate

~m- EfIi- ep in A~ in Effi- ep in Ap in ciency ep in Ap in cieney e»• ciency epR

e!°~ ppm ppm % ppm ppm % ppm ppm i%

7.8 8.1 8.1 8.l ~,.7

8.72 7.78

10.00 8.72 7.78

32.00 55.50 Nil 32.00 55.50

48.80 11.20

100.00 48.80 11.20

8.0 8.33 8.0 8.06 8.0 8.80 7.9 7.96 8.0 7.87

41.75 48.50 30.00 51.00 53.25

33.20 22.40

52.00 18,40 14,80

7.9 7.87 8.0 7.87 8.0 8.72 8. I 8.72 7.9 7.96

53.28 53.25 32.00 32.00 51.00

14.80

i4.80

48.80 48.80 18.40

8.1 7.9 8.0 8.0 8.0

8.15 8.15 9.26 8.33 8.10

46.25 46.25 18.50 41.75 47.50

4.64 4.64

61.80 13.91 2.06

7.8 7.96 7.9 7.96 7.9 9.26 7.9 8.33 7.9 8.33

51.00 51.00 18.50 41.78 41.75

--5.15 --5.15 61.80 13.91 13.91

7.9 8.00 7,9 7.60 8.0 8.50 7.9 8.00 8.2 7.80

50.00 60.00 37.50 50.00 55.00

--3.09 --23.69

22.68 --3.09

-- 13.39

8.1 8.1 8.1 8.1 8.2

10.00 10.00 11.13 10.00 10.56

Nil Nil

--28.25 Nil

-- 14.00

100.00 100.00 100.00 100.00 100.00

7.9 8.33 7.9 8.52 7.9 9.63 7.9 8.89 7.9 8.60

41.75 37.00

9.25 27.75 35.00

7.22 17.76 79.37 38.30 22.20

7.9 8.00 7.9 8.00 8.0 8.20 8.0 8.20 7.9 8.10

50.00 50.00 45,00 45.00 47.50

- 1 1 . 1 0 - 1 1 . 1 0

Nil

level. Fluoride at 2, 5 and 20 ppm, tartrate at 2 and 5 ppm, and oxa- late at all levels of anion concentrations (except the 10-ppm level) proved to be ineffective in reducing phosphorus retention in soff. In some of the cases phosphorus retention was encouraged. Other anions, however, reduced the retention of phosphorus at all levels and the performance of silicate and arsenate was consistent and better than that of citrate and tartrate.

(iii) I n i t i a l p H of t h e s o l u t i o n adj u s t e d to 7.0. The presence of an/ons in association with phosphorus reduced the retention of phosphorus by soil considerably. Spectacular effect was achieved in case of citrate, as the entire amount of added phosphorus could be recovered over the entire concentration range of citrate used. Citrate tartrate, arsenate, and silicate showed their maximum efficiency at the 10-ppm level of anion concentration while fluoride showed its best performance at the 20-ppm level and at this concentration

436 D . L . D E B A N D N. P . D A T T A

T A B L E 4

Role of associating anions on the retention of phosphorus in t H a c k soi l :

Fluoride Arsenate Silicate Anion

BOYIC.

Ln ppm ep in

ep~ p p m Ap in p p m

Effi- c iency

%

ep in e»~i p p m

Ap in p p m

Effi- c iency

% epti

ep in p p m

Ap in p p m

Effi- c ieney

%

Init ial p H o / themixed solution is ad]usted to 4.0

2 7.6 3 .47 163.25 - - 2 . 4 4 7.5 4.54 5 7.6 3.66 158.50 0.55 7.6 5.56

10 7.4 3 .30 167.50 - - 5 . 1 2 7.5 6 .02 15 7.6 2.60 162.60 - - 1 . 9 7 7.5 5.56

20 7.6 3 .85 153.75 3 .55 7.6 6.39

Inifial

2 5

10 15 20

pH o~ the mixed solution is ad/usted to 6.o

7.7 4.24 144.00 - - 5 . 0 2 7.8 5.46

7.7 4 .57 135.75 1.01 7.6 6.02 7.7 4.31 142.25 - - 3 . 7 4 7.6 6.67

7.7 4.73 131.75 3.93 7.6 6.48 7.7 5.57 110.75 19.26 7.6 6.48

Init ial

2 5

10

15 20

p H o/the mixed solution is ad/usted to 7.0

7.8 4.45 138.75 - - 7.80 7.7 6.02

7.8 4 .90 127.50 0.98 7.7 6.02 7.8 4 .75 131.25 - - I . 9 5 7.6 7.22 7.7 4.66 133.50 - -3 .71 7.6 7.32 7.7 5.00 125.00 2.93 7.6 7.87

136.50 111.00

99 .50 111.00 90.25

113.50

99.50 83.25

88 .00

88,00

99.50 99.5'0

69 .50 67 .00 53.25

14.41

30 .48

37.72 30.48 43.55

17.25 27.47 39.33 35 .86

35.86

22.82 22.82 46 .22

48.17

58.89

7.2 7.7

7.1

7.0 6.5

7.9

7.9 7.9 7.9

8.0

8.0

8.0 8.1

8.1 8.1

4,30

5,85 5,18

5.33 5.70

5.69 6.45 6.66

6.07

6.07

6.38

6.86 7.03

6.14 6.32

142.50 103.75

120.50 116.75

107.50

110.25 88.75 83.50 98.25

98 .25

90.50 78.50 74 .25

96.80 92.00

10.63 35 .05

24.49 26.86

32 .68

19,62

35 ,32 39 .15 28.88

23.38

29.84

39 .20 42.51 25.16 28.67

Remarks: See Table 2.

arsenate showed its second maximum. It is interesting to note that oxalate was ineffective at all levels of anion coneentration at pH 7.0 in reducing the phosphorus retention by soil. With the exception of the 2-ppm level of fluoride, all the other anions showed a gain in ef- ficiency in reducing the retention of phosphorus by soil at all the levels of anions used. The performance of silicate at the 10-ppm level was comparable to those of citrate.

Black soil The amount of phosphorus retained by Black soil from a solution

of the 10-ppm P-concentration in absence of any associating anion at pH 4.0, 6.0 and 7.0 is given in Table 3.

The behaviour of the anions is de~eribed below for each pH system.

ASSOCIATING ANIONS AND P - R E T E N T I O N IN SOIL. I I 437

concentration of phosphorus used is 10 ppm (solution basis) in each case

Citrate Oxalate

épH eF in ppm

Ap in ppm

Tartrate

Effi- ep in A~ in ciency ep~

% ppm ppln

Effi- ciency

%

el, in epu ppm

Ap in ppm

Effi- ciency

%

7.7 7.7 7.8 7.7 7.9

5.56 111.00 5,56 111.00 8.52 37.00 5.56 111.00 5.19 120.25

30.48 30.48 77.10 30,48 24.65

7.9 6.74 7.9 5.37 7.9 6.37 7.7 5.37 7.7 4.82

106.50 115.75 115.75 115.75 129.50

33.31 27.49 27.49 27.49 18.82

7.9 4.07 7.9 4.54 7.8 5.37 7.8 5.00 7.9 4.54

148.25 136.50 115.75 125.00 136.60

7.0t 14.41 27.49 21.66 14.41

8.2

8.0

8.0 8.1

8.1

4.75 131.25 5.37 115.75 5.93 101.75 4.75 131.25 4.75 131.25

4.29 15.61 25.83

4.29 4.29

8.0 5.00 7.9 5.56 8.0 5.00 8.0 4.82 7.9 5.56

125.00 111.00 125.00 129.50 111.00

8.85 19.07 8.85 5.57

19.07

8.0 5.80 8.0 5.10 8.0 6.00 8.0 5.60 8,1 5.00

105.00 122.60 100.00 112.50 125.00

23.45 10.68

27.10 17.98 8.85

8.2 8.2 8.2 8.3 8.3

7.78 7.78 9.45 7.22 7.41

55.50 55.50 13.75 69.50 64.75

57.14 57.14 89.70 46.22 49.92

7.9 5.19 7.9 5.74 7.9 5.93 7.9 5.00 7.9 5.10

120.25 106.50 101.75 125.00 122.50

6.63 17.36 21.06

2.93 4.88

8,0 5.30 8.0 5.00 8.0 5.70 8.1 5.30 8.1 5.30

117.50 125.00 107.50 117.50 117.50

8.78 2.93

16.58 8.78 8.78

TABLE 3

Retention of phosphorus in B l a e k soi! at different pH in absence of any assoeiat ing anion

P-eonc, in Conc. of P in Ini t ia l pH of Equi l ibr ium ppln in equi- P adsorbed in

ppm soln. pH of soln. l ibrated soln. soll in ppln

10 4.0 7.8 3.625 159.38 10 6.0 8.0 4.515 137.13 10 7.0 8.l 4.850 128.75

(i) I n i t i a l p H of t h e s o l u t i o n a d j u s t e d to 4.0. It can be seen from Table 4 that associating anions reduced the retention of phos- phorus by soil in most cases considerably. Fluoride and arsenate showed their maximum efficiency at the 20-ppm level, citrate and oxalate showed their best performance a t the 10-ppm level, tartrate and silicate showed their maximum effectiveness at the 2- and 5-ppm

438 D . L . D E B A N D N. P. D A T T A

T A B L E 6

Role of associating anions on the retention of phosphorus in Red soll:

Fluoride Effi-% Arsenatel Effi-% " Silieate Anion I ëp in Ap in es in Ap in

Effi- conc. e~ in Ap in eiency epg ciency epi~ ciency

n ppm %I4 ppm ppm ppm I ppm ppm ppm % [

Initial

2

5

10 15 '20

Initial

2

5

10 15 20

Initial

2

5

10 15 20

pH of themixed solution is ad~usted to 4.0

6.6 1.90 202.50 - -1 .25 6.5 3.98 5.7 2.58 185.50 7.25 6.7 3.80 5.5 3.00 175.00 12.50 6.8 4.82

5.7 3.30 167.50 16.25 6.8 4.36 6.0 3.65 15'8.75 20.63 6.9 5.65

p H o/¢he mixed solution is adjusted to 6.o

6.5 2.40 190.00 0.78 6.7 4.26 6ù6 3.30 167.60 12.48 6.4 4.36 6.7 3.73 156.75 18.07 6.5 4.75 6.8 3.78 155.50 18.72 6.6 5.09 6.8 3.95 151.25 20.93 6.6 5.65

pH o/the mixed solution is adjusted to 7.0

6.8 2.76 181.00 --0.91 6.7 4.07 6.8 3.50 162.50 9.45 6.7 4.45 7.0 4.05 148.75 17.15 6.7 4.91 7.0 4.51 137.25 23.59 6.7 5.19 7,0 4.60 135.00 24.85 6.7 5.46

150.50 155.00 129.50 141.00 108.75

143.50 141.00 131.25 122.75 108.75

148.25 138.75 127.25 120.25 113.50

24.75 22.50

35.25 29.50 45.63

24.96

26.26

31.33 35.75 43.08

17.43 22.75

29.19 33.11

36.89

4.7

6.3 4.6 4.5 4.5

6.6 7.0 7.0 7.1 7. I

6.8 6.6 7.1 7.2

7.3

3.35 3.96 4.44 4.51 4.81

3.96 4,64 4.81 4.81 6.33

4.07 4.68 5,32 4,84 5.12

166.25 151.00 139.00 137.25 129.50

151.00 134.00 129.75 129,75 116,75

148.25 133.00 117.00 129.00 122.00

16,88 24.50

30.50 31.38 35.13

21.06 29.90 32.11 32.11 38.87

17.43 25.97

34.93 28.21

32.13

Remarks: See Table 2.

level of anion concentration, respectively. With the exception of fluoride, all the other anions showed a gain in efficiency in reducing the phosphorus retention by soil at all levels of anion concentration used. The performance of citrate at 10 ppm level was best among the anions.

(ii) I n i t i a l p H of t h e s o l u t i o n a d j u s t e d to 6.0. Associating anions were eIfective in reducing the retention of phosphorus by soil almost over the entire concentration range. Citrate, oxalate, arsenate and silicate showed their maximum efficiency at the 10-ppm level and fluoride and tartrate showed their best performance at the 20-ppm level of anion concentration. Tartrate showed its second maximum at the 5-ppm level. With the exception of the 2-ppm and 5-ppm levels of fluoride, all the anions were effective at all concentrations in reducing the retention óf phosphorus by soll. The overall perfor- mance of arsenate and silicate was better than other anions.

ASSOCIATING A N I O N S A N D P - R E T E N T I O N IN SOIL. I l 439

d6ncentration of phosphorus used is I0 ppm (solution basis) in each oase

Citrate Tartrate Oxalate

/ Effi- Effi- ep in A~ in Effi- el, in Ap in ciency

ep in Ap in ciency e~~t ciency ep~l ppm ppm %o1-1 ppm ppm % ppm ppm % %

4.7 4.7 4.7 4.7

4.5

5.00 3.89 4.75 3.52 3.70

125.00 152.75 131.25 162.00 157.50

37.50 23.63 34.38 19.00 21.25

6.7 3.52 6,7 3.24 6,9 3.98 6.9 2.96 7.0 2.96

162.00 169.00 150.50 176.00 176.00

19.00 15.50 24.75 12.00 12.00

6.0 3.24 5.5 3.24 5.8 3.61 5.5 3.61 5.4 4.26

169.00 169.00 159.75 159,75 143.50

15.50 15.50 20.13 20.13 28.25

7.0 6.9 7.2 7.0 7.3

4.07 3.05 3.70 3.05 4.07

148.25 173.75 157.50 173.75 148.25

22.49 9.23

17.68 9.23

22.49

6.6 3.89 6.5 3.33 6.8 4.07 7.2 3,33 7.2 4.07

152.75 166.75 148.25 166.75 148.25

20.15 12.87 22.49 12.87

22.49

7.1 3.10 7.0 3.30 7.0 3.70 7.0 3.10 7.3 4.00

172.50 167.50 157.50 172.50 150.00

9.88 12.48 17.68 9.88

21.58

7.1 7.2 7.5 7.6 7.6

5.56 5.93 6.30 4.82 5.37

111.00 101.75 92.50

129.50 115.75

38.29 43.47 48.65 27.93 35.63

6.7 3.52 6.7 3.05 6.9 4.07 7.0 3.05 7.0 3.33

162.00 173.75 148.25 173.75 166.75

9.73 3.15

17.43 3.15 7.07

7,0 3.60 6,9 3.70 7.0 3.90 7.3 3.20 7.3 3.60

160.00 157.50 152.50 170.00 160.00

10.85 12.25 15,05 5,25

10,85

(iii) I n i t i a l pH of the s o l u t i o n a d j u s t e d to 7.0. Associating anions proved their worthiness in reducing the phosphorus retention in soff over the entire concentration fange leaving aside the exception of fluoride. Silicate, citrate, tartrate and oxalate showed their maxi- mum efficiency at the 10-ppm level while arsenate and fluoride showed their best performance at the 20-ppm level of anion concen- tration. Fluoride was ineffective at the 2-, 10-, and 15-ppm levels of anion concentration. The performance of citrate at I 0 ppm was found to be best among anions.

Red soil

The amount of phosphorus retained by Red soll from a solution with a concentration of 10 ppm P in ab8ence of any associating anions at pH 4.0, 6.0, and 7.0 is given below in Table 5.

440 » . L. DEB AND N. P. DATTA

TABLE 5

Retention of phosphorus in l t e d so i l at different pH in absence of associating anion

P-conc. in Conc. of P in Ini t ia l pH of Equi l ibr ium equil ibrated P adsorbed in

pprn soln. pH of soln. so]n in ppm soll in ppm

10 4.0 6.7 2.00 200.00 10 6.0 6.6 2.34 I91.50 10 7.0 7.4 2.825 179.38

The behaviour of the anions in association with phosphorus has been presented in Table 6 and the salient features have been de- scribed below for each pH system.

(i) I n i t i a l p H of t h e s o l u t i o n a d j u s t e d to 4.0. The presence of associating anions reduced the retention Of phosphorus by soil considerably in majori ty of the cases. Fluoride arsenate, silicate and oxalates howed their maximum efficiency at the 20-ppm level, where- as citrate and tartrate showed their maximum efficiency at the 2- and 10-ppm level of anion concentration, respectively. With the only exception of the 2-ppm level of fluoride, all the assoeiating anions at all levels showed a gain in efficiency in reducing the retention of phosphorus by soil. The anions did not differ rauch in their efficiency. The performance of the anions over the entire concentration range was almost of the same order.

(ii) I n i t i a l p H of t h e s o l u t i o n a d j u s t e d to 6.0. The associ- ating anions reduced remarkably the phosphorus retention in soi I. In all the cases, anions reduced the phosphorus retention and all the six anions showed their maximum efficiency at the 20-ppm level of anion concentration. Citrate and tartrate showed their second maxi- mum at 2 ppm and 10 ppm, respectively. Overall performance of arsenate and silicate was better than the rest.

(iii) I n i t i a l p H of t h e s o l u t i o n ad j u s t e d to 7.0. The presence of associating anions reduced the phosphorus retention in most cases. Citrate, tartrate, oxalate and silicate showed their maximum efficiency at the 10-ppm level while fluoride and arsenate showed their maximum efficiency at the 20-ppm level of anion concentration, With the only exception of the 2-ppm level of Ihoride, all the six anions showed their effectiveness in the reduction of phosphorus by soil at all the levels of anion concentration. The overall performance of citrate was best among the anions.

ASSOCIATING ANIONS AND P-RETENTION IN SOIL. II 441

DISCUSSION

It is evident from the results of this investigation that the effect of the associating anions, at different coneentration, on phosphorus retention in soil depends considerably on the nature and properties of soll and the pH of the reacting medium. In AUuvial soil, the associating anions have shown eomparatively higher effectiveness in counteraeting the phosphate retention reactions than in Black and Red soils. Such effect is attributable to its low phosphorus retention ability due to the less activity of phosphorus retention mechanisms.

In general, it has been observed that citrate is most efficient over the entire pH range of the experiment, which is in confirmation with our earlier observations a. But it is not understood why citrate showed comparatively poor performance at pH 6.0. The performance of other anions differs from soil to soil. Tartrate shows its best effectiveness in an acid medium in Red and Black soils, but in Alluvial soil its performance at neutral pH is found to be best. Over the en- tire pH range of the experiment in Red and Black soils silicate seems to be equally effective but its effectivity in AUuvial soil shows a de- cided tendency to increase with the increasing pH of the reaction medium. Oxalate counteracts phosphate retention mechanisms more effectively in acidic medium while arsenate seems t O be more effective in alkaline medium. Thus it appears that the effectivity of the as- sociating anions in different soils differs depending on the nature of reaction between phosphate retention mechanisms and anions under different pH of the reaction medium. At comparatively higher acidie medium, phosphate retention is m.ainly due to active Fe, A1 and the anion-exchange mechanisms of soil. The effectiveness of organic anions in acidic medium is mainly due to their ability to form stable complexes with active Fe and A1 of soil and thereby prevent the solution phosphorus to form basic iron and aluminium phosphate. So the retention of P under such conditions is mainly caused by the anion-exchange mechanisms, since organic anions cannot inactivate them (D e a n and R u bi n s 2). The superiority of organic anions over inorganic anions in reducing phosphorus retention in soil confirms out earlier observations 4 and substantiates the findings of S t r u - t h e r s and S i e l i ng 11 and S w e n s o n et al la. Fluoride also forms stable complexes with active A1 in an acid medium ( T u r n e r and R i c e la and S p e c h t and M a c I n t i r e lo). In addition to this, Iluo-

4 4 2 D. L. DEB AND N. P. DATTA

ride has got a strong affinity for the anion exchange complex of soil ( D i c k m a n and B r a y 6 and K u r t z el al.S). It is surprising to ob- serve that fluoride has not shown any notable evidence of counter- acting the phosphorus retention reactions in soll even at higher concentration, although it was thought in our earlier work the inef- fectivity of fluoride is mainly due to its low concentration. The inef- fectiveness of fluoride may be assumed to be possibly due to the fact that the inactivation of A1 alone is not enough to reduce phosphorus retention in soil due to the dominance of active Fe. It may also be assumed that as the aluminium fluoride complex is not very stable under alkaline reaction, the rise in pH of the medium after equi- librium makes the Iluoride complex unstable and A1 is set free which then reacts with phosphate leaving very little phosphorus in equi- librium solution. The comparatively hefter performance of fluoride in Red soll may be explained by assuming that the aluminium fluoride complex in Red soil is comparatively more stable as the pH of the equilibrated solution remains acidic especially when the initial pH has been adjusted in the acid range. The mechanisms by which silicate inactivates the phosphate retention reactions in soil has already been discussed in our earlier paper 4. Arsenate forms insolu- ble compounds with Fe and A1 under slightly acidie to acidic con- ditions and at higher concentration it competes with phosphate for occupying a place in the positively charged spots of the soil colloid and thereby prevent phosphorus retention.

At a neutral to slightly acid medium, the phosphate retention is mainly caused by the adsorbed Ca and free CaCO3 of soil. As the maonitude of phosphorus retention in soil at such pH is compara- tively low andsince at such pH precipitation reactions are mainly responsible for inactivating Ca, the associating anions which are capa- ble of forming comparatively more insoluble comp0unds with Ca than with phosphate, are likely to show higher effectiveness in counter- acting phosphorus retention in soil as can be seen from the per- formance of citrate, arsenate and silicate. Fluoride has hown in A1- luvial and Red soils comparatively higher effectiveness at neutral condition which is possibly due to the inactivation of exchangeable Ca. The ineffectiveness of fluoride in Black soll is assumed to be due to its high content of exchangeable Ca and frëe CaCO3 and the latter possibly favours the formation of fluorapatite at higher pH. Oxalate also forms a stable eompound with Caun der neutral to alkaline con-

ASSOCIATING ANIONS AND I~-RETENTION IN SOIL. II 443

dition as can be seen by its performance in Red and Black soils. It seems to be difficult to explain why oxalate has failed to exert any, benificial effect in Alluvial soll under neutral reaction medium. On the other hand, it has increased phosphate retention by soil.

The effect of anion concentration in counteracting phosphorus re- tention in soll is not very clear. It was thought that the higher concentration of assocJating anions would be able to counteract the phosphorus retention reactions in soil more effectNely than the corresponding treatments with a lower anion concentration, It has been observed that associating anions, in general, have exerted their maximum beneficial effect at the 10-ppm level of anion concen- tration. It appears, therefore, that the concentration of associating anions should be at least of the same order as that of phosphorus in solution so as to be effective in counteracting the phosphorus re- tention reactions in soil. It is not understood why assoeiating anions at higher concentrations could not produce any remarkable per- formanee in this respect.

SUMMARY

The ef fec t iveness of assoc ia t ing an ions in r educ ing p h o s p h o r u s r e t e n t i o n in soff depends cons iderab ly on t h e i r phys icochemica l n a t u r e a n d t he cond i t ions u n d e r which t h e y reac t w i t h soll c o n s t i t u e n t s especia l ly t h e p H of t h e r eac t ion med ium. Organic anions, in general , are found to be super ior to inorganic an ions a n d t h e p e r i o r m a n c e of s i l icate is be s t a m o n g t h e inorganic an ions a n d in some cases i ts pe r f o r m ance is c o m p a r a b l e to t hose of organic anions. Organie an ions are h igh ly effect ive in r educ ing p h o s p h o r u s r e t e n t i o n in soll in a n aeid m e d i u m whi le t he inorganic an ions are i o u n d to be more effect ive u n d e r s l ight ly acidic to n e u t r a l condi t ions . Citrate , however , is f ound to be m o s t effect ive u n d e r a n e u t r a l condi t ion . The c o n c e n t r a t i o n of assoc ia t ing an ions should be a t leas t of t h e sa lne order as t h a t of p h o s p h o r u s in so lu t ion in o rder to be m o s t effeet ive Jn r educ ing p h o s p h o r u s r e t e n t i o n in soff. H i g h e r levels of an ion c o n e e n t r a t i o n do no t exer t any r e m a r k a b l e effect on p h o s p h a t e r e t e n t i o n reac t ions .

Received April 29, 1966

REFERENCES

1 Bray , R. H. and Kur tz , L. T., Determination of total, organic and available forms of phosphate in soils. Soll Sci. 59, 39-45 (1945).

2 Dean, L. A. and Rub i ns , E. J., Anionexchange in soils. I. Exohangeable phosphorus and the anion exchange capacity. Soil Sei. 63, 377-387 (1947).

4 4 4 ASSOCIATING ANIONS AND P - R E T E N T I O N IN SOLL. II

3 Deb , D. L. and D a t t a , N. P., Effect of degree of base saturat ion and in i t i a lphospha te enr ichment of soll Oll the utilization of soff and fertilizer phosphorus by rye. Plant and Soll 27, (in press) (1967).

4 Deb , D. L. and D a t t a , N. P., Role of associating anions on phosphorus retention in soll: I. Under variable phosphorus coneentration. Plant and Soff .oB, 303-314 (1967).

5 D i c k m a n , S. R. and B r a y , R. H., Colorimetric determination of phosphate, Ind. Eng. Chem. AnM. Ed. 12, 665 (1940).

6 D i c k m a n , S. R. and B r a y , R. H., Replacement of adsorbed phosphate from kaolinite by fluoride. Soil Sei. 5.o, 263-273 (1941).

7 K o e n i n g , R. A. and J o h n s o n , C. R., Colorimetric determination of phosphorus in biological materials. Ind. Eng. Chem. AnM. Ed. 14, 155-156 (1942).

8 K u r t z , T., De T u r k , E. E., and B r a y , R. H., Phosphate adsorption in Illinois soils. Soil Sei. ö l , 111-124 (1946).

9 R u b i n s , E. J. and D e n n , L. A., Anion exehange in solls. II. Methods of s tudy. Soil Sei. BS, 389-397 (1947).

10 S p e c h t , R. C. and M a c l I l t i r e , W. H., Fixation, leaching and plant uptake of fluorine from additions of certain fluorides in representat ive sandy solls. Soil Sci. 02, 172-177 (1961).

11 S t r u t h e r s , P. H. and S i e l i n g , D. H., Effect of organic anions iI1 phosphate precipitation by iron and a luminium by pH. Soil Sei. 69, 205-213 (1950).

12 S w e n s o n , R. N., Cole, C, V., and S i e l i n g , D. H., Fixation of phosphate by iron and a luminium and replacement by organic and inorganic ions. Soff Sei. üT, 3-22 (1949).

13 T u r n e r , R. C. and Rice , H. M., Role of fluoride ion in the release of phosphate adsorbed by a luminium and iron hydrooxides. Soll Sei. 7~, 141-148 (1952).