https://orcid.org/0009-0000-0860-7104
Figures
Abstract
Understanding the distribution and release dynamics of potassium (K) in calcareous soils is essential for efficient nutrient management in arid and semi-arid regions. This study investigates K forms and their release behavior in ten agricultural soil pedons from the Darab region in southern Iran, characterized by diverse physiographic settings and soil orders (Entisols, Inceptisols, Aridisols, and Vertisols). Dominant clay minerals identified through X-ray diffraction included illite, smectite, and chlorite. The surface horizons showed greater average concentrations of soluble K (0.63%), exchangeable K (5.06%), and non-exchangeable K (6.5%), whereas structural K was most abundant in subsurface horizons (89.32%). Cumulative K release, assessed through successive CaCl₂ extractions, ranged from 267.7 to 572.4 mg/kg in surface soils and 145.2 to 433 mg/kg in subsurface layers. Release kinetics of non-exchangeable K were effectively described by the power function and pseudo-second-order models, indicating both rapid and sustained release phases. Among the studied soils, pedon 6 (Inceptisol) exhibited the highest release rate, as reflected in its b parameter suggesting enhanced K availability and faster equilibrium attainment. These findings underscore the critical influence of soil mineralogy on potassium dynamics and support the development of site-specific fertilization strategies.
Citation: Javan M, Abtahi A, Zareian G, Haghighatnia H (2025) Soil mineralogy and potassium forms distribution and release in some calcareous soils of the Darab Region, Iran. PLoS One 20(12): e0334155. https://doi.org/10.1371/journal.pone.0334155
Editor: Abhay Omprakash Shirale, ICAR National Bureau of Soil Survey & Land Use Planning, INDIA
Received: May 5, 2025; Accepted: September 23, 2025; Published: December 19, 2025
Copyright: © 2025 Javan et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All data are in the manuscript and Supporting information files.
Funding: The author(s) received no specific funding for this work.
Competing interests: Authors have declared that no competing interests exist.
1. Introduction
Potassium (K) is recognized as an essential nutrient for the growth and metabolism of plants, animals, and humans [1–4]. Although K accounts for approximately 2.6% of the Earth’s crust [5,6], it predominantly exists in mineral forms within the soil, particularly in clay minerals such as muscovite, biotite, illite, and feldspar [7–10]. If not adequately replenished through fertilization, especially under intensive cropping systems, soil K reserves gradually diminish, resulting in reduced plant performance and lower agricultural productivity [11].
In recent decades, the depletion of soil K has emerged as a major challenge in highly cultivated regions [12]. The quantity and bioavailability of K are strongly influenced by the physical and chemical characteristics of soils, including mineral composition, clay type and content, organic matter, and climatic variables such as temperature and moisture regimes [13]. These factors cause substantial variation in K status among soils. Thus, identifying and quantifying the different forms of K is critical to understanding its availability and guiding nutrient management strategies.
Soil K is commonly categorized into four distinct forms: soluble, exchangeable, non-exchangeable, and structural [1,14–16]. While plants primarily absorb potassium from the soluble and exchangeable pools present in the soil solution and on colloidal surfaces [9], these forms represent only a small fraction of the total potassium content. The vast majority of soil K—typically ranging between 90% and 98%—is retained in non-exchangeable and structural forms [17]. As depicted in Fig 1, the relative contribution of each form varies depending on soil type and mineralogical composition.
Several factors influence the transformation and distribution of K among these forms, including clay mineralogy, texture, organic matter, soil water status, and temperature fluctuations [12,13,15,18,19]. In particular, soils with higher proportions of 2:1 clay minerals (e.g., illite and smectite) are more likely to retain and slowly release K over time. In the calcareous soils of Iran—especially those under intensive agriculture—exchangeable K may not reliably represent plant-available K [20,19]. This is due to complex interactions between soil constituents and the limited mobility of K ions in high-pH conditions [21,22]. Therefore, evaluating K release from non-exchangeable forms becomes essential, particularly when assessing subsoil fertility across depth profiles [23].
Several studies have highlighted the contribution of non-exchangeable K as a medium-term K source under K-depleted or fertilized conditions [21,24]. However, limited research has been conducted on deeper soil layers, despite evidence suggesting that plant roots access K from multiple depths [17]. Moreover, the relationship between the type and quantity of clay minerals and K availability has garnered increasing interest in Iran and other arid to semi-arid regions [16,17,25–27]. One of the persistent challenges in these environments is the progressive reduction of soil K in the absence of targeted K fertilization [11,28].
Given the above context—and the scarcity of comprehensive studies focusing on K dynamics in calcareous soils of the Darab region in Fars Province, southern Iran—this study was undertaken with two primary objectives: (i) to assess the distribution of different forms of potassium across selected agricultural soils in relation to their mineralogical composition and degree of soil development, and
(ii) to investigate the kinetics of potassium release across soil horizons, with emphasis on the role of mineralogical properties.
2. Materials and methods
2.1. Study area
This study was conducted in the Darab region of Fars Province, southern Iran, which is one of the study areas investigated by the Soil and Water Research Department, Agricultural and Natural Resources Research Center, AREEO, IR Iran. It should be noted that the third author is academic staff of AREEO and no permission was required to access the field site. According to a previous study, ten soil profiles were selected in Darab regions, southern Iran with different physiography and the same agricultural capabilities. Soil profiles were dug, described, and classified according to Keys to Soil Taxonomy [29]. In the sampling points, information such as landform unit, soil parent material, slope, erosion status, drainage status, and land use were recorded. Then diagnostic horizons were distinguished and soil samples (a total of 38) were taken and transferred to the laboratory for soil analysis after air-drying and sieving (<2 mm).
2.2. Physicochemical properties of soils
Soil particle-size distribution was determined using the hydrometer method [30]. Soil pH was measured using the saturated paste extract, and electrical conductivity (EC) was measured in soil saturation extract using a conductometer. Organic carbon (OC) was determined by the wet oxidation method [31]. Cation exchangeable capacity (CEC) was measured by replacing exchangeable cations by NaOAc and exchanging Na+ with NH4OAc [32].
2.3. Mineralogical analysis
Before mineralogical analysis, samples were washed with distilled water to remove soluble salts and gypsum. Then carbonates, organic materials, and iron oxides were removed using 1 N Na acetate (pH 5), 30% H2O2, and dithionite citrate bicarbonate solution, respectively [33–35]. After the separation of clay fraction, samples were saturated with Mg and K, using 1 N MgCl2 and 1 N KCl, respectively. The Mg and K saturated samples were saturated by ethylene glycol and heated at 550°C, respectively. In addition, samples were treated with 1 N HCl to discriminate kaolinite and Fe-chlorite. These five treatments (Mg saturated, K saturated, Mg ethylene glycol (Mg-EG), K-550°C, and HCl treatments) were analyzed by XRD (X-ray diffraction). The relative abundance of clay minerals based on peak intensities was semi-quantitatively measured following the method of Johns et al. [36].
2.4. K forms
Different forms of K were determined for all 38 samples. Soluble K in the saturated extract was measured. Exchangeable K was determined through extraction with 1M NH4OAC at pH = 7 for 5 min [37] and non-exchangeable K with boiling 1 M HNO3 for 1 hour [38]. Total K was determined following digestion (383 K) of a 0.5-g soil sample with 10 mL of 48% HF and 1 mL of aqua regia (one part of concentrated HNO3 with three parts of concentrated HCl). Then structural K was calculated by subtracting total K from K extracted with boiling HNO3. K was measured on all filtered extracts using a Corning 405 flame photometer [39]. All analyses were carried out in triplicate and the mean values are presented.
2.5. Kinetic of K release
After preliminary analysis, 20 surface and subsurface soil samples were selected for the K release experiment. Kinetics of non-exchangeable K release was studied by successive extraction of 2g soil sample with 0.01 M CaCl2 [40–42] during 2, 4, 8, 24, 48, 72, 96, 120, 144, 168, 192, 360, 528, 696, 864, 1032 and 1200 minutes in two replications. The concentration of K in the supernatant was measured by a flame photometer (Corning 405). The release of non-exchangeable K with time was fitted to the following equations:
Power function equation:
Elovich equation:
Parabolic diffusion equation:
Psuedo second order
Where y was the quantity of K released (mg/kg) at time t, and (qe, k, a, b) was constants [17,43]. An essential concept of these equations is the invariant b, which is representative of the release rate of K. These mathematical models were evaluated through least squares regression analysis to ascertain which equation most accurately represented K release from soils. Coefficients of determination (r2) were derived from least squares regression of observed versus anticipated values. The rate constants of K release from soils were computed based on these models.
2.6. Data analysis
Statistical analysis of data was conducted utilizing the SPSS 20.0 program (SPSS Inc., Chicago, IL, USA) and all statistical tests were considered significant at p < 0.05. The kurtosis and skewness were ascertained for the evaluation of the data normality. Tukey HSD post hoc test was employed to ascertain the significance of variables among different groups. All graphs were generated in Microsoft Office Excel 2013.
3. Results
3.1. Soil classification
Based on soil temperature and moisture regimes, along with horizon development, the studied profiles were classified into four soil orders: Entisols (pedons 1, 2, 3, 5, 8, and 10), Inceptisols (pedons 4 and 6), Aridisols (pedon 9), and Vertisols (pedon 7). These soils formed under varying physiographic conditions, including gravelly colluvial fans, piedmont alluvial plains, and lowland settings. Given the limited rainfall in Fars Province, soil development is often minimal, and soil characteristics closely reflect those of the parent material. Selected physical and chemical properties of the soils are provided in Table 1. Clay content ranged from 15.9% to 44.2%, both extremes observed in Entisols (pedons 2 and 8). Cation exchange capacity (CEC) varied from 11.2 Cmol(+)/kg (Entisols, pedon 10) to 25.8 Cmol(+)/kg (Aridisols, pedon 9). The highest organic carbon content was recorded in the subsurface of pedon 5 (Entisols), while the greatest electrical conductivity (EC) was observed in pedon 9 (Aridisols). Soil pH values ranged from 7.4 to 8.2, with the highest in pedon 5.
3.2. K forms
As shown in Fig 2, the distribution of K forms varied significantly across pedons and horizons. The structural K fraction was highest in pedon 9 (Aridisols) at 4791 mg/kg, whereas the lowest soluble K value (1.65 mg/kg) was found in pedon 10 (Entisols), both in subsurface horizons. The maximum values for soluble, exchangeable, non-exchangeable, and structural K across all profiles were 42.4, 289, 357, and 4791 mg/kg, respectively. Pedon 9, with high CEC (21.7 Cmol(+)/kg) and clay content (33.6%), contained the highest exchangeable, non-exchangeable, and structural K concentrations. Pedon 8, with moderate CEC (13.5 Cmol(+)/kg) and 21.4% clay, exhibited the greatest soluble K. Statistical analysis revealed significant positive correlations between CEC and structural K (r = 0.640, p < 0.01), non-exchangeable K (r = 0.511, p < 0.01), and exchangeable K (r = 0.552, p < 0.01) (Table 2). In contrast, soluble K was negatively correlated with CEC (r = –0.349, p < 0.05). Similarly, positive correlations were found between clay content and exchangeable K (r = 0.511), non-exchangeable K (r = 0.540), and structural K (r = 0.630), while sand content was negatively associated with exchangeable K (r = –0.322) and non-exchangeable K (r = –0.496). EC also correlated positively with exchangeable K (r = 0.353, p < 0.05), whereas no significant correlation was found for OC and pH with any K form. Fig 2 illustrates K form distribution in surface and subsurface soils. Pedon 8 had the highest percentage of soluble K in surface soil, while pedon 6 showed the highest in subsurface soil. Pedon 3 exhibited the greatest exchangeable and non-exchangeable K percentages in both horizons, likely due to favorable temperature–moisture regimes and dominant illite mineralogy [12,44]. Non-exchangeable K was calculated by subtracting exchangeable K (extracted with 1M NH₄OAc) from K released by boiling HNO₃. It differs from structural K, as it resides in interlayer positions of 2:1 clay minerals such as vermiculite and illite and can be gradually mobilized under depletion conditions. In most pedons, the percentage of exchangeable and non-exchangeable K was higher in surface soils than in subsurface layers, except for pedons 7, 9, and 10. Structural K was generally more abundant in subsoils, with exceptions again being pedons 7, 9, and 10. These patterns suggest enhanced mineral weathering in surface soils, facilitated by more favorable temperature and moisture conditions.
3.3. Clay mineralogy
Clay mineral analysis (Table 3) identified illite, smectite, and chlorite as dominant, with minor quantities of vermiculite and kaolinite (<15%). The prevalence of illite was particularly evident, often decreasing with depth, except in pedon 10, where illite content increased—likely due to surface enrichment by wind-deposited materials or biotite transformation under fluctuating moisture–temperature cycles [12].
Smectite dominated the surface horizons of pedons 7, 9, and 10, while illite prevailed in others. In the subsurface, pedons 5, 4, 7, and 9 showed a shift toward smectitic dominance. Vertisols (pedon 7) and Aridisols (pedon 9) contained the highest smectite percentages (35–45%). The presence of smectite in well-developed soils supports its pedogenic origin [13], although its formation may also result from transformation under high pH and Mg-rich environments. Smectite’s enrichment in subsurface horizons may reflect downward migration through eluviation processes [12]. Chlorite, found in substantial quantities across most pedons, showed irregular depth distribution. In pedon 1, chlorite was the dominant mineral at both depths, likely of inherited origin. Previous studies in southern Iran have attributed chlorite, illite, and kaolinite to lithological inheritance [12,13,25,45] . Palygorskite was observed only in pedon 9, characterized by high salinity and the presence of calcic–gypsic horizons. Palygorskite is often found in saline and alkaline conditions, where groundwater influences its formation. High concentrations of Mg²⁺ and silica in groundwater are crucial for palygorskite development [12,46].
3.4. Kinetics of K release
K release over time, studied via 0.01 M CaCl₂ extraction (Figs 3 and 4), followed a biphasic pattern: an initial rapid phase (2–192 min) attributed to surface site exchange, and a subsequent slower phase (192–1200 min) reflecting K release from interlayer and edge positions [47]. The stronger bonding of K within these sites, along with the larger hydrated radius of Ca² ⁺ relative to K ⁺ , slows ion exchange over time [20,48,49]. Modeling of K release data using pseudo-second-order, parabolic diffusion, Elovich, and power function equations revealed that the power function and pseudo-second-order models provided the best fit based on R² and SE values (Table 4) [20,50,51]. At the same time, the results were also fitted with zero and first-order equations, but the fitting results were poor. The b parameter of the power function was less than 1 in all cases, indicating that K release declined over time [52]. The highest a and b values were observed in pedons 5 (Entisols) and 6 (Inceptisols), respectively, with pedon 6 exhibiting. This pedon, containing 35–45% illite, 7.8% non-exchangeable K, 4.8% exchangeable K, and 1.12% soluble K, showed rapid and efficient K release, likely necessitating regular K fertilization to maintain soil fertility. According to Mengel and Uhlenbecker [53], the b parameter correlates with plant K uptake, emphasizing the importance of monitoring both exchangeable and non-exchangeable K. Without adequate K replenishment, especially in arid and semi-arid regions, soil K reserves may decline significantly over time [28]. Pseudo-second-order kinetics further confirmed these observations, with high R² values (0.9927 for surface and 0.9886 for subsurface soils) and low SE. The lowest rate constant (k = 0.0025) and highest equilibrium K (qe = 555.56) were recorded in pedon 2 (Entisols), suggesting a high K release potential but short equilibrium time. This profile had the lowest soluble and non-exchangeable K percentages, highlighting the importance of K fertilization during the growing season.
4. Discussion
The relative abundance of clay minerals in the studied soils followed the general trend: illite > smectite > chlorite > vermiculite > kaolinite > palygorskite. Given the calcareous nature of the soils and the arid to semi-arid climate of the Darab region, most illite, chlorite, and kaolinite appear to be inherited from parent materials. In contrast, smectite and a portion of illite likely formed through pedogenic processes influenced by local climate, moisture availability, and soil development intensity. Pedon 9 (Aridisols), located in a lowland physiographic unit and characterized by 33.6% clay and CEC of 21.7 Cmol(+)/kg, exhibited the highest levels of exchangeable, non-exchangeable, and structural potassium. The dominance of smectite in this soil likely contributes to this observation, as smectite’s high surface area enhances K retention [54]. The presence of palygorskite in this pedon further supports its formation under saline conditions. Among the Entisols, pedon 8 had the highest percentage of soluble K, while pedon 3 contained the highest exchangeable and non-exchangeable K. These patterns can be attributed to higher clay content, greater CEC, and the dominance of illite. Illite is known to contribute to both exchangeable and non-exchangeable K pools and provides a buffering effect against K depletion [12,19]. In some contexts, its interaction with vermiculite may further enhance K availability, especially in humid conditions [8].
In most pedons, non-exchangeable K was more concentrated in the surface horizons compared to the subsurface, except in pedons 7, 9, and 10. This distribution suggests more intensive mineral weathering at the surface, driven by moisture and temperature fluctuations [55]. Weathering of micas and K-rich primary minerals likely contributes to the release of K into non-exchangeable forms at shallower depths [56]. On the other hand, structural K was generally more abundant in subsurface layers—further evidence of surface mineral breakdown and redistribution. A strong positive correlation was found between exchangeable K, CEC, and clay content, consistent with previous studies [57–61]. Since exchangeable K represents the portion held on mineral exchange sites, its availability depends largely on the type and amount of clay—particularly 2:1 and 2:1:1 clay minerals prevalent in these soils [62]. The inverse relationship observed between soluble K and CEC further supports the idea that stronger cation exchange capacity leads to greater K retention. K release experiments showed a two-phase kinetic pattern: a rapid initial release from external surfaces, followed by a slower, sustained release from interlayer and edge sites. This biphasic behavior reflects the binding strength of K in various mineral positions. Initially, K is readily exchanged with Ca²⁺ at external surfaces. Over time, the release slows due to the larger hydrated radius of Ca² ⁺ compared to K ⁺ , which impedes its ability to displace interlayer-bound K [57]. Interestingly, this slower exchange process may help maintain a more stable nutrient environment, supporting gradual plant uptake over time. Among the kinetic models tested, the power function and pseudo-second-order equations best fit the release data, as indicated by high R² and low SE values. The b parameter of the power function was consistently <1, indicating a declining release rate over time [52]. Pedon 6 (Inceptisols), which contained 35–45% illite and relatively high levels of non-exchangeable and exchangeable K, had the highest b suggesting efficient and rapid K release. Pseudo-second-order modeling yielded R² values of 0.9927 and 0.9886 for surface and subsurface soils, respectively. Pedon 2 (Entisols) showed the highest predicted K release capacity (qe = 555.56) and the lowest rate constant (k = 0.0025), indicating high K availability but a short time to equilibrium. These findings highlight the need for targeted K fertilization strategies, particularly during periods of high plant demand.
5. Conclusion
This study demonstrated that soil mineralogy, particularly the presence and distribution of illite and smectite, plays a crucial role in controlling potassium (K) dynamics in calcareous soils of the Darab region. Understanding how various forms of K behave across different soil types and depths provides valuable insight for improving nutrient management in arid and semi-arid environments.
- The dominant sequence of clay minerals across the studied soils was: illite > smectite > chlorite > vermiculite > kaolinite > palygorskite. While illite, chlorite, and kaolinite were mainly inherited from parent materials, smectite and some illite appeared to have both inherited and pedogenic origins.
- Among the studied pedons, pedon 9 (classified as an Aridisol), located in a lowland physiographic setting and containing 33.6% clay and a CEC of 21.7 Cmol(+)/kg, showed the highest concentrations of exchangeable, non-exchangeable, and structural K. Smectite was the dominant mineral in this profile, and the presence of palygorskite likely reflected the soil’s high salinity.
- The highest proportion of soluble K was recorded in pedon 8 (Entisol), whereas pedon 3 (also an Entisol) had the highest levels of exchangeable and non-exchangeable K. These findings are likely linked to higher clay content, elevated CEC, and the presence of illite as the dominant mineral.
- Non-exchangeable K was generally more concentrated in surface horizons, particularly due to enhanced weathering of primary and secondary minerals under surface temperature and moisture fluctuations. In contrast, structural K was typically higher in subsoil layers, providing further evidence of surface mineral transformation.
- Potassium release followed a two-phase kinetic pattern: an initial rapid release from external mineral surfaces, followed by a slower release from edges and interlayer positions. Over time, the exchange of K with calcium became less efficient due to the larger hydrated radius of Ca² ⁺ , leading to a gradual decline in the release rate.
- Release kinetics were best described by the power function and pseudo-second-order models. In all soils, the b parameter was less than one, indicating a decreasing release rate over time. Pedon 2 (Entisol) exhibited the highest K release potential, with the largest values for the a. Interestingly, this soil had the highest structural K content but the lowest non-exchangeable and soluble K, suggesting that a large reservoir of K was rapidly mobilized over a short time. The high qe and low k values further confirmed that K equilibrium was reached quickly in this profile.
In conclusion, the findings underscore the importance of considering both mineralogical composition and K release behavior when designing site-specific fertilization programs. Tailored management strategies are especially vital in dryland agricultural systems where maintaining long-term soil fertility is critical and nutrient losses can be difficult to recover.
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Supporting information
S1 File. raw data-Supporting Information files.
https://doi.org/10.1371/journal.pone.0334155.s001
(XLSX)
Acknowledgments
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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