The Dissolution of Different Types of Potassium
Fertilizers Suitable for Fertigation
M. Elam(1), S. Ben-Ari(1) and H. Magen(2)
(1) R&D, Potash Division, Dead Sea Works Ltd. (2) Potash Marketing Division, Dead Sea Works Ltd.
Introduction
Large amounts of potassium are needed during short irrigation intervals and therefore,
for fertigation, we are looking for a fertilizer with a fast dissolution rate and a high final
K2O concentration.
Three major potassium fertilizers - the chloride (KCl), sulphate (K2SO4) and nitrate
(KNO3) were chosen for comparative study. KCl, which is the cheapest, is the most
widely used in general agriculture. K2SO4 and KNO3 do not contain chloride, an
advantage in certain applications and the nitrate also supplies available nitrogen.
Solubility
The solubility curves shown for ambient temperatures (Fig.1) are for the pure salts. The
chloride is the most soluble up to 25°C. The solubility of the nitrate increases sharply with
temperature but, on the other hand, at ambient and lower temperatures, its solubility
decreases sharply and becomes significantly lower than KCl. The solubility of the sulphate
is the lowest over the entire temperature range.
However. it is more important to consider the K2O content of those salts (Table 1).
These data are, again, for pure salts and their saturated solutions. (In parentheses - data for
fertilizers).
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Elam et al.
The dissolution of different types of potassium fertilizers suitable for fertigation
Fig. 1: Solubility curves of K fertilizers.
Solubility (Wt. %)
35
KCl
30
K2SO4
25
KNO3
20
15
10
5
0
5
10
15
20
25
30
Temperature (C)
Table 1: K2O content of K fertilizers.
Temperature
KCl
K2SO4
KNO3
Solid
63.2 (61)
54.0 (50)
46.6 (44)
30
17.1
6.2
14.7
20
16.1
5.4
11.2
10
15.0
4.9
8.1
It's clearly seen that the KCl has the highest content of K2O, especially at lower
temperatures. This strongly affects the volume of the storage tank solution needed. Thus at
10oC, the tank volume needed to prepare a KNO3 or a K2SO4 solution increases twofold
or threefold respectively, as compared to the same quantity of KCl. It is obvious that when
designing a fertigation system, the lowest temperature during the irrigation season should
be taken into account.
It should be mentioned that a recent patent claims an enhanced solubility of K2SO4 of
more than 8% K2O can be achieved. This requires pretreatment of the salt with surfactants
at elevated temperatures.
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
Experimental
The dissolution was measured by means of a pharmaceutical test station that was
modified to our needs. The salt was added to a still de-ionized water in a thermostaticaly
controlled water bath at 20 or 10oC. The measurement of dissolution started with the
beginning of stirring - usually at 100 rpm. The salt concentration in solution was measured
with the aid of a conductivity electrode connected to a conductimeter. As the dissolution of
the chloride and the nitrate is highly endothermic, the temperature change during the
dissolution was also measured.
The measurements were controlled by a computer and the data were recorded.
The conductivity data were then transformed to concentrations with the aid of
calibration curves taking into account the temperature change.
The size of the fertilizer granules was 6/9 Tyler mesh (2 -3.35 mm): Compacted KCl
(from Dead Sea Works), compacted K2SO4 (from Kali & Saltz) and granulated KNO3
(from Haifa Chemicals).
Dissolution Rate, General
An example of dissolution curves can be seen in Fig.2 for K2SO4 at 20oC for different
final concentrations. As the final concentration increases, so does the dissolution time.
Presentation of the concentrations as the fraction of the final concentration (Fig.3)
normalize the dissolution curves.
Fig. 2: Dissolution of K2SO4, at 20÷C
10
Concentration (Wt. %)
0.5%
8
1.0%
6
2.0%
3.8%
4
7.0%
2
9.1%
0
0
10
20
30
40
50
60
Time (min)
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
Fig. 3: Dissolution of K2SO4, at 20÷C
Fraction Dissolved
1
0.8
0.6
0.4
0.2
0
0.5% 1.0% 2.0% 3.8% 7.0% 9.1%
0
10
20
30
40
Time (min)
At low final concentrations the curves are nearly identical and the dissolution times then
increases with concentration. The dissolution rate depends in parts on the exposed surface
area of the salts. Because of the limited volume of the dissolution cell, the surface area
remains approximately constant, above a given quantity of the added salt. Thus, the
dissolution time increases with the final concentration because the ratio of the volume to
the exposed surface area increases.
In order to quantify the dissolution process and obtain values for comparison, a
parameter - t90 which is the time needed to dissolve 90% of the salt added was calculated.
A plot of the square root of t90 versus the final concentration shows a linear dependence
(Fig.4). A square root relationship is characteristic of a diffusion controlled or mass
transfer controlled process.
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
Fig. 4: Solubility time (90%, 100 rpm, 10 & 20°C)
7
KCl (20)
6
K2SO4 (20)
5
KNO3 (20)
4
KCl (10)
3
K2SO4 (10)
2
1
KNO3 (10)
0
5
10
15
20
25
Concentration (%)
It is clearly seen that the dissolution of the KCl is the fastest, whereas that of K2SO4 is the
slowest. Also, the dependence of the dissolution time on the temperature is of lesser extent
for KCl than for the other fertilizers.
Dissolution Rate, detailed
A few detailed examples are now presented after the general comparison between the
three salts given above. Fig.5 depicts the dissolution curves at 20°C for salts which contain
about 4% K2O at their final concentration. The change of the temperature of the solution
during dissolution is also shown, manifesting the endothermic nature of KCl and KNO3
dissolution. The difference between the t90's of various fertilizers is remarkable. It is about
double for KNO3 compared with KCl and six times more for K2SO4. At 10oC (Fig.6) the
differences are even higher. This is because the solutions are then nearer their saturation
concentration.
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
1
21
KCl
0.8
20
K2SO4
0.6
19
0.4
0.2
0
KCl
4.4
27
3.9
7.0
% K2O
% Sat
t 90
% Salt
0
10
20
K2SO4
3.8
70
23.2
7.0
30
KNO3
4.2
38
7.3
9.1
18
Temperature (c)
Fraction Dissolved
Fig. 5: Dissolution of K fertilizers (~4% K2O, 20°C, 100 rpm)
KNO3
17
16
50
40
Time (min)
1
11
KCl
0.8
10
K2SO4
0.6
9
0.4
% K2O
% Sat
t 90
% Salt
0.2
0
0
10
20
KCl
4.4
29
5.0
7.0
30
K2SO4 KNO3
3.8
4.2
82
53
38.7
12.5
7.0
9.1
40
8
7
Temperature (C)
Fraction Dissolved
Fig. 6: Dissolution of K fertilizers (~4% K2O, 10°C, 100 rpm)
KNO3
6
50
Time (min)
It is more significant and more practical to compare the dissolution of the three
fertilizers at 80% of their saturation concentration. We chose the degree of 80% saturation
since it represents general field conditions. Fig.7 shows it at 20°C. The dissolution time of
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
KCl is still much shorter and furthermore, the K2O content is much higher; about 13% for
KCl as compared to 4% of K2SO4 and 9% for KNO3. At 10oC the higher K2O content as
compared to KNO3 is enhanced. It is nearly double (Figure 8).
1
22
KCl
0.8
20
K2SO4
0.6
18
0.4
KCl K2SO4 KNO3
0.2
0
0
10
% K2O 12.9
4.3
9.0
t 90
% Salt
25.2
8.0
15.6
19.2
8.0
20.4
20
30
40
16
14
Temperature (C)
Fraction Dissolved
Fig. 7: Dissolution of K fertilizers (80% saturation, 20°C, 100 rpm)
KNO3
12
50
Time (min)
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
1
12
KCl
0.8
10
K2SO4
0.6
8
6
0.4
KCl
% K2O
t 90
% Salt
0.2
0
0
10
12.0
11.2
19.0
20
30
K2SO4 KNO3
3.9
35.3
6.8
6.5
20.6
13.8
40
4
Temperature (C)
Fraction Dissolved
Fig. 8: Dissolution of K fertilizers (80% saturation, 10°C, 100 rpm)
KNO3
2
50
Time (min)
At 10°C, the temperature of the KCl solution decreases more than the KNO3 solution
because of the higher KCl concentration.
Influence of Stirring
Fig.9 shows dissolution curves of KCl at 20oC and at final concentration of 80% of
saturation (20.4 Wt.%). For the sake of clarity the time scale is logarithmic. Thus, the
major dependence of the dissolution rate on stirring can be appreciated. While the rotation
rate is decreased fourfold, the t90 increases 50 times. A similar behavior was noted for the
other salts. Such behavior is of major practical importance.
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
Fraction Dissolved
Fig. 9: Rotation rate dependence (KCl, 20÷C, 20.4 Wt%)
1
36 rpm
0.8
49 rpm
64 rpm
0.6
81 rpm
0.4
100
0.2
0
0.1
144 rpm
1
10
100
1,000
Time (min)
This phenomenon can be explained from the basic equation of the dissolution rate
(dM/dt) expressed as
dM = DS(Co-c)
dt
δ
On dissolving, a concentration, Co, equal to that of the saturated solution is built up in
the layer immediately adjacent to the solid. At some small distant δ away, the
concentration is equal to that of the bulk solution - c. The difference (Co-c) is the driving
force for dissolution. D is the diffusion coefficient and S - the exposed surface area of the
solid.
The dissolution rate (dM/dt) is determined by the diffusion of the solute across the
concentration gradient. Keeping the final concentration small with respect to saturation,
sink condition are approximated. The diffusion layer - δ, whose thickness is of the order of
1-100 microns, depend on stirring rate. Usually, for such systems as those described
above, the diffusion layer decreases with the increase of square root of rotation rate - ω1/2.
δ = k1(η/ω)1/2
Where η is the kinematic viscosity. Thus the former equation is transformed to:
dM/dt = DS(Co-c)k2ω1/2
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
The dissolution of different types of potassium fertilizers suitable for fertigation
Elam et al.
At different rotation rates and for certain quantity M, of the salt dissolved, the same c
(or the same Co-c) is achieved at different times. For a value of c that corresponds, to say
90% of the final concentration, the time, t90, is inversely proportional to the square root of
the rotation rate
1/t90 = k3ω1/2
This relation is verified experimentally (Fig.10). It is assumed that the apparent surface
area does not change significantly with time, or at least is similar for all rotation rates at
t90.
Fig. 10: Rotation rate dependence (20.4 Wt%, KCl, 20÷C)
0.2
0.15
0.1
0.05
0
5
6
7
8
9
10
11
12
13
√ω (min)
-½
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel
Elam et al.
The dissolution of different types of potassium fertilizers suitable for fertigation
Conclusions
It can be concluded that for crops not sensitive to the chloride anion or under leachable
condition, KCl is the most suitable fertilizer for fertigation because.
* Its dissolution is the fastest.
* Its K2O content is the highest.
* Its sensitivity to temperature change is the smallest.
* It is the cheapest.
The very marked effect of the stirring rate on dissolution of the salts should be
emphasized. Practically, it is recommended to apply very vigorous, turbulent stirring, so
the fertilizer particles are suspended, floating in solution.
References
1. Properties of Aqueous Solutions of Electrolytes, Edited by I.S. Zaytsev
and G.G. Asayev, CRC Press, Boca Raton, Fl., 1992.
2. Solubilities of Inorganic and Organic Compounds, Edited by H. Stephen
T. Stephen, Pergamon Press, Oxford, 1963.
3. W.A. Hanson, Handbook of Dissolution Testing, Pharmaceutical Technology
Publication, Springfield, Or., 1982.
4. V.G. Levich, Physicochemical Hydrodinamics, Prentice Hall, Englewood
Cliffs, NJ, 1962.
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Dhalia Greidinger International Symposium on Fertigation, 26 March-1April, 1995, Haifa, Israel