2. TITLE AND SUBTITLE

Phosphate absorption by soils, 1: Influence of time and ionic environment on

phosphate absorption

3. AUTHOR(S)

Rajan, S.S.S.; Fox, R.L.

4. DOCUMENT DATE j .NUMBER OF PAGES 6. ARC NUMBER

1972 12 p. ARC

7. REFERENCE ORGANIZATION NAME AND ADDRESS

University of Hawaii, Department of Agronomy and Soil Science, College of Tropical Agriculture, Honolulu, Hawaii 98622

8. SUPPLEMENTARY NOTES (Spona:rlng Otaanlzatlon, Publiehora, Availability) Hawaii Agricultural Experiment Station Journal Series No. 1472)(In(In Commununity in Soil Science and Plant Analysis, v. 3, no. 6, p. 493-504)

9. ABSTRACT

The influence of reaction time and ionic environents on phosphate adsorption were

studied using one calcareous soil from India, and one calcareious and two latosolsfrom Hawaii. Phosphorus adsorption by soils has an initial rapid phase followed

by a slow process. For plant nutrition studies, in which empha;is is on P concen­tration of solutions from which plants derive P, isotherms shou~d be constructed using data obtained after near-equilibration has been attained. This condition does not obtain in a few hours and m y require 6 days or more. Calcium chloride

wasas suspending electrolyte always gave lower phosphate solubility than when KCl used as electrolyte. Phosphate retention increased with increasing ionic strength. The necessity for obtaining clear supernatant solutions and the desirability for maintaining reasonable constant equilibrium conditions make 0.01 M CaCI2 a reasonable choice for constructing P sorption isotherms, even though 0.01 M#aCl 2 is not representative of Ca concentration in many soil solutions. Saturation extracts of soils investigated here were in the range 0.0002 to 0.005 M Ca. Adsorption of calcium by highly weathered soils was high, suggesting spec-ific adsorption. Calcium adsorption was increased by phosphate additions to a Hydrandept.

10. CONTROL NUMBER I1, PRICE OF DOCUMENT

PN-AAC-362

,. DESCRIPTOs ,3. PROJECT NUMBER

AdsorptionCalcium Ionic mobility

PhosphatesReaction time Tropics

14. CONTRACT NUMBER CSD-2833 211(d)

Isothermal treatment r. TYPE OF DOCUMENT

AID 90,l1 4-74)

,4 ' }"I.- j (.

C024. IN SOIL SCIENCE AND PLANT ANALYSIS, 3(6), 493-50h (1972)

PHOSPHATE ADSORPTION BY SOILS 1. INFLUENCE IF TIME AND IONIC

ENVIRONMENT ON PHOSPHATE ADSORPTION

YEY WORDS: Adsorption isotherms, Tropical soils, calcium

S. S. S. Rajan and R. L. Fox

Department of Agronomy and Soil Science College of Tropical Agriculture, University of Hawaii

Honolulu, Hawaii 96822

ABSTRACT

The influence of reaction time and ionic environments, on phosphate

adsorption were studied using one calcareous soil from India, and one cal­

careous and two latosols from Hawaii.

Phosphorus adsorption by soils has a initial rapid phase followed by

a slow process. For plant nutritioT, studies, where emphasis is on P con­

centration of solutions from which plants derive P, isotherms should be

constructed using data obtained pter near-equilibration has been attained.

This condition does not obtain in a "-w hours and may require 6 days or

more.

Calcium chloride as suspending electrolyte always gave lower phos­

phate solubility than when KCI was used as electrolyte. Phosphate reten­

tion increased with increasing ionic strength. The necessity for obtaining

clear supernatant solutions and the desirability for maintaining reason­

able constant equilibrium conditions make 0.01 M CaC12 a reasonable

IContribution from tho Agricultural Experiment Station of the

University of Hawaii, Journal Series No. 1472.

493

Copyright I 1972 by Morcel Oekker, Inc. NO PART of this work may be reproduced or utilized In any form or by any means, electronic or mechanical, including xerography, photocopying, microfilm, and re­cording, or by any informatlon storage and retrieval system, without the written permission of the publisher.

RAJAN AND FOX

choice for constructing P sorption isotherms, even though 0.01 H CaC12

is not representative of Ca concentrations inmany soil solutions.

Saturation extracts of soils investigated here were in the range 0.0002

to 0.005 M Ca.

Adsorption of calcium by highly weathered soils was high suggesting

specific adsorption. Calcium adsorption was increased by phosphate

additions to a Hydrandept.

INTRODUCTION

Colloids of highly weathered tropical soils, especially those rich in

hydrated oxides of iron and aluminum are very adsorptive for phosphate.

Consequently, P concentrations in solution are closely associated with

adsorptive properties of soils. Knowledge of phosphate adsorption char­

acteristics is important for making rational statements about phosphate

availability and phosphate fertilizer requirements. Phosphorus concentra­

tions in soil solutions of highly weathered agricultural soils are usually

0.001 ppm to 0.1 ppm, but may be much higher in or near zones of fertilizer

placement. This necessitates extension of P adsorption studies to higher

concentrations than are usually found in well equilibrated soils.

The present study is the first of three reports on a detailed inves­

tigation of phosphate adsorption by several Hawaiian and Indian soils in

relation to (a)reaction time and ionic environment (b)concentration and

temperature and (c)desorption of hydroxyl, sulfate and silicon.

MATERIALS AND METHODS

Influence of Reaction Time onP Adsorption

The influence of reaction time on phosphate adsorption was studied

using three soils namely Lualualei, Wahiawa and Akaka soils. These soils

differ greatly in their mineralogical and chemical properties (Table 1).

4.94

PHOSPHATE ADSORPTION BY SOILS. I

TABLE 1

Some Characteristics of the Experimental Soils

Soils

Black Soil Lualualei Wahiawa Akaka

(Indian Black Earth)

Particulars

Soil Order Vertisol Vertisol Oxisol Inceptisol

Rainfall (cm/year) 53 50 100 450+

P.rent material Granite Alluvium Basalt Volcanic Ash

Important M M K, Sesq. Amor. Sesq.

Secondary mineralh

pH (H20 Paste) 7.29 7.80 5.92 3.99

ApH -1.11 -1.30 -.0.32 +0.24

Organic carbon % 0.49 0.96 1.63 9.21

Surface P (Ug/g soil)** 20 29 1 350

M - Montmorillonite, K = Kaolin, Sesq. = Sesquioxides, Amor. Sesq. =

Amorphous Sesquioxides

** Surface P is a measure of initially adsorbed (native) soil P.

Soil samples (1.5 g,<1 mm) were introduced into 50 ml centrifuge

tubes with 30 ml of 0.01 M CaC12 containing Ca(H2PO4 )2 . The initial P

concentrations in the CaCl 2 solution were 35, 70 and 425 ppm for the

Lualualei, Wahiawa and Akaka Soils respectively. Two drops of toluene

were added to each sbmple and the suspensions were shaken one hour twice

° daily. Temperature was i5 C. Samples were removed after 1, 2, 4, 6, 8

and 10 days. They were centrifuged at 250 C. P was determined in the

P removed from solution was considered to have been supernatant solution.

adsorbed.

I495

RAJAN AND FOX

Influence of Ionic Environment on P Adsorption

Along with the Wahiawa and Akaka Soils from Hawaii, a Black Soil

collected from Coimbatore (Southern part of India) was used in a study of

the effect of ionic environments on P Adsorption. Four concentrations

(0.0, 0.001 M, 0.01 M and 0.1 M) of KCl and CaCl2 were used. When the

supporting electrolyte was KC1, P was added as KH2PO4 and when the elec­

trolyte was CaCl2 , P was added as Ca(H2P04 )2. Initial P concentrations

were 7, 40 and 233 ppm for the Black Soil of India, Wahiawa soil and Akaka

soil respectively. The soils were equilibrated for 6 days after which pH,

P, K and Ca were estimated in the supernatant solutions.

All the experiments were done in duplicate and the results presented

are means of two values.

RESULTS AND DISCUSSION

Use of sorption isotherms is gaining increasing acceptance for

estimating phosphate availability and requirments of soils 1,2,3,4,5,

6,7. By this type of procedure, phosphate adsorbed by soils at an equi­

librium P concentration associated with any given yield is an estimate of

the P requirement for that yield. Equilibration time and ionic environ­

ments - the kind of cations and ionic strength - greatly influence P

adsorption and as a consequence P remaining in solution. Therefore,

standardized methodology is required to obtain comparable results.

Influence of Reaction Time on P Adsorption

Relative P adsorption was calculated at different times of equili­

bration. In all soils the adsorption patterns were characterized by an

initial fast reaction followe: hy a slow process (Figure 1) which agrees

'10 '9

among soils. The rates of P adsorption arranged according to the

soils are Akaka (x-ray amorphous hydrated oxides) > Wahiawa (kaglin

496

with earlier observations8 . Some interesting comparison can be made

PHOSPHATE ADSORPTION BY SOILS. I

10

IL-- AKAKA o(o,8 I-0-"I--e

0 WAHIAWA LUALUALEI

6 100 2 TIME (DAYS)

FIG. 1

Fractional P sorption by three soils plotted as a function of reaction

time (based on 10 days = 1)

and iron oxide) > Lualualei (2:1 and CaCO3). This is also the order

for the magnitude of P immobilization by various mineralogical systems

suggested by Fox et al.11 .

The proportion of P added in relation to the adsorption maxima

of soils (adsorption maximum wau calculated employing Langmuir's

equation) is given in Table 2. Rapid P adsorption by the Akaka soil

can be attributed to the low proportion of P added in relation to the

adsorption maximum and probably to rapid intraparticle diffusion of P

in this soil compared to that of Wahiawa soil.

Although the Lualualei soil was easily dispersed inwater and

P added in relation to adsorption maximum was less than for the

Wahiawa soil, the rate of phosphate adsorption was slowest for this

soil. The plausible explanation is the nature of reactive sites,

associated with carbonates of calcium in this soil.

49T

P

RAJAN AND FOX

TABLE 2

Proportion of P Added in Relation to P Adsorption Maxima of Soils -Adsorption Maximum was calculated Using Langmuir's Adsorption Equation

Soils P Adsorption Vg P added / g soil P added Maxima (Vg/g Soil) P adsorption maximum

Lualualei 600 700 1.17

Wahiawa 900 1,400 1.56

Akaka 8,000 8,500 1.06

Data from the equilibration time study are presented as a plot

of adsorption vs time (Figure 1) and as P concentration vs time

(Figure 2). Data plotted as in Figure 1 is adequate for presenta­

tion of initial phosphate reactions. The adsorption was 85% completed

after 24 and 48 hours equilibration for latosols and montmorillonitic

soils respectively (based on 10 days = 100%).

From the point of view of plant nutrition, however, solution

concentration plotted against reaction time is more appropriate since

interest here is centered on the concentration of P in the solution

from which the plant derives its nutrients. Figure 2 shows a great

decrease, in relative terms, between 2 and 6 days. In the "high" P

fixing Akaka soil, P concentration after 6 days was only 1/3 that

after 2 days equilibration. Reasonably stable levels of P in solution

are attained only after 6 days equilibration. This confirms an earlier

report by Fox and Kamprath I0 .

A significant point for making fertilizer recommendations is

that the relative position of the three soils with regard to P concen­

trations were different when compared at 2 and 6 days.

49 8

I PHOSPHATE ADSORPTION BY SOIIB.

430

230 SOILS -- Luoluolol

30 .o--.o &--

Wohiowc Akoko

0

- IG22 (51) IL - _, "-7-Z0

0

4 (0400)

6 10Oo 2

REACTION TIME (doys)

FIG. 2

Numbers in paren­in relation to reaction time.P remaining in solution

indicate the amount of P sorbed after 6 days equilibration.

theses

Observations of slow equilibration by Hawaiian soils have been

3' 8. This may be attributed to slow diffusion of P

reported earlier

through extremely small pores of stable aggregates. Amorphous materials

These have been implicated in phosphate sorption

coat many surfaces. 2 . It is probableare silicon depletedespecially for those soils that

that slow phosphate equilibration is associated with the occurrence

of

thick, amorphous, gel-like coatings.

Influence of Ionic Environment on P Adsorption

Calcium chloride and KCl are commonly used as ionic environments

Of the two, Ca in solution during the equilibration of P

with soils.

Phosphate additions also always increased P adsorption (Table

3).

499

J

RAJAN AND FOX

TABLE 3

Effect of Concentration of Two Salts on pH and P Concentration of the Equilibrium Solution, and Adsorption of P, Ca and K by Three Soils

Added P Cations Adsorbed Salt concentration in solution Adsorbed (ig/g) pH

(M/1) (g/ml) (pg/g soil) K Ca

Black Soil (India) KCI none 2.76 81 -- 8.02

0.001 2.36 86 28 -- 8.21 0.010 1.66 100 190 -- 7.59 0.100 1.33 106 900 -- 7.30

CaCl2 none 2.82 78 -- -- 8.00 0.001 2.14 90 -- -82 7.71 0.010 1.29 107 -- 1376 7.21 0.100 1.12 111 6160-- 6.70

Wahiawa Soil KCl none 6.57 668 .-- 6.08

0.001 4.46 711 ---11 5.73 0.010 2.69 746 56 -- 5.20 0.100 1.77 765 540 -- 5.19

CaCl2 none 5.37 693 -- -- 5.76 0.001 3.09 --738 501 5.32 0.010 1.69 766 -- 1544 5.07 0.100 1.61 768 -- 8800 4.78

Akaka Soil KCI none 7.79 4504 -- -- 5.23

0.ol 5.78 4551 -153 -- 4.31 0.010 3.13 4604 -18 .- 4.24 0.100 1.31 4641 440 -- 4.30

CaCl2 none 2.22 4616 -- -- 4.08 0.001 2.12 4625 -- 610 (87)* 4.05 0.010 1.49 4637 -- 2728 (1550) 4.05 0.100 1,15 4644 -- 12600 (10,000) 3.98

*Values in parenthesis indicate pg of Ca adsorbed by Akaki soil when the soil was equilibrated with CaCl2 solutions containiag no P.

increased calcium retention (Table 3, Akaka soil). Equilibrating

15 6solutions have been NaH2P04 , Ca(H2PO04)216 ,17, KH2P04 ,

78 ,16,18,

NaOac + NaH2PO41 ,3 , CaCl2 + KH2PO 19,20 or CaC1 2 + Ca(H 2P04 )2

2 ,5,l0.

Although the depressing effect of Ca in solution, as compared with K

500

I PHOSPHATE ADSORPTION BY SOILS,

1 3 1 4 ,or Na on P solubility has been known for some timel 2 , the fact

,that so many different ionic environments have been used is evidence

that the point is not sufficiently appreciated.

Soils suspended in CaC12 solutions have lower pH than in KC1

(Table 3). However, attributing higher P adsorption to lower pH is

not a satisfactory explanation, because pH changes do not necessarily

Afluence P adsorption21 . Another suggested mechanism is co-adsorption

.of phosphate with exchangeable catioisl

3 Amounts of Ca retained by

soils increased in the following order: Black soil (2:1 clay) < Wahiawa

soil (1:1 clay)< Akaka soil (amorphous hydroxides) (Table 3). The order

for K retention was the reverse. Specific adsorption of phosphate is known

to increase negative charge and increase cation retention by soils22 ,23 ,24 .

However, this mechanism of adsorption does not account for the big dis­

crepancy between Ca and K retention. Apparently, specific adsorption of

calcium is involved here. An indication that this occurs in Tropical

soils was reported by Mikami and Kimura25 . This and other evidence

for specific adsorpti;n of calcium through covalent forces has been

presented recently (Uehara, G., L. D. Swindale and R. C. Jones.

Mineralogy and behavior of Tropical Soils. Manuscript submitted for

Special Publication, International Institute of Tropical Agriculture,

Ibaden, 1972).

To a limited extent, increased ionic strength also enhanced P

adsorption. In most soils, increasing iontc strength is associated with

decreasing pH (Table 3). Definite proof is not available to show whether

the higher adsorption is directly related to ionic strength or to the

increase in acidity.

A practical point of interest is that the change in solution P

coicentration is high with variation in ionic strength, and especially

at the low range of ionic strength.

501

RAJAN AND FOX

The procedures used for constructing P sorption isotherms results

in dilution of dissolved salts in soil solutions. Most investigators

have attempted to standardize electrolyte concentrations by suspending

soils in 0.01 M CaCl2. If this is not done, values for P concentration

in solution may be high by a factor of 2 or more (Table 3).

Calcium chloride is usually the preferred salt because Ca is the

dominant cation in most agricultural soils and phosphate fertilizers.

Calcium chloride extracts of mineral soils are clear: an important con­

sideration when determining P in the fractional parts per million range.

The soils investigated contained from 0.0002 to 0,004 M Ca. The range of

Ca in the saturation extract for 8 other Hawaiian and Indian soils

was .0007 to .005 M which makes the rationale of using 0.01 M CaCl2

somewhat dubious. Nevertheless, the selection of 0.01 M CaCl2 is

probably fortunate because it will compensate for small fluctuations in

Ca contents of solutions caused by adsorption.

SUMMARY AND CONCLUSION

The influence of two factors, reaction time and ionic environments,

on phosphate adsorption were studied using three Hawaiian soils and one

Indian soil.

The rates of phosphate adsorption varied with soils. In general,

adsorption patterns where characterized by an initial fast reaction

followed by a slow process. When the object of constructing phosphate

adsorption isotherms is understanding initial surfa!e reaction, it may

suffice to allow only enough time for completion of the fast sorption.

For studies of plant nutrition, especially when comparisons are being

made among soils of varying phosphorus fertilization histories, isotherns

should be constructed after near equilibrium has been attained.

502

PHOSPHATE ADSORPTION BY SOILS. I

The ionic environment influences phosphate adsorption greatly.

Equilibration in solutions containing calcium instead of potassium and

increasing ionic strength always decreased phosphorus concentration of

equilibrated solutions. As a standard ionic environment, 0.01 MCaCl2

is a logical choice because calcium is a dominant cation in most­

agricultural soils and use of calcium chloride always provided a clear

Also, when a high concentration of CaCl2 is used,supernatant solution.

fluctuation in calcium concentration caused by adsorption of calcium

by soils does not affect phosphate adsorption appreciably. Adsorption

of calcium by Latosols was surprisingly high suggesting some specific

adsorption mechanism.

ACKNOWLEDGMENT

The senior author wishes to thank the Center for Cultural and

Technical Interchange between East and West (East West Center), Honolulu

for a scholarship which maae this study possible.

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Soil Sci. Adelaide, Australia. 2, 301 (1968)

Soil Sc. and Plant Nutrition. 10, 20 (1964)

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