


  

Kurdistan Journal of Applied Research (KJAR) 

Print-ISSN: 2411-7684 | Electronic-ISSN: 2411-7706  

 

Website: Kjar.spu.edu.iq | Email: kjar@spu.edu.iq 

 
  

 

 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |95 

 

 

Potassium Adsorption Phenomenon 

in Calcareous Soils of Shahrazur 

Plain 

 
  

 

Article Info  ABSTRACT 

Volume 6 – Issue 1- June 2021 

DOI:  

10.24017/science.2021.1.10 

Article history: 

Received 5/6/2021 

Accepted 1/7/2021 

 
     A laboratory study for adsorption of potassium (K) determination was 

conducted on six soils located in Sharazur plain from the Kurdistan Region of Iraq 

in 2021 using the batch technique method. Potassium (K) adsorption isotherms 

were achieved by equilibrating 5.0 g of soil samples with eight grades of K (0 to 

300 mg L-1) as KCl in 50 ml of 0.01M CaCl2 solution. To match the data of 

adsorption, Freundlich, Langmuir, and Temkin adsorption isotherms were used. 

The results show that the amount of adsorbed K ranged between (45.78 to 52.49) 

% added K. The Freundlich model fit the equilibrium K adsorption data better for 

the Serwan location of soil (silty loam), as demonstrated by a greater coefficient of 

determination (R2 =0.90). The value of heterogeneity factor 1/n for the Freundlich 

model ranged from (0.34 to 0.47) kg mg-1, which was less than one. The sorption 

processes for all of the studied soils were normal adsorption. The constant of the 

Langmuir isotherm (KL) aligned from (0.107 to o.425) L mg
-1. Smaller KL values 

mean that more adsorbed K would be transformed to a non-exchangeable form, 

either through the creation of crystalline K or through ion occultation. The RL 

values indicate the type of isotherm, the values of RL> 1 that means the adsorption 

nature to be unfavorable. The Temkin equilibrium binding constant (AT) was high 

for all studied soils except the soils of Bestan Sur and Grdigo locations, the high 

value of AT indicates high binding energy. The Temkin constant (bT) ranged from 

(10.46 to 13.47) J mole-1 that was related to the nature of the adsorption energy, a 

positive value indicates that the adsorption process is exothermic. 
 

Keywords: 

Potassium adsorption, Freundlich 
model, Langmuir model, Temkin 

model. 

Copyright © 2021 Kurdistan Journal of Applied Research. All rights reserved. 

1. INTRODUCTION 
 

One of the fundamental nutrients for the growth of the plant is potassium (K), and when it 

exists at an insufficient level in soils, plant growth is seriously hampered. Potassium is available 

to plants in the form of inorganic cations (K+) [1]. Over sixty (60) enzymes involved in 

carbohydrate production, sugar translocation, numerous enzyme actions, yield, efficiency 

parameters, disease tolerance, processes to resolve abiotic tension, cell permeability, and 

Kamil Sabir Saeed  

Protected Agriculture Department  

Bakrajo Technical Institute  

Sulaimani Polytechnic University  

Sulaimani, Iraq  

kamil.saeed@spu.edu.iq  

  

mailto:kamil.saeed@spu.edu.iq


 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |96 

 

stomata opening are stimulated by potassium. It also affects the microbial population in the 

rhizosphere [2]. Various soil processes, such as the physical, chemical, and biological 

properties, influence K availability [3]. The release of potassium influences by the interaction 

of soil mineralogy, soil texture, and biological processes [4]. 

Potassium is present in the soil in four different types. Soil minerals like feldspar and mica 

make up (90 to 98) percent of the total potassium in the soil.  Plants can only use a small portion 

of this potassium supply. The second type of soil potassium is non-exchangeable potassium, it 

is related to the 2:1 clay mineral and ranges from 1% to 10% of total soil K. In the soil, the non-

exchangeable potassium source functions as a potassium reserve. Non-exchangeable K release 

has a major impact on soil K fertility. The exchangeable or readily available potassium, which 

makes up 1 to 2 percent of the soil potassium, is found in the soil solution or on cation 

exchangeable sites [5]. The root system of the plant readily absorbs potassium from the soil 

solution, which is then replaced by potassium from the exchangeable sites. Since crops respond 

to applied potassium earlier, the rate of K+ adsorption after fertilizer addition differs among 

soils, and thus its unpredictability is since different soils have varying potassium adsorption 

properties. Clay colloids can adsorb up to 57 % of the applied potassium, depending on the 

amount and form of clay minerals [6]. Kinetic and thermodynamic variables influence the 

amount of potassium adsorbed on clay particles [7]. 

According to Jalali [8], calcareous soils with enough exchangeable K and K-bearing minerals 

are capable of releasing enough K for optimal plant production. However, the exchangeable K 

in calcareous soils is not always reliable as a source of potassium for plants. The phenomenon 

of K adsorption in soils has altered the soil-plant system fertilization efficacy by converting 

available K into an unavailable form for plants. Due to the availability of different compelling 

potassium pools in the soil which could be mobilized by the chemical resistance of the mineral 

in the soil, understanding the process of K adsorption in the soil is essential [9]. A simple 

approach for determination K adsorption may aid in understanding the relationship among the 

adsorption of the alternative K soil characteristics [10]. The balance between K in the interlayer, 

the exterior, and K in the edge sites of the mineral crystal lattice is maintained by the solubilized 

K in soil solution which controls the potassium adsorption mechanism. The key elements 

impacting the equilibrium include clay mineral forms, soil (pH), soil organic matter (SOM, Al 

(OH)3, moisture state of soil, cation exchange capacity (CEC), fertilizer rate, and the process of 

tillage [11]. Due to the complexity of the adsorption of potassium by the soil, it is difficult to 

explain it by the occurrence of a single reaction. Many equations have been advanced for the 

calculation of soil potassium. The Freundlich, Langmuir, and Temkin adsorption isotherms are 

often utilized to illustrate the relationship between the potassium quantity fixed per weight unit 

in the soil and the potassium concentration in the solution. The Freundlich equation was initially 

used by Russell and Prescott in 1961, and it is the earliest adsorption equation in the soil 

literature. It's a mathematical statement for an adsorption model that the sympathy term declined 

rapidly with increasing adsorption. The Freundlich equation effectively predicted K adsorption 

for a limited range of conditions [12]. The energy of adsorption decreases as the sum of 

adsorption increases, according to the Freundlich equation. The Temkin isotherm includes a 

component that accounts for adsorbent-adsorbate interactions and it is based solely on the 

exterior analysis [13]. The Temkin isotherm is intensely used for an exterior energy system that 

is heterogeneous [14].

 

 

2-METHODS AND MATERIALS 

 

2.1 Study area and soil sampling  

 A study was conducted on the soil of Sharazur plain, which was located between the 

provinces of Suleimania and Halabja, situated in the southeastern part of Kurdistan 

Region, Iraq (Figure 1). The soil samples were randomly taken at 0 to 30 cm depths 

of the six major agricultural locations of the plain of Shahrazur during the spring 

season of 2021. For soil analysis and sorption experiments, the soil samples were air-



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |97 

 

dried then it was grounded and screened through a (2) mm screen. According to Gee 

and Bauder [15], the pipette method was applied to obtaining the soil Particle Size 

Distribution (PSD) of the soil samples. A pH meter Professional Benchtop, model 

(BP3001), and EC meter HERMAN PAULSEN, model (D-2000 HAMBURG) was 

used for measuring the soil reaction (pH) and electrical conductivity (ECe) for the 

soil suspension 1:2 respectively. The soil samples Cation Exchange Capacity (CEC) 

was determined by saturating the soil with 1M ammonium acetate (NH4OAc) at pH 

8.1 as an extraction solution according to the method described by Suarez [16]. The 

content of Organic Matter (O.M.) was determined using Nelson and Sommer [17] 

technique. A quick titration method, based on the approach technique of Rayment 

and Higginson [18] approach, was used to quantify the total carbonate minerals in 

soil expressed as carbonate minerals equivalent. The Active Equivalent Carbonates 

(AEC), were estimated by the 0.5 M NH4-oxalate method as described by 

Drouimeau [19]. 

 

2.2 Potassium adsorption experiment procedure 

For the construction of the potassium adsorption isotherm, each soil sample of 5.0 g 

was placed into a plastic bottle of 100-ml and then equilibrated with so ml of 0.01 M 

CaCl2 with (soil/solution ratio of 5/50), using various concentration of potassium (K) 

which were (0, 25, 50, 100, 150, 200, 250, and 300 mg L-1 K as KCl). The 

suspensions were then shaken for 2 hours at 180 rpm and 298 K in a horizontal flask 

shaker before being allowed to equilibrate overnight. Every solution was centrifuged 

at 250 rpm for 5 minutes after that, it was filtered through Whitman filter paper No. 

42 and tested for K concentration using a Flame Photometer JENWAY model PFP7. 

By subtracting the quantity measured in the solution which was supernatant from the original 

amount of added K, the amount of K adsorbed was estimated, as shown below:  

 𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑 𝐾 =  𝐶𝑜 −  𝐶𝑒 . . . . . . . . . . . (1) 

Where Co = the initial concentration of added K (mg L
-1) and Ce= concentration of K (mg L

-1) 

in the equilibrium supernatant solution. 

The removal efficiency was determined by computing the percentage adsorption using the 

formulae in Mandal et al. [20]. 

 𝐾 𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑(%) =
(𝑐𝑜𝑛. 𝑜𝑓 𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑 𝐾 (𝑚𝑔 𝐿−1 )𝑥 100)

𝐶𝑜  (𝑚𝑔 𝐿
−1)

. . . . (2) 

According to Vanderborght and Van Grieken [21], the amount of adsorbed Kat equilibrium (mg 

kg-1)  was determined as follows: 

𝑞𝑒 =
(𝐶𝑜 − 𝐶𝑒 )𝑉

𝑊
… … … . . . . . . . . . . . … … (3) 

Where qe represents the quantity of K adsorbed from the solution (mg kg-1),  



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |98 

 

 
Figure 1:  Geographical location of studied sites. 

V represents the volume of the solution (L), and W represents the weight of the soil sample used 

in the experiment (kg). The linearized forms of the Freundlich, Langmuir, and Temkin 

isotherms were used to calculate the results. 

 2.3 Freundlich adsorption isotherm 

Freundlich isotherm was one of the first to propose an adsorption isotherm equation [22]. The 

Freundlich isotherm is an empirical model for non-ideal adsorption on homogeneous surfaces, 

as well as multilayer adsorption with an uneven distribution of heat of adsorption and 

sympathies all over the surface that was heterogeneous [23]. Based on sorption onto a 

heterogeneous surface, the empirical Freundlich equation is: 

qe = KF Ce
(1/n)

…………………(4) 

By taking logarithms, Eq.(4) can be rearranged to obtain a linear form. Equation (5): 

𝑙𝑜𝑔𝑞𝑒 =  𝑙𝑜𝑔 𝐾𝑓  +  
1

𝑛
𝑙𝑜𝑔𝐶𝑒 . . . . . . . . . (5 ) 

 Where Kf is the energy of sorption or distribution coefficient (mg kg
-1). In the adsorption 

process, n is the dimensionless constant, and 1/n is a function of the strength of adsorption in 

the adsorption process [24]. The partition between the two phases is concentration-independent 

if the Freundlich affinity value n equals 1. If 1/n is smaller than 1, the value shows normal 

adsorption. However, if the value is more than 1, it suggests cooperative adsorption [25]. The 

Freundlich sympathy value, n is important for consideration sorption processes and 

heterogeneity of the system. The process of sorption is favorable when the value of n is between 

1 and 10, [26, 27]. A 1/n value approaching unity denotes a more homogenous system, while a 

1/n value near zero denotes a more heterogeneous system [28]. 

 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |99 

 

2.4 Langmuir adsorption isotherm 

The Langmuir model assumes monolayer adsorption of solutes on homogenous sorption sites, 

and the Langmuir equation defines the distribution of K between the solid-liquid interface 

equilibrium. 

The Langmuir isotherm was the most extensively used linear expression for studying the 

interactions between solute concentrations in the liquid and solid phases at equilibrium 

conditions [29, 30]. Equation (6) describes the Langmuir isotherm model. 

1

𝑞𝑒
=

1

𝑏
 +  

1

𝑏𝐾𝐿 𝐶𝑒
 . . . . . . . . . . . . . . (6) 

Where qe represents the amount of adsorbed potassium per unit of soil weight (mg kg-1), Ce 

represents the concentration potassium in the solution at equilibrium (mg L-1) point, KL 

represents the bonding energy or affinity constant of potassium to the soil (L mg-1), and b 

represents the soil's highest monolayer coverage ability while K is adsorbed (mg kg-1). A 

consecutive line with a slope of 1/bKL and an intercept of 1/b when plotted against 1/Ce. RL is 

the dimensionless constant called the equilibrium parameter for Langmuir isotherm [31]. 

𝑅𝐿 =
1

(1 +  𝐾𝐿 𝐶𝑜 )
 . . . . . . . . . . . (7) 

where KL is a constant relative to bonding energy (L mg
-1) and Co is the initial K concentration 

(mg L-1), RL values indicate the shape of isotherm [32]. The RL value indicates whether the 

adsorption is favorable (0 <RL< 1), unfavorable (RL> 1), linear (RL = 1), or irreversible (RL = 0). 

According to Reyhanitabar et al. [33], the buffering capacity (MBC) which was at its highest 

value was estimated as the product of KL and b. 

𝑀𝐵𝐶 = 𝐾𝐿  ∗  𝑏 . . . . . . . . . . . . . (8) 

2.5 The Temkin isotherm 

The Temkin isotherm was estimated that the heat of adsorption decreases in a linear rather than 

logarithmic manner. Furthermore, it is restricted to chemical adsorption and has homogeneous 

binding energy [34]. 

Equation (9) can be used to describe the Temkin isotherm. 

𝑞𝑒  =  
𝑅𝑇

𝑏𝑇
 𝑙𝑛(𝐴𝑇𝐶𝑒 ). . . . . . . . . . . . . . . . (9) 

𝑞𝑒 =  
𝑅𝑇

𝑏𝑇
 𝑙𝑛𝐴𝑇 +  

𝑅𝑇

𝑏𝑇
 𝑙𝑛𝐶𝑒 …….….(10) 

The linear form of the Temkin isotherm model is given by the following [35]: 

𝑞𝑒 =  𝐵𝑙𝑛𝐴𝑇  +  𝐵𝑙𝑛𝐶𝑒 . . . . . . . . . . . (11). 

where qe = the mass of K adsorbed per unit mass of the soil (mg kg-1), Ce is equilibrium solution 

K concentration (mg L-1), AT is the Temkin isotherm equilibrium binding constant (L kg
−1), and 

B is a constant linked to the heat of sorption (J mol−1) computed as: 

𝐵 =  
𝑅𝑇

𝑏𝑇
 . . . . . . . . . . . . . . . . . (12) 

where bT is the Temkin constant linked to the nature of the adsorption energy (J mol
−1) a positive 

value of bT indicates that the adsorption process is exothermic while a negative value of bT is an 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |100 

 

indication of the endothermic process [36] R is the universal gas constant (8.314 J mol−1K−1), T 

stands for the temperature with an absolute value of 298 kelvin. A plot of qe versus lnCe 

(equation 10) results in a straight line of slope RT/bT and intercept 
𝑅𝑇 

𝑏𝑇
𝑙𝑛𝐴𝑇  

enables the determination of the isotherm constants AT  and  bT.

 

3-RESULTS AND DISCUSSION 

 

 Table 1 demonstrates some of the soil characteristics (physical and chemical) that were studied 

previously. The concentration of equilibrium K in the soil solution ranged from 1.65 to 3.76 mg 

L-1 for 25 mg L-1 added K, and from 170.15 to 181.81 mg L-1 for 300 mg L-1 added K. The 

direct proportionality that existed was demonstrated by the linear relationship between 

additional K and equilibrium K (mg L-1) (Figure 2). With increasing K concentrations, the 

percent of K adsorbed did not increase equally (Figure 3). The range (%) of the adsorbed 

potassium (K) was lined in a range between 84.953 to 93.421% for the added K concentration 

which was lowest (25 mg L-1) and from 39.398 to 43.284% for the maximum added 

concentration of K (300 mg L-1). All the studied soils indicated a decrease in percentage 

adsorption K after initially adding with different K levels for the three first K levels and the 

presence of adsorbed K for all studied soils at the latest three added K concentration levels, are 

close to each other and in parallel. These results are similar to the finding by Auge et al [37]. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |101 

 

 

 

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Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |102 

 

 

Figure 2: Potassium adsorption characteristics of (A) Bestan Sur, (B) Serwan, (C) Grdigo, 

(D) Khurmal. (E) Kharpany, and (F) Kanypanka locations. 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |103 

 

 

Figure 3: Percent K adsorbed as a function of different K concentration levels of studied soils. 

3.1 Comparison of different adsorption isotherms 

The sorption isotherms precision alters depending on the soil. In comparison to the Langmuir 

and Temkin models, the Freundlich model exhibited an advanced match (Figure 4) of 

equilibrium K adsorption for the soil of silty loam type at Srwan site, as demonstrated by the 

(R2) values coefficient (Table 2). These findings are consistent with Kassa et al. [38], who 

indicated that the Freundlich model performed better than the other model in describing K 

adsorption. This was due to qualitative changes in the mineralogy and chemistry of the fractions 

that were separated [39]. 

Freundlich's isotherm assumes low adsorption energy, for the constant Kf, and 1/n (Table 2) as 

a measure of adsorption extent [24]. The buffering capacity of the soil is performed by the 

constant value of 1/n. [40]. The 1/n value is a heterogeneity parameter, and it is a function of 

the strength of adsorption in the adsorption process, with less 1/n values indicating more 

heterogeneity [26]. As a result, the soils of the whole studied locations have a high 

heterogeneity, and a normal adsorption process [25] the n parameter values reflect even if the 

process of sorption is favorable or not [37]. For favorable sorption processes, the value of n is 

lies between one and ten [37,27]. Depending on the values of n parameters in this study the 

sorption processes for the whole studied soils are favorable (Table 2). These findings are 

consistent with Mam Rasul [41] findings. The adsorption capacity (KF) Freundlich constants 

describe the amount of potassium (K) held on non-specific sites they are ready to be released 

for uptake by plants during cropping season [24, 40]. It was 74.75 to 156.39 mg kg-1 on average, 

with a mean value (112.41 mg kg-1). The Langmuir isotherm assumes that all adsorption sites 

have the same affinity for the adsorbate molecules. As a result, the existence of adsorbed 

molecules at a nearby location is noteworthy [26]. From the intercept and slope of the straight 

equations of figure 5, depending on the Langmuir isotherm model's the maximum monolayer 

coverage capacity (b) ranged from 500.00 to 714.29 mg kg-1 (Table 2), all the studied soils 

indicated high monolayer coverage capacity possessing the best fit to Langmuir model. These 

findings are consistent with those of Auge et al. [37].  

 

 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |104 

 

Figure 4:  Freundlich adsorption isotherm for (A) Bestan Sur, (B) Serwan, (C) Grdigo, (D) 

Khurmal. (E) Kharpany, and (F) Kanypanka locations 

 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |105 

 

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The maximum monolayer capacity (b) can be used to calculate how much fertilizer should be 

applied to unfertilized soil [25]. The Langmuir isotherm constant (KL) ranged from 0.107 to 

0.425 Lmg-1. Smaller KL values, according to Mehandi and Taylor [42], mean that the adsorbed 

K is more accessible to transform to non –exchangeable form, either through the production of 

crystalline K or through occultation through K ions.  KL indicates the measurement of adsorbent 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |106 

 

sympathy, according to Del-Bubba et al. [43], A high KL value, according to Anderson and Wu 

[44] indicates the power of attaching to the minerals of the soil was clay. The adsorption nature 

of the potassium (K) for all studied soils is unfavorable because of the diffusion coefficient 

values (RL>1) (Table 2). These results not matching with the results of Mam Rasul [41], who 

found that the adsorption nature of potassium for calcareous soil in the Kurdistan Region of Iraq 

is linear because in his study the value of RL=1.  The maximum buffering capacity (MBC) of 

the studied soil ranged between (85.00 to 250) L kg-1 (Table 2). MBC is a capacity factor that 

assesses the soil's ability to release the restored K ions from adsorbed K to soil solution as 

mentioned by  Rehman et al. [45]. The ability of a soil to give K to the soil solution, according 

to Fried and Shapiro [46], is an important component in determining a soil's K status. 

 
 
Figure 5: Langmuir adsorption isotherm for (A) Bestan Sur, (B) Serwan, (C) Grdigo, (D) Khurmal. (E) 

Kharpany, and (F)  Kanypanka locations. 

The Temkin isotherm equilibrium parameters are calculated from the intercept and slope of the 

straight-line equation of figure 6. The binding constant, AT (L kg
-1) value, ranged from (0.003 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |107 

 

to 0.479) L kg-1 (Table 2), the soil of Kanypanks location had a high binding constant. The 

values of bT ranged from 10.46 to 13.47 J mole
-1 with the mean of 11.86 J mole-1, A positive 

value of bt implies that the adsorption processes in the soil of the analyzed areas are exothermic. 

Furthermore, high constant values related to sorption heat, B (J mol-1) demonstrate a high 

contact between the adsorbent and that of adsorbate, implying the mechanism of the exchange 

of ion [26]. These findings are consistent with the results of Mam Rasul [41]. 

.

 

 

Figure 6: Temkin adsorption isotherm for (A) Bestan Sur, (B) Serwan, (C) Grdigo, (D) Khurmal. (E) 

Kharpany, and (F)  Kanypanka locations. 

 

 



 

Kurdistan Journal of Applied Research | Volume 6 – Issue 1 – June 2021 |108 

 

4. CONCLUSION AND RECOMMENDATION 

The importance of isotherms in assessing soil K adsorption characteristics was revealed by the 

outcomes of this study. The low 1/n values of the soils investigated suggest that the soil has a 

low buffering capacity. Exchangeable potassium has high binding energy on exchangeable sites. 

These showed that the crop in the examined soils had a limited supply of K. As a result, it can 

be observed that K fertilizer is required to restore the soil solution K in the examined soils. This 

suggested that crops in the examined soils were inadequate in K, proposing that K should be 

one of the soil management options. 

 

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