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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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