A D S O R P T I O N OF P O T A S S I U M AS INFLUENCED
BY C O N C E N T R A T I O N A N D p H OF THE
SOL UTIO N
By S. P. MITRA and DHARAMPRAKASH,
Sheila Dhar Institute of Soil Science, University of Allahabad, India.
[MS. Received 30th October, 1956.]
ABSTRACT
The adsorption of potassium by clay minerals under varying conditions
of concentration and pH has been studied. It seems from the experimental data that montmoriUonite shows the greatest adsorption at all
pH values. The adsorption is found to vary with the concentration of
potassium in the solution, pH of the solution and the nature of the
cations in the exchange complex. Data are presented for kaolinite,
montmorillonite, halloysite and natrolite.
INTRODUCTION
It is well known that clay minerals adsorb certain cations such as
K +, NH4 + and Rb +. The present paper includes some studies on
the adsorption of potassium by some minerals at different concentrations from solutions of KH2PO4, K2HPO4 and KzPO4, i.e.,
within a p H range of 5.2 to 11-8.
EXPERIMENTAL
The adsorption experiments were conducted on the ovendried
samples. Four g of minerals were treated with KH2PO4, K2HPO4
and K~PO4 solutions of varying concentrations, shaken for an hour
in a mechanical shaker, kept for two days and filtered through a
buchner funnel. The clear filtrate was analyzed for potassium which
was estimated as potassium sodium cobaltinitrite.
DISCUSSION
The results in Table 1 clearly show the adsorption of potassium by
montmorillonite, kaolinite, halloysite and natrolite, both from acid
and alkaline salts of potassium of concentrations varying from 0.02
M to 0.001 M. The adsorption is a function of concentration. It
decreases with a decrease in the concentration of the salt added.
The decrease in the amount of potash adsorbed is only proportional
in a few cases to the decrease in concentration. Montmorillonite
adsorbs the greatest amount of potassium at p H values from 5.2 to
151
152
S. P. MITRA A N D D. PRAKASH
o
9~,~
~ o ~ o ~
e.qOOlP~.~-~,
0 0 0 0 0 0
~'~0
4,
0
%
0
o ~ _o
~o~
9. - ~
~ t ~
. o o
0
~'~0
=
im
0
0
E
0
0
~'~0
.<
<
0
0
.,e
, ~. ~00
O~"g~0 0 0 0
0
0
O0
O~o
0
<
POTASSIUM ADSORPTION BY CLAYS
153
11-8, while kaolinite and halloysite adsorb a much smaller amount.
Natrolite adsorbs a greater amount of potassium than either kaolinite
or halloysite. It seems that generally the adsorption of potassium is
greater in an alkaline medium than in an acid medium. The type of
cation dominant in the exchange complex also seems to influence the
ability of the mineral to adsorb potassium, for Wiklander, (1950) who
studied the fixation of potassium by clays saturated with different
cations, found that if Ca z+, Mg 2+, Na +, etc., were the ions already
present, they could be replaced by K +, while this replacement was
more difficult if NH4 + or H + were the cations already present. It
has also been shown by Peech and Bradfield (1934) that the abilities
of the soils to adsorb potassium from neutral salts decreased as the
degree of base saturation decreased.
Kashmir montmorillonite contains a very high percentage of exchangeable calcium and the high amount of calcium seems to be very
effective in increasing the adsorption of potassium by montmorillonite. This is further supported by the results obtained by Ayers
(1941) for the adsorption of potassium at various degrees of calcium
saturation by the Hilo-Coast soil. The results indicate that increasing the amounts of exchangeable calcium in the soil irtereases its
ability to adsorb K +. Gilligan (1938), Harris (1937) and McIntire
(1936) have demonstrated that liming increases the adsorption and
fixation of potassium. Although the present work does not lead to
any definite conclusion as some other factors are undoubtedly involved, for the b.e.c, of montmorillonite is considerably above that
of the other minerals and the role of anion adsorption is difficult to
evaluate, yet it appears definite that the adsorption of potassium is
related to the exchangeable calcium content of the mineral, and this
in turn is presumably in some manner linked with the anion adsorption.
REFERENCES
Ayers, A. S. 1941. Soil Sci., 51, 265.
Gilligan, G. M. 1938. Bull. DeL Agr. Exp. Sta, 215, 1-20.
Harris, H. C. 1937. SoilSci., 44, 265-75.
MacIntire, W. H. 1936. J. Amer. Soc. Agron., 2.8, 202-15.
Peech, M. and Bradfield, R. 1934. Bull. Amer. Soil surv. Assn., 15, 10l-6.
Wiklander, L. 1950. Soil ScL, 69, 261-8.