Pl. Kirilov,and
L. Brakalov
Journal of the UniversityK.ofKamburova,
Chemical Technology
Metallurgy, 44, 3, 2009, 249-252
LIQUID FERTILIZERS FROM
POTASSIUM PHOSPHATES – COMPOSITION AND PROPERTIES
K. Kamburova, P. Kirilov, L. Brakalov
University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
Received 05 May 2009
Accepted 18 June 2009
E-mail: [email protected]
ABSTRACT
Solubilityin the system KH2PO4 – K2HPO4 – H2O was measured at T=25oC by the isothermal method. Saturated
solutions were prepared by dissolution of mixtures of KH2PO4 and K2HPO4 at different ratio ù(K2O) : ù(P2O5) =
= 0.663 (100 % KH2PO4) – 1.327 (100 % K2HPO4) in distilled water. Data on some physico-chemical properties of the
saturated solutions - density, pH, electrical conductivity and corrosion activity are given. According to the requirements
to foliar liquid fertilizers the most suitable phosphorus potassium (PK) solution is estimated. It was used for preparing PK
liquid fertilizer with micronutrients. A clear, green solution, stable for a long time at temperatures down to 0oC was
prepared.
Keywords: liquid foliar fertilizers, potassium phosphates, physico-chemical properties of PK foliar fertilizers, PK
foliar fertilizers with micronutrients.
INTRODUCTION
The aim of the study is preparing a highly concentrated, stable, clear PK liquid fertilizer with micronutrients, applicable for foliar feeding of the plants. For
this purpose it is necessary: to measure the solubility at
25oC in the system KH2PO4 - K2HPO4 - H2O, to estimate the main physico-chemical properties of the saturated solutions and to select the most suitable PK solution according to the requirements to foliar liquid fertilizers.
Foliar feeding, a term referring to application of
essential plant nutrients to above ground plant parts,
has been documented as early as in 1844. The purpose
of foliar feeding is not to replace the convenient soil
fertilization. Foliar fertilization is usually a supplement
to soil fertilization and puts the plants in a position to
utilize soil-derived nutrients better. Foliar fertilizers are
used to improve the yield and quality of the crops, to
meet peak nutrient demands during critical growth stage
periods and to stimulate the plants to absorb more nutrients from the soil.
Foliar-applied nutrients have the benefit of being 4 to 30 times more efficient than soil - applied and
there is no risk of ground water contamination [1]. If
foliar feeding is done correctly, visual results may be
seen within 48 hours.
Not all fertilizers are suitable for use as a foliar
spray. A true solution of a foliar fertilizer has the following characteristics: high concentration, no settling
out, clear appearance, low vapor pressure, neutral pH,
low salt index, high buffer capacity, virtually non-corrosive, no creeping crystallization, high rate of absorption and penetration of nutrients, low temperature of
crystallization, long term stability.
The primary objective of a foliar application is
to allow for maximum absorption of nutrients into the
plant tissue. Most suitable for this purpose are KH2PO4
and K2HPO4 as PK fertilizers and urea - as a nitrogen
fertilizer [2, 3].
249
Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009
Therefore, foliar fertilizer formulations should
meet certain standards in order to minimize foliage
damage [4]. One of these requirements is low salt index.
The above mentioned salts have the lowest salt index of
all convenient mineral fertilizers [5]. In our previous
paper we have shown a method for calculation the salt
index of mixtures of KH2PO4 and K2HPO4 and their solutions [6].
According to the data for the equilibrium in the
system P2O5 – K2O – H2O it is seen that mixtures of
KH2PO4 and K2HPO4 have maximum solubility at different ratios depending from the temperature [7].
There are no data for the physico-chemical properties of the ternary system KH2PO4 - K2HPO4 – H2O.
RESULTS AND DISCUSSION
The data for the solubility and physico-chemical
properties of the solutions are given as a function of the
ratio K/P in Table 1 and Figs. 1-4.
Solubility in the system at 25oC increases with
increasing K/P ratio and reaches a maximum value
S=201.9 g/100g H2O at K/P = 1.2. According to the
data of Wendrow and Kobe [7] the highest concentrations of P2O5 and K2O in the saturated solutions at T=
25oC are 28.3 % P2O5 and 33.3 % K2O. Our results:
EXPERIMENTAL
Solubility. Solubility in the system KH2PO4 –
K2HPO4 – H2O was measured at T=25oC by the isothermal method at the range ù(K2O) : ù(P2O5) = 0.663
(100 % KH2PO4) – 1.327 (100 % K2HPO4). Reagent
grade (Merck) salts have been used. Further in the text
the ratio ù(K2O) : ù(P2O5) will be signed as K/P.
Density. Density was measured with 25 cm3 GayLussac type pycnometer at 20°C.
pH. pH was measured with a combined glass electrode and pH meter Seibold G 104. All measurements
were made at 25°C. The apparatus was calibrated with
reference buffer solutions.
Conductivity. Electrical conductivity was measured with METTLER TOLEDO- Seven Multi Conductivity device at 25°C.
Corrosion activity. Corrosion was evaluated by
inserting carbon steel (Bulgarian steel St. 3) coupons in
closed beakers filled with 200 ml saturated PK solutions for a duration of 30 days at room temperature
(Tmin = 23.9°C, Tmax = 27.8°C). The coupons were previously polished, rinsed with deionized water, treated
in ethyl alcohol-ether mixture and dried.
Each coupon was pre-weighed to an accuracy of ±
0.0001 g and immersed in the solution for 30 days. After
exposure they were removed, dried and photographed,
cleaned by dipping in 10 - 15 % HCl + 0.5 % inhibitor
(ferasin) solution for 7 min, again dried and re-weighed.
The corrosion rate of the coupon was calculated in mm
per year based upon the mass loss of the material [8, 9].
250
Fig. 1. Solubility in the system KH2PO4 – K2HPO4 – H2O at
25°C as a function of mass ratio ω(K2O)/ ω(P2O5).
Fig. 2. Density of the saturated solutions at 25°C in the
system KH2PO4 – K2HPO4 – H2O as a function of mass ratio
ω(K2O)/ ω(P2O5).
K. Kamburova, Pl. Kirilov, L. Brakalov
Table 1. Solubility and physico-chemical data in the system KH2PO4 - K2HPO4 – H2O at 25°C.
¹
K/P
1
2
3
4
5
6
7
8
9
0.663
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.327
ù(KH2PO4)s
%
100
93.12
75.13
58.56
43.18
28.93
15.63
3.24
-
ù(K2HPO4)s
%
6.98
24.87
41.44
56.82
71.07
84.37
96.79
100
S
g/100gH2O
25.1894
26.1530
34.8439
48.1260
73.5265
124.3111
201.8982
173.3216
165.7215
pH
4.01
4.79
5.58
6.19
6.92
8.06
9.51
10.39
10.98
ñ20
g/cm3
1.1527
1.1583
1.2063
1.2795
1.4018
1.5811
1.7743
1.7254
1.7193
ù(KxHyPO4)aq
%
20.12
20.73
25.84
32.49
42.37
55.42
66.88
63.41
62.37
ù(P2O5)aq
%
10.49
11.43
12.74
15.41
19.35
24.41
28.44
26.07
25.42
ù(K2O)aq
%
6.96
8.00
10.12
13.86
19.35
26.85
34.14
33.89
33.73
ù(PK)aq
%
17.45
18.43
22.86
29.27
38.70
51.26
62.58
59.96
59.15
ó
mS/cm
128.7
137.5
172.5
213.0
235.0
165.7
55.1
70.1
75.9
Km
g/m2h
0.1977
0.1864
0.1751
0.1351
0.1156
0.0878
0.0679
0.0655
0.0696
RC
mm/y
0.2203
0.2077
0.1951
0.1506
0.1288
0.0978
0.0756
0.0729
0.0776
Legend: K/P - mass ratio ù(K2O)/ù(P2O5) in the solid mixture (KH2PO4 + K2HPO4); ù(KH2PO4)s - mass part of
KH2PO4 in the solid mixture; ù(K2HPO4)s - mass part of K2HPO4 in the solid mixture; S – solubility; ñ20 – density;
ù(KxHyPO4)aq – mass part of (KH2PO4 + K2HPO4) in the solution; ù(P2O5)aq and ù(K2O)aq – mass parts of P2O5
and K2O in the solution; ù(PK)aq – total mass part of (P2O5 + K2O) in the solution; ó – electric conductance; Km
– mass index of corrosion; RC – rate of corrosion
28.4 % P2O5 and 34.1 % K2O at K/P = 1.2 are very
close to those of Wendrow and Kobe.
The dependence of the density of the K/P ratio is
analogous to that for the solubility with maximum value
at the same ratio K/P = 1.2. In the studied range of
K/P ratio, pH increases from 4 to 11 with an S–shaped
curve. The solution with the highest concentration of
P2O5 and K2O (at K/P = 1.2) has a pH = 9.51.
Soluble salts in irrigation water are measured in
term of electrical conductivity (EC). The higher the salt
content in dilute solutions the greater the EC. In general
high EC values are considered detrimental to plant growth.
Water quality should be monitored on a frequent
basis in order to avoid potential problems with soluble
salts. The conductivity of the investigated solutions increases with the K/P ratio, reaches a maximum value at
K/P = 1 and after that decreases. The lowest value ó =
55.1 mS/cm is at K/P = 1.2.
The rate of corrosion for all samples was very
low, especially for the solutions with high pH values.
These solutions are practically non-corrosive.
From the presented data it is seen that the solution with K/P = 1.2 is with the best complex physicochemical properties according to the requirements to
foliar liquid fertilizers.
This solution was used for preparing a PK liquid
fertilizer with micronutrients by adding a solution of
micronutrients. The micronutrients Fe, Mn, Cu and Zn
Fig. 3. pH of the saturated solutions at 25°C in the system
KH2PO4 – K2HPO4 – H2O as a function of mass ratio ω(K2O)/
ω(P2O5).
Fig. 4. Conductance of the saturated solutions at 25°C in
the system KH2PO4 – K2HPO4 – H2O as a function of mass
ratio ω(K2O)/ ω(P2O5).
251
Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009
were helated with Na2EDTA. The composition of the prepared foliar fertilizer is: 22 % P2O5, 24 % K2O, 0.05 % Fe,
0.025 % B, 0.025 % Mn, 0.02 % Cu, 0.012 % Zn and
0.0025 % Mo. It is a clear, green solution, stable at
temperatures down to 0 ºC. This foliar fertilizer can be
used alone or a part of a two package NPK foliar fertilizer (one package with PK + a micronutrients fertilizer
and second with a nitrogen liquid fertilizer), and also
as a stock solution for producing NPK foliar fertilizers
+ micronutrients by adding nitrogen fertilizer/s.
CONCLUSIONS
According to the requirements to foliar liquid
fertilizers the most suitable PK solution is that with a
ratio ù(K2O)/ù(P2O5) = 1.2. It is with the highest content of the primary nutrients P and K and with the best
complex physico-chemical properties. This solution can
be used for preparing PK liquid fertilizers with micronutrients. Clear, green solutions with high content of
P2O5 and K2O, stable for a long time at temperatures
down to 0oC can be produced.
252
REFERENCES
1. R. Dixon, Fluid Journal, 40, 11, 2003, 22-23.
2. R. Reuveni, V. Agapov, M. Reuveni, M. Raviv, Journal of Phytopatology, 142, 3, 2008, 331-337.
3. L.G. Albrigo, J.P. Syversten, Fluid Journal, 34, 9,
2001, 8-11.
4. J. J. Mortvedt, Fluid Journal, 9, 2, 2001, 8-11.
5. Pennzoil near the Concentration Production of Potassium Phosphate, Phosphorus & Potassium, 68,
November/December, 1973, 44 -49.
6. K. Kamburova, Pl. Kirilov, J. Univ. Chem. Technol.
Met. (Sofia), 43, 2, 2008, 227 – 230.
7. B. Wendrow, K. Kobe, Chem. Rew., 54, 6, 1954,
891-924.
8. ASTM G31 – 72(2004) Standard Practice for Laboratory Immersion Corrosion Testing.
9. G. Haynes, R. Baboian (Eds.), Laboratory corrosion tests and standards, ASTM STP 886, American Society for Testing ahd Materials, Philadelphia, 1985.