Potassium Chloride water-soluble fertilizer applied through irrigation

1. Potassium chloride in fertigation
H. Magen (1)
(1) ICL Fertilizers
Paper presented at the 7th International Conference on Water and Irrigation,
13-16 May, Tel Aviv, Israel
Abstract
The rapid proliferation of fertigation in modern agriculture has brought about a growing demand for
factory mixed fertilizer solutions, as well as wide-scale dissolution of solid fertilizers by farmers in the field.
Rules of chemistry and physics control the rate of dissolution and, hence, the nutrient content in the
fertilizer solution. Potassium chloride (KCl) is the cheapest potassium-containing fertilizer. Data concerning
the dissolution of potassium chloride and other potassium sources are presented below.
Introduction
Fertigation is the application of solid or liquid mineral fertilizers via pressurized irrigation systems, creating
nutrient-containing irrigation water. Although the practice of commercial fertigation started only in the mid - 20th
century, there is evidence that the concept of irrigation with dissolved nutrients in water was well known in the
past. The first reported example dates back to ancient Athens (400 BC) where city sewage was used to irrigate tree
groves (Young and Hargett, 1984).
One of the major factors in promoting modern fertigation was the development of micro-irrigation systems (MIS)
such as drip irrigation, jets and micro-sprinklers. Field experiments in Israel in the early 1960s showed that when
localized sections of a field are irrigated, as in MIS, standard broadcasting of fertilizers is ineffective. The limited
root zone and the reduced level of mineralization in the restricted wetted zone are the main reasons for the reduced
nutrient availability to the plant. When this was recognized, fertigation was integrated in almost all MIS.
Israel is an unmatched example of the use of fertilizers by fertigation. The Israeli farmer uses an average of 100, 55
and 75 kg/ha/y of N, P2O5 and K2O, respectively. Over 50% of the N and P2O5 is applied by fertigation. Of the
33,000 tons used annually, approximately 10,000 tons are applied as clear liquid N-P-K, N-K or P-K solutions or
soluble complex fertilizers, and another 5,000-10,000 tons are applied as solid KCl dissolved in the field.
Fertigation is by far the most common, and in some cases the only, method of fertilizing in greenhouses, orchards,
vegetables and drip irrigated field crops such as cotton, maize, jujube, etc.
Various sources have shown the advantage of applying K through irrigation water. Potassium ions are adsorbed at
the cation exchange sites of soil colloids, but it was shown that lateral and downward mobility of potassium occur
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2. when applied via drip irrigation (Goode et al. 1978; Kafkafi and Bar-Yosef 1980). Haynes (1985) showed that the
distribution of potassium was more uniform than that of either nitrate or phosphate.
Preplanting fertilizer application usually include 15-25% of the recommended N, 20-30% of the recommended K
and 100% of the recommended P, Ca, Mg and micro-elements (Sanders 1991). Further nitrogen and potassium is
given by fertigation during the growing period to complete the plant requirements.
Potassium is applied through fertigation by using various sources of K salts such as potassium chloride (KCl),
potassium sulfate (K2SO4), potassium nitrate (KNO3) and mono-potassium phosphate (KH2PO4). Among the less
common K fertilizers are potassium carbonate (K2CO3) and potassium silicate (K-Si). The K fertilizer is chosen by
price, solubility, anion type and ease of use. This paper discusses various features of KCl fertigation.
Compatibility of Fertilizers with Fertigation
There are no standards regarding fertigation fertilizers currently in force, therefore, the following are suggested:
complete solubility (< 0.2% insolubles in water), high nutrient content in the saturated solution, fast dissolution in
irrigation water, insolubles of non-clogging mineral and bacterial type only, no chemical interactions between the
fertilizer and irrigation water, and absence of undesired anions
Solubility of K fertilizers. Solubility is defined as the amount of salt (grams) per volume (liter). Potassium chloride
(KCl) is the most soluble potassium fertilizer up to a temperature of about 20°C (fig. 1); at higher temperatures
potassium nitrate (KNO3) is more soluble. Both salts have an endothermic reaction when dissolved (the solution
cools as the fertilizer dissolves). This phenomenon limits the solubility of KNO3 more than that of KCl. The
solubility of fertilizers is reduced when two or three fertilizers are mixed together. The maximum concentration
can be determined by using triangular diagrams, from which any ratio can be calculated for a given temperature
(Wolf et al. 1985).
KNO3
KCl
KH2PO4
K2SO4
0 5 10 15 20 25 30 35
0
100
200
300
400
500
Temp (°C)
Solubility (gr. / liter)
Fig. 1: Solubility of various K fertilizers at different temperatures.
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3. Nutrient content of K fertilizers. Nutrient content is defined as the value received by multiplying solubility times
the percentage of the nutrient in the fertilizer. KCl yields the highest nutrient content at 10°C (fig. 2), achieving a
concentration of 15% K2O, compared to only 8% with KNO3, and even less with K2SO4 and KH2PO4.
Fast dissolution. This parameter is important when considering industrial dissolution processes dissolution at
field level for the calculation of irrigation timing and intervals. Elam et al. (1995) showed the difference between
KCl, KNO3 and K2SO4 dissolution rates and the change in temperature after dissolving the fertilizers (fig. 3). The
graph shows that the dissolution time (t90, the time needed to dissolve 90% of the salt added, in minutes) of KCl is
much shorter and the K2O content is much higher, about 13% for KCl in 8 minutes, as compared to 4% for K2SO4
in 25.2 min. and 9% for KNO3 in 15.6 min.
%ofplantfood(K2O;anion)MOP KNO3 MKP SOP
0
150
300
450
600
0
2
4
6
8
10
12
14
16
Solubility(gr./l)
Solubility
K2O (%)
Anion (%)
Fig. 2: Solubility and nutrient concentration at saturation of K fertilizers at 10°C (50°F); MOP = muriate of potash
(KCl), MKP = mono-potassium phosphate (KH2PO4), SOP = sulfate of potash (K2SO4).
Insolubles of non-clogging type
KCl is marketed either as white or red MOP (muriate of potash). The source of the red color is the presence of
insoluble (in water) Fe-Oxide compounds in the material. When dissolving red potash, It is clearly seen that the
saturated solution contains a red colored fraction of insoluble material, thus raising the question of its ability to be
applied through fertigation
Chemical interactions between the fertilizer and irrigation water
The formation of precipitates in irrigation water due to the addition of fertilizer, is one of the most common
problems farmers encounter at field level. The most common precipitates are Ca-P compounds at pH>7.0, when P
fertilizers are added. Fertilizing with K and N fertilizers are limited when irrigation water contain high
concentration of Ca and or SO4. At such conditions, salting out of K2SO4, CaSO4 and NH4SO4 occur. Since
chloride salts are highly soluble, precipitation of its salts practically does not exist in such systems.
A more accurate method of predicting precipitation under various conditions of pH, and concentrations of Ca, Mg,
Fe and PO4 is by the use of the computer program GEOCHEM-PC (Parker et al., 1995). The program can predict
the precipitation of any salts in irrigation water, and thus plays an important role in fertigation management.
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4. Laboratory experiments with nutrient solutions showed a good correlation between the program’s predictions and
the actual results (Magen, 1995).
0 10 20 30 40 50
0
0.2
0.4
0.6
0.8
1
2
4
6
8
10
12
Time (min)
FractionDissolved
Temperature(C)
KCl
K2SO4
KNO3
at equilibrium KCl K2SO4 KNO3
% K2O 12.9 4.3 9.0
t 90 8.0 25.2 15.6
% Salt 20.4 8.0 19.2
Fig. 3: Dissolution kinetics (full points) and change in temperature (hollow points) when dissolving K fertilizers
(80% saturation, 20°C, 100 rpm).
Presence of undesired anions
Table 1 shows types of anions and their relative consumption by plants.
Table 1: Cations and anions from different K fertilizers and their uptake by plants
Fertilizer Cation Anion Anion absorbed by
plants as .... nutrient
KCl K+ Cl-
secondary
K2SO4 K+ SO4
-2
secondary
KNO3 K+ NO3
-
macro
KH2PO4 K+ H2PO4
-
macro
Chloride is consumed by plants at low quantities, therefore, under heavy fertilization with Cl containing fertilizers
and sub-optimal leaching conditions, it will accumulate and create salinity problems. Specific guidelines and
tolerance levels were assessed (Maas, 1986), enabling research and farmers to monitor and adopt proper
management to minimize this problem.
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5. Prepared liquid fertilizers
Clear liquid fertilizers used for fertigation include urea, ammonium nitrate, ammonium sulfate, either individually
or in combination as the N source, orthophosphate as the P2O5 source and KCl, K2SO4 and KH2PO4 as the K2O
sources. By mixing two or three different nutrients, the solubility of each nutrient declines (table 2).
Table 2 presents some of the characteristics of selected liquid fertilizers of industrial preparation.
Under field conditions, mixing and heating are limited, resulting in a much lower nutrient content. An experiment
describing the maximal nutrient content in field prepared solution, was conducted in our laboratory, in order to
develop a simple mixing tables for the end user. At the present work, we have examined the preparation of KCl
solutions with minimal stirring, at a controlled temperature of 10°C.
Different amounts of fertilizer grade potassium chloride (61% K2O) (4%, 6%, 7%, and 10% K2O concentration
w/w) were added to 100 ml of tap water at 10°C in a vessel jacketed with cooling liquifd at 10°C. After addition of
the fertilizer the solution was stirred for one minute and the stirring then stopped. The temperature change with
time was recorded as well as the time to reach complete dissolution.
The temperature changes with time for four different amounts of KCl is shown in table 3. For the first three
additions clear solutions were obtained after 5-8 minutes. The fourth addition, 10% K2O, did not dissolve
completly even after one hour, although the temperature had returned to 10°C after 17 minutes.
Table 2: Selection of various formulas of liquid fertilizers (source: Sne, Ministry of Agriculture. Israel, 1989).
Fertilizers Formula Salting out
Temperature (C°)
pH
(1:1000)
NH4NO3+H3PO4+KCl 19-5-0 6 0.0
7-7-7 15 3.1
4-0-12 5 4.5
Urea+NH4NO3+ H3PO4+KCl 0-10-10 5 0.3
8-8-8 13 0.6
8-0-12 12 7.6
NH4NO3+H3PO4+KNO3 7-0-7 14 3.5
6-6-6 3 3.5
3-0-9 12 3.5
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6. Table 3: Temperature change with time to reach clear K solution
Time
(minutes)
4% K2O 6% K2O 7% K2O 10% K2O
Temp (C)
0 10 10 10 10
0.5 7 6 5 2
1 8 6 5 3
2 8 7 6 5
3 8 8 7 6
4 8 8 8 6
5 9# 8 8 6
6 9 8# 8 7
7 9 8 9 8
8 9 9 9# 8
9 9 9 9 8
10 10 9 9 8
15 10 10 10 9
17 10##
# - clear solution
## - undissolved material present after one hour
On the basis of these results, the most concentrated solution of KCl that can be prepared in simiulated field
conditions, at 10°C with minimal stirring, is 0-0-7, and this will take approximately 8 minutes to reach full
dissolution.
Conclusion
KCl was compared with other K fertilizers regarding their suitability to fertigation. The chemical characteristics
of KCl were discussed and assessed. KCl’s high nutrient percentage in saturated solution, as well as its rapid
dissolution and compatibility with other fertilizers applied in stock solutions, makes it ideal for fertigation use.
In some cases, monitoring of Cl-
content in soil and plant is recomended to eliminate its accumulation.
Acknowledgments
Thanks are due to Dr. M. Lupin, IMI (TAMI) Institute for research & Development Ltd., who conducted and
summarized the KCl dissolution at field conditions experiment.
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7. 7
References
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