Supplementary data.

Factors such as topography, mineralogy, physicochemical properties, and climate can affect the distribution of soil potassium (K) forms. However, often the special effects of physiographic units are neglected. Therefore, this study aimed to investigate the factors controlling the distribution of chemical fractions of K in different physiographic units of calcareous soils (piedmont plain, flood plain, alluvial plain, lowland, badland, and plateau) in southern Iran. The XRD analyzing patterns showed that the distribution of K forms is controlled by K-bearing minerals (i.e., illite) in finer particles of the soils. Physiographic units significantly affect the distribution of K forms due to different microclimates (such as soluble, exchangeable, and non-exchangeable). In addition, different correlations between the K forms and some physicochemical properties of the soils such as soil texture (i.e., silt (r = 0.511** to 0.527**) and sand (r = -0.357* to -0.389*)), electrical conductivity (EC) (r = 0.617** to 0.723**), sodium absorption ratio (SAR) (r = 0.478** to 0.577**), pH (r = 0.347* to 0.519**), and gypsum (r = 0.372* to 0.475**) were found in soils of the study area. It is recommended that cultivation sites are chosen with a deeper understanding of land conditions e.g., slope, elevation, microclimatic conditions, soil development, and clay mineralogy.地形、矿物学、理化性质及气候等因素，均可影响土壤钾素（K）形态的分布规律，但过往研究往往忽视了地貌单元的独特效应。为此，本研究以伊朗南部石灰性土壤的不同地貌单元（山前平原、泛滥平原、冲积平原、低地、劣地及高原）为研究对象，旨在探究调控土壤钾化学形态分布的关键因素。X射线衍射（XRD）分析图谱显示，土壤细粒组分中的含钾矿物（如伊利石）主导了钾素形态的分布特征。由于微气候条件存在差异，地貌单元对钾素形态（包括可溶性钾、交换性钾与非交换性钾）的分布具有显著影响。此外，本研究区域的土壤中，钾素形态与多项土壤理化性质间呈现出不同程度的相关性：包括土壤质地（粉粒(r = 0.511** ~ 0.527**)与砂粒(r = -0.357* ~ -0.389*)）、电导率(EC)(r = 0.617** ~ 0.723**)、钠吸附比(SAR)(r = 0.478** ~ 0.577**)、pH值(r = 0.347* ~ 0.519**)及石膏含量(r = 0.372* ~ 0.475**)。研究建议，在选择种植地块时，应深入了解坡度、海拔、微气候条件、土壤发育程度及黏土矿物学特征等立地条件。