Journal of
Soil Science and Environmental Management

  • Abbreviation: J. Soil Sci. Environ. Manage.
  • Language: English
  • ISSN: 2141-2391
  • DOI: 10.5897/JSSEM
  • Start Year: 2010
  • Published Articles: 315

Full Length Research Paper

Potassium adsorption characteristics of five different textured soils under enset (Ensete ventricosom cheesman) farming systems of Sidama zone, South Ethiopia

Kibreselassie Daniel Auge
  • Kibreselassie Daniel Auge
  • Department of Plant and Horticulture Science, Hawassa University, Ethiopia.
  • Google Scholar
Tekalign Mamo Assefa
  • Tekalign Mamo Assefa
  • Ministry of Agriculture, Ethiopia.
  • Google Scholar
Wassie Haile Woldeyohannes
  • Wassie Haile Woldeyohannes
  • Department of Plant and Horticulture Science, Hawassa University, Ethiopia.
  • Google Scholar
Bizuayehu Tesfaye Asfaw
  • Bizuayehu Tesfaye Asfaw
  • Department of Plant and Horticulture Science, Hawassa University, Ethiopia.
  • Google Scholar


  •  Received: 27 October 2017
  •  Accepted: 12 December 2017
  •  Published: 31 January 2018

 ABSTRACT

A laboratory study was conducted to determine the potassium (K) adsorption characteristics of five soils under enset (Ensete ventricosom cheesman) farming system in Sidama. Potassium adsorption isotherms were constructed by equilibrating 5 g soil samples with 5 levels of K (0 to 78mg/L) as KCl in 10 mmoles/L CaCl2 solutions. Suspensions were shaken for half an hour. Adsorption data were fitted to Freundlich, Langmuir and Temkin adsorption equations. On an average, amount of K adsorbed ranged from 29.2 to 67.82% of added K. Significant positive relationship existed among percent K adsorbed, CEC, percent clay and pH. Langmuir equation gave a better fit of equilibrium K adsorption data for sandy clay loam and silty clay loam soils, while Freundlich equation gave a better fit for clay and loam soils. Furthermore, Temkin isotherm gave a better fit of equilibrium K adsorption data for clay loam soil. Sandy clay loam and loam soils had low 1/n values. The n values on the other hand indicated that the sorption is favorable only for these soils. The Freundlich adsorption capacity, kf were found low as compared to available soil potassium. The negative values of Langmuir isotherm maximum monolayer coverage capacity (a) for clay loam and clay soils agreed with the absence of best fit to the model. Sandy clay loam and silty clay loam which indicated best fit had high monolayer coverage. Value of Temkin isotherm equilibrium binding constant (AT) was high for clay loam soil and this was manifested by high coefficient of determination. Low values of constant related to heat of sorption (B) were determined for sandy clay loam and loam whilst clay loam, silty clay loam and clay soils indicated high binding energy (B).

Key words: Sorption isotherm, Freundlich, Langmuir, Temkin model, monolayer coverage, equilibrium binding constant, heat of sorption.

 


 INTRODUCTION

Enset (Ensete ventricosum Cheesman) farming system is a part of sustainable production and enset has been cultivated in Ethiopia since ancient times (Garedew et al., 2017). It is most commonly grown in home-gardens, frequently intercropped with peas or beans, which is suitable to compensate for the low protein level in enset foods (Abebe et al., 2010). Enset is also intercropped with cereal crops (maize), root crop (sweet potato), vegetable (cabbage) and coffee.  Potassium is an essential element for crop production and productivity. It is required for the activation of over 60 enzymes involved in the formation of carbohydrates, translocation of sugars, various enzyme actions, yield, quality parameters, tolerance to certain diseases, mechanisms to overcome abiotic stress, cell permeability and several other functions (Askegaard et al., 2004). Plants require immediately available forms of K such as exchangeable and soluble forms. Availability of K is however affected by physical, chemical and biological processes of soil (International Plant Nutrition Institute, 1998). Soils K availing capacity can be assessed using different techniques. Among these, the study of K adsorbing potential or transforming available K forms into unavailable ones is one of the techniques.
 
Adsorption is the accumulation of a chemical species at the interface between solution and the solid phase (Sposito, 1989). It affects the mobility and fate of nutrients in the soil. Soils K adsorption potential depends upon the amount and type of clay minerals (Pal et al., 1999). Moreover, equilibrium among the potassium retained by the interlayer sites, surface and edge sites of mineral crystal lattice and potassium in soil solution also affect soils’ K adsorption potential. The mobility of soil potassium is affected by the dynamic equilibrium that exists in soil system. This dynamic equilibrium is affected by clay minerals types, pH, soil organic matter (SOM), hydroxide aluminum, soil moisture status, cation exchange capacity (CEC), fertilization and tillage system (Pannu et al., 2003). Among the clay mineral types influencing the dynamic equilibrium, illite and vermiculite were found to have positive associations with quantities of adsorbed K (Goulding, 1987). Due to this, the fate of K fertilizer in soils differs for different soils and the responses of crops to applied K are erratic and unpredictable.
 
Studies show that adsorption of K increases as the K imbalance is increased by addition of fertilizer and the degree of adsorption varies from soil to soil. Pal et al. (1999) reported the range of percent K adsorbed to be 5 to 67% of added K and also reported the negative effect of high K adsorption capacity of soils on the availability of K to crops. This implies that the rise in percent of K adsorbed by a given soil indicates the soils poor capacity to avail K to crops. To visualize the relationship between the quantities of K fixed per unit soil weight and the concentration of K in solution, several equations or adsorption isotherms were developed. Among the isotherms, Langmuir and Freundlich equations are mostly employed and described the adsorption phenomena satisfactorily (Boschetti et al.,1998). The Freundlich equation is an empirical equation which corresponds to a model of adsorption where the affinity term decreases exponentially as the amount of adsorption increases. Potassium adsorption was described well often by Freundlich equation over a limited range of concentration (Barrow, 1978). The theory of Langmuir is restricted to cases where only one layer of molecule can be adsorbed on the surface. Therefore, the non-conformity to the Langmuir model suggests the presence of several types of K sorption sites in the soils, each with different selectivity for K (Hannan et al., 2007).
 
The study of K adsorbing potential of soils could help clarify the relationship that exists between K adsorption capacity and other soil properties (Murashkina et al., 2007). It is undertaken by investigating the relation existing between amounts of soil solution K and its amount adsorbed by the soil (sorption isotherm). This provides information on whether any of the applied plant nutrients fix, react or make complex with the soil and helps optimize fertilizer to enhance crop productivity (Hunter, 1980). Thus, K sorption isotherm can be used to evaluate the ability of soils to supply K to plants and to determine interchangeable K in order to understand its dynamics in soil (Yunda et al., 1997). It can also be used to describe the exchange of K from the soils by other ions particularly, Ca (Bedrossian and Singh, 2002). To date, there has been a general perception that soils of Ethiopia have sufficient amount of potassium. This perception is mainly due to the earlier work done by Murphy (1968) and hence; soil K adsorption studies were limited in Sidama, South Ethiopia. The present study was therefore conducted to assess and describe the K adsorption characteristics of soils under enset farming systems of Sidama, South Ethiopia. The results of this study are expected to illuminate light on the effects of some soil properties on the potassium adsorption characteristics of soils in the study area.

 


 MATERIALS AND METHODS

Study area
 
The study was conducted in Awassa-Zuriya, Dale and Hula districts of Sidama zone, Southern Ethiopia (Figure 1) in 2016. Sidama administrative zone is located within 5°45’- 6°45’N latitude and 38°39°E longitude, covering a total area of 6,538.17 sq km of which 97.71% is land and 2.29% is covered by water (SZPEDD, 2004). It lies in the area varying from flat land (19.9 to 24.9°C) to highland (15 to 19.9°C) (Sidama Development Corporation, 2000). The regional and zonal capital, Hawassa, which is located in the northern tip of Sidama zone, has a distance of 275 km from Addis Ababa. In the present study, sample districts from the zone were randomly selected because nearly all areas in the zone have good potential for enset production irrespective of productivity variation due to rainfall and altitude discrepancy. The sites are located between 6°28'15.5" - 7°04'50.3"N latitude and 38°20'7.8" - 38°32'36.5"E longitude.
 
 
Soil sampling
 
Samples were taken from enset farm fields in Sidama, South Ethiopia from the depth of 0 to 50 cm. From samples collected, five samples were chosen based on OECD guideline for the testing of chemicals (2000). The samples represent Awassa-Zuriya district, Jara hinesa and Oudo Wetame; the Hula district, Teticha and Adola kura and the Dale district, Moto dorsiso villages. The soil samples then were air-dried and ground to pass through 2 mm sieve before use.
 
Soil analysis
 
The pH was determined by 1:2.5 soil-water suspension by a glass electrode, EC was determined by conductivity meter at 25°C (Jackson, 1967) and CEC was determined by the 1 N NH4OAc extraction method (Lu, 1999). The exchangeable potassium was determined using Mehlich III extractant (Mehlich, 1984). Particle size distribution was determined by the sedimentation procedure using hydrometer method after dispersing the soil with sodium hexametaphosphate (Bouyoucos, 1951). Organic carbon was determined by the wet oxidation method of Walkley-Black (1934).
 
Potassium adsorption experiment procedure
 
Soils were analyzed by the quantity/intensity procedure based on the method outlined in Beckett (1964). A stock solution of potassium chloride, 10 mmole/L KCl and calcium chloride, 10 mmole/L CaCl2 were prepared separately. From these separate solutions, graded concentrations of potassium (0.0, 0.2, 0.4, 1.0 and 2.0 mmole/L in 10 mmole/L CaCl2) were prepared and used for equilibration study. From each of the soil samples, duplicates of 5 g soils were weighed and quantitatively transferred into each of the five plastic tubes. Then, 50 ml of the graded concentration solutions was added to the correspondingly labeled plastic tubes. The suspensions were then shaken with a horizontal flask shaker for half an hour (180 rpm and 25 ± 1°C) and allowed to equilibrate overnight. After equilibration, suspensions were filtered using Whatman No. 42 filter paper, and the equilibrium solutions were analyzed for potassium by Flame Photometer. Adsorption isotherms were constructed based on the method described by Rowell (1994). The amount of K adsorbed was obtained by subtracting the amount determined in supernatant solution from the initial amount of K added as in the following.
 
Concentration of Adsorbed K = CKi – CKf       (1)
 
Where CKi = initial concentration of added K and CKf = concentration of K in supernatant solution percent K adsorbed (%) = (concentrations of K adsorbed × 100)/concentrations of added K.
The K adsorption data were fitted into the following adsorption isotherm equations: Freundlich Adsorption equation:
 
Log (x/m) = Log kf + 1/n Log C                    (2)
 
Where x/m is the mass of adsorbed K per unit mass of soil mg/kg, C is the equilibrium K concentrations of solutions (mg/L), kf and 1/n are constants obtained from the intercept and slope respectively.
 
Langmuir adsorption equation
 
1/(x/m) = 1/a + 1/akLC                               (3)
 
Where C is the equilibrium solution K concentration (mg/L), x/m is the mass of K adsorbed per unit mass of soil (mg/kg), kL is a constant related to bonding energy of K to the soil, and ‘a’ is the soil’s maximum monolayer coverage capacity during K adsorption. The values of ‘a’ and ‘kL’ are obtained from the intercept (a) and the slope (b), respectively.
 
The Temkin equation:
 
x/m = BlnAT + Bln C                                  (4)
 
where, x/m is the mass of K adsorbed per unit mass of soil (mg/kg), C is equilibrium solution K concentration (mg/L), ‘AT’ is Tempkin isotherm equilibrium binding constant and ‘B’ is constant related to heat of sorption. The values of ‘AT’ and ‘B’ are obtained from the intercept and the slope, respectively. For all isotherms, graphing and mathematical
computations were undertaken using Excel sheet facilities.
 
Statistical analysis
 
 
Data analyses were performed with the statistical analysis system (SAS Institute, 2012). Correlation analysis was used to determine the relationship with percent K adsorbed, CEC, clay and pH.

 


 RESULTS AND DISCUSSION

Soil physico-chemical properties
 
Power of hydrogen (pH) of soils ranged from 4.8 to 7.5 (Table 1). According to EthioSIS (2014), the Awassa- Zuriya district, Jara hinesa kebele clay loam and the Dale district clay samples showed neutral reaction. The Hula district Teticha kebele sandy clay loam and Adola kura kebele loam samples indicated strongly acidic reaction (pH<5.5) (EthioSIS, 2014). On the other hand, Awassa-Zuriya district Oudo Wetame kebele silty clay loam had high pH (7.5) as compared to the rest soils. The soil OC contents ranged from 1.23 to 4.12%. According to the rating by Landon (2014), very low value was determined for Awassa-Zuriya district, while the Hula and Dale district soils had low values. Soil OC influences the physical, chemical and biological properties of soils such as structure, water retention, nutrient contents and retention as well as soil microbiological activities. The CEC (cmol (+) kg-1) of the samples ranged between 25.98 and 36.9. Based on Landon (2014), the range intervenes in the high rate (25 to 40) of the CEC. According to EhioSIS (2014), the Hula district Teticha and Adola kura kebele soils had low levels of exchangeable K due to low percent clay and pH, while high levels of exchangeable K were determined in Awassa-Zuriya district soils. The Dale district soil had optimum level of soil K (Table 1).  The particle size distribution of the soils ranged from 22 to 42%, 26 to 45% and 16 to 51 for clay, silt and sand, respectively. Electrical conductivity of the studied soils ranged between 0 and 2 dS/m and it has negligible salinity effect on crop plants according to the rating by Rhoades and Loveday (1990).
 
 
Soils potassium adsorption characteristics
 
 
Potassium adsorption isotherms of the soils studied are presented in Figure 2. The soils showed noticeable variation in their K adsorption characteristics. The percent of K adsorbed was not uniformly increased with increasing K concentrations (Figure 3). For the initial added concentration of K (7.8 mg/L), K adsorbed varied from 35.89 to 69.3% and for the highest added concentration of K (78 mg/L), it varied from 29.5 to 68.9%. The silty clay loam, sandy clay loam and loam soils indicated decreased adsorption after initially adding K and following this, slightly increasing trend in K adsorption was observed. The clay soil increased K adsorption for all added K levels. Apart from this, the clay loam soil increased adsorption for the first three K levels and decreased adsorption upon K levels increase. The equilibrium K concentration ranged from 2.4 to 5 mg/L for 7.8 mg/L of added K and from 24.2 to 55 for 78 mg/L of added K. Here, the lowest equilibrium K was detected for Awassa-Zuriya district Jara hinesa silty clay loam soil while the Hula district Adola kura loam soil had the highest Equilibrium K.

 

 
The high K in equilibrium solution for the Hula district Adola kura kebele loam soil could be due to relatively low clay percent (22) (Bangro et al., 2012) and strongly acidic pH (4.8) (Table 1). The linear relationship between added and equilibrium K showed the direct proportionality that existed (Figure 2). Furthermore, the soils indicated maximum adsorption behavior up to 53.8 mg/kg (silty clay loam for 78 mg/L added K) and the adsorption of K was found to increase with the amount of added K irrespective of soil types and texture. Soils varied in percent of adsorbed K for different concentrations of added K. On average, highest adsorption (67.82%) was recorded for silty clay loam Awassa-Zuriya district Oudo Wetame kebele soil. High K adsorption potential of this soil is due to high silt fraction (45%), clay fraction (39%), CEC (36.9) and pH (7.5) of the soil. Following this, Awassa-Zuriya district Jara hinesa clay loam soil adsorbed 66.38% of added K. This could be due to high pH (7.3) and percent silt (40), percent clay (39) and CEC (33.91).
 
 Further, the clay soil adsorbed 65.89% of added K and it might be attributed to high clay proportion (42%) and CEC (31.64). Bangro et al. (2012) also found similar correlation that soils with high clay and CEC values fixed more K as compared to soils with low clay and CEC values. These soils may contain dominant amount of illite (hydrous mica) type of clay minerals since such clay types increase K adsorption capacity of soils (Mengel and Busch, 1982). Lastly, the Hula district Teticha kebele sandy clay loam soil adsorbed 32.69% of added K and followed by Adola kura kebele loam soil which adsorbed 29.2% of added K. Relatively low percentages of K was adsorbed by these soils when compared with the first three is likely due to low clay percentages and strongly acidic (4.9) and pH (4.8) values, respectively (Table 1). From the results, it could be concluded that adsorption was mainly governed by CEC (Jafari and Baghernejad, 2007) clay content (Zhang et al., 2009) and pH (Loannou et al., 1994) of the soils. In general, the soils were found to adsorb high proportion of added K than letting it stay in equilibrium solution manifesting their high K fixation capacity.
 
Potassium adsorption study from graphical analysis of Freundlich, Langmuir and Temkin equations plots
 
The degree of accuracy of the sorption isotherms varied from soil to soil. The coefficient of determination (R2) values in Table 2 indicated that Langmuir equation gave a better fit (Figure 5) of equilibrium K adsorption data for silty clay loam soil. This is due to the homogeneity of sorption sites in the soil that allows only complete monolayer adsorption of solutes. On the other hand, the coefficient of determination (R2) value indicated that Freundlich equation gave a better fit (Figure 4) of equilibrium K adsorption data for clay soil. This implied that clay soil had unlimited adsorption sites having heterogeneous surfaces (Hutson and Yang, 2000). Lastly, Temkin isotherm gave a better fit (Figure 6) of equilibrium K adsorption data for clay loam soil. Since Freundlich isotherm assumes low energy of adsorption, its constants ‘kf’ and ‘1/n’ (Table 2) may be taken as a measure of the extent of adsorption and rate of adsorption or energy of adsorption (Voudrias et al., 2002). According to Kenyanya et al. (2013), the constant 1/n (Eq. 2) also represents the buffering capacities of soils. In the present study, it ranged from 0.76 to 1.22 kg/mg with a mean of 1.00±0.21 kg/mg. In the range, sandy clay loam and loam soils had low 1/n values due to high sand proportion.
 
 
 
 
 
The values of 1/n also indicate a heterogeneity parameter where smaller 1/n values reveal greater heterogeneity (Dada et al., 2012). Accordingly, sandy clay loam and loam soils had high heterogeneity (Table 2). The n values indicate whether the sorption is favorable or not (Goldberg, 2005). For favorable sorption processes, it lies between one and ten (Goldberg, 2005). From the data, only sandy clay loam and loam had the n values that lie between one and ten, indicating that the sorption is favorable only for these soils since they assume more than a single layer of adsorbed molecules. The adsorption capacity, kf (Eq 2) represents the amount of potassium held on non-specific sites that is ready to be released for uptake by plants during cropping season (Kenyanya et al., 2013). It ranged from 0.61 to 1.205 mg/kg and had a mean of 0.92±0.27 mg/kg. These values were low as compared to available soil potassium (Table 1), suggesting that part of exchangeable potassium is held on exchange sites by high bonding energy. For Langmuir isotherm, it is assumed that the adsorption sites have equal affinities for molecules of adsorbate. Therefore, the presence of adsorbed molecules at one site will not affect adsorption of molecules at an adjacent site (Dada et al., 2012).
 
In the present report, the maximum monolayer coverage capacity (a) from Langmuir isotherm model ranged between -1000 and 333 mg/kg. In the range, the monolayer coverage capacity of clay loam and clay soils was found to be below zero. These negative intercepts suggest that the adsorption behavior of the tested systems does not follow the assumption of the Langmuir approach. These soils also had no best fit to coefficient of determination (R2) of Langmuir model. On the other hand, soils possessing best fit to Langmuir model (sandy clay loam and silty clay loam) indicated high monolayer coverage. The Langmuir isotherm constant (kL) ranged from -0.011 to 0.017 L/mg. From the Temkin plot shown in Figure 6, values of Temkin isotherm equilibrium binding constant, AT (L/kg) ranged between 0.481 and 0.743. In this range, the clay loam soil had high binding constant and high coefficient of determination. Furthermore, low values of constant related to heat of sorption, B (J/mol) indicate a weak interaction between adsorbent and adsorbate supporting a mechanism of ion exchange (Dada et al., 2012). In the soils studied, the sandy clay loam and loam had low values of B, while clay loam, silty clay loam and clay soils had comparatively high values of B.
 
 
High repression existed due to high clay proportion in latter soils which could hamper the movement of K into soil solution where the uptake of nutrients takes place. Strong and significant positive correlation occurred between percent potassium adsorbed and soil properties such as pH, clay content and CEC of soils (Table 3 and Figure 7). The results are in line with the findings of Loannou et al. (1994) who reported positive association between pH and potassium adsorption as a result of formation of new sites where competition between H+ and K+ for these sites decreased due to high pH. Furthermore, the results coincide with the findings of Zhang et al. (2009) who found similar correlation that soils containing high clay content and CEC fixed more K as compared to those with low clay content and CEC. The amount and type of clay minerals affect the distributions of K between exchange and solution phases. Study on relationships occurring between adsorbed K and soil solution K may help in formulating precise fertilizer recommendations based on the adsorption capacity of the soils (Samadi, 2003).
 


 CONCLUSION AND RECOMMENDATIONS

Although, apparent variations were observed in adsorption behaviors; all soils adsorbed the added K to high degree. This indicated the high potential of studied soils to decrease crop availability of applied K. This indicates that crops in the sampling districts were in K deficient condition and hence K recommendation should be one of the soil management choices. A better fit of equilibrium K adsorption data to Langmuir, Freundlich and Temkin isotherms was different for different textured soils. This helps in describing the adsorption behaviors of soils based on their textures. Thus, the present research findings indicated the significance of the isotherms to assess soils K adsorption behaviors. The low 1/n values of Hula district Teticha kebele sandy clay loam and Adola kura kebele loam soils indicate low K buffering capacity and high heterogeneity of the soils. In addition to this, low values of the Freundlich adsorption capacity, kf, as compared to available soil potassium (Table 1 and 2) for Awassa-Zuriya district clay loam and silty clay loam and for Dale district clay soils suggested the binding of part of exchangeable potassium on exchange sites by high energy. These manifested the limitedness of K availability to crops in the studied areas.
 
Hence, it can be concluded that the studied districts would require K fertilization to replenish the soil solution K. The weak interaction between adsorbent and adsorbate indicated by constant related to binding energy (B) for Hula district sandy clay loam and loam soils implies the exposedness of K to be lost via leaching. However, potassium availability in these soils is relatively better as compared to those soils with high values of B such as clay loam, silty clay loam and clay. Over all, Awassa-Zuriya and Dale district soils require K application since K flow is favored to the direction where its fixation is boosted due to high binding potential. The Hula district also requires K fertilization since less binding potential may favor leaching of K. Significant positive correlation existed among percent K adsorbed, CEC, percent clay and pH which is convincing since the rise in contents of these soil properties creates room for large number of negative sites which adsorb and or fix K from soil solution. 


 CONFLICT OF INTERESTS

The authors declare that there is no conflict of interest.

 


 ACKNOWLEDGEMENTS

The authors are greatly indebted to International Potash Institute (IPI) for covering all research expenses and their devotion to support food insecure society through very indispensible and timely researches on potassium status of soils.



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