Soil Potassium Dynamics and its Relationship with Clay Mineralogy in Agricultural Soils of Shahrekord Piedmont Plain

Background and Objectives:  The identification of the type and composition of clay minerals in soil is of particular importance, as these minerals play a fundamental role in potassium dynamics and the soil’s capacity to supply this essential element to plants. Clay minerals can influence the uptake, retention, and release of potassium, and therefore play a crucial role in fertilization management and maintaining soil fertility. The present study aims to investigate the relationship between different forms of soil potassium, including soluble, exchangeable, and non-exchangeable potassium, as well as the amount of released non-exchangeable potassium, in relation to the clay mineral composition of agricultural soils.
 
Materials and Methods:  15 soil samples were collected from the 0–30 cm depth of agricultural soils in the Shahrekord Piedmont Plain, based on variations in exchangeable potassium, non-exchangeable potassium, cation exchange capacity and clay content. The physical and chemical properties of the soils were analyzed. Also, the exchangeable potassium of the soils was determined using 1 N ammonium acetate, and the non-exchangeable potassium was determined using 1 M boiling nitric acid. The amount of non-exchangeable potassium released after 2017 hours using the sequential extraction method with 0.01 M calcium chloride (CaCl₂). The composition of clay minerals were examined. The clay samples were saturated with potassium and magnesium, and to identify the types of clay minerals, Mg, Mg–ethylene glycol, Mg–glycerol, K, and K-heated treatments at 110, 350, and 550 °C were analyzed using an X-ray diffraction (XRD) instrument.
 

 



Results: The results showed that the range of soluble, exchangeable, and non-exchangeable potassium concentrations were 11.5–99.7, 108.5–612.7, and 491- 2065 mg kg⁻¹, respectively. The range of non-exchangeable potassium released after 2017 hours varied from 225.5 to 479.0 mg kg⁻¹. The dominant soil minerals included; micas, chlorites, kaolinite, smectites, quartz, and a small amount of mixed mica–chlorite minerals, with variations in their relative abundances. In soils with similar clay and silt contents, differences in potassium release behavior are mainly attributed to the type and proportion of clay minerals. Soils with the lowest levels of exchangeable and non-exchangeable potassium exhibited the highest non-exchangeable potassium release, whereas soils with high non-exchangeable potassium content showed a lower release percentage. The results showed that different forms of potassium had significant relationships with certain soil properties; Soluble and non-exchangeable potassium, as well as the cumulative amount of non-exchangeable potassium released after 2017 hours, were positively correlated with electrical conductivity (r=0.54**, r=0.45**, r=0.25**); soluble and non-exchangeable potassium were negatively correlated with cation exchange capacity (r=-0.18* , r=-0.26*); soluble potassium was positively correlated with organic matter (r=0.21*); and exchangeable potassium was negatively correlated with calcium carbonate equivalent (r=-0.34**). The different forms of potassium and the amount of non-exchangeable potassium released after 2017 hours showed no significant correlation with the total clay content. Different forms of potassium and the amount of non-exchangeable potassium released after 2017 hours showed no significant correlation with total clay content, indicating that potassium dynamics in these soils are influenced more by the type and composition of clay minerals than by the clay content itself.
 
Conclusion: The results of this study indicated that potassium dynamics in the agricultural soils of the Shahrekord plain are primarily influenced by the type and composition of clay minerals. In other words, the quantity and rate of non-exchangeable potassium release in soils depend more on the abundance and type of potassium-bearing minerals, such as micas and smectites, rather than merely the total clay content. Soils with high smectites content exhibited faster non-exchangeable potassium release, whereas micas-rich soils had high non-exchangeable potassium content but a lower release rate. These findings highlight the importance of understanding clay mineralogy and the different forms of potassium for designing optimal potassium fertilization programs. To meet crop potassium requirements and avoid excessive fertilizer application, it is necessary to consider not only exchangeable and non-exchangeable potassium levels but also the clay mineral composition and the behavior of potassium release.