Abstract

An investigation into the mineralogical controls on potassium (K) availability in calcareous soils cropped to maize (Zea mays L.) was carried out using two field trials located at 2 sites of central Iraq, Hilla and Al-Tali’a, Babil Governorate. Surface layer (0–30 cm) soil samples were retrieved before the start of the plot measurements, and physical and chemical properties, potassium species, and clay mineral composition by X-ray diffraction (XRD) were quantified. The Center of Gravity (Cg) index was employed to assess the degree of mica–smectite alteration and its impact on potassium availability. However, potassium fixing is strong in the soils, as show that available potassium levels (0.30–0.35 cmolc kg⁻¹) were below the critical limit of 0.36 cmolc kg⁻¹ despite relatively high total potassium contents (0.61–0.76 cmolc kg⁻¹). Forty-six–55% of potassium was non-exchangeable, highlighting significant structural fixation in kaolinite-type minerals. XRD revealed predominantly montmorillonite (35–45%), chlorite (26–32%) and illite (up to 30%), with lesser amounts of kaolinite (3–8%). Cg values (0.87–1.26) reflected different stages of mica alteration, but their relationship with the K is weak due to strong fixation processes and competition for retention between Ca²⁺ and Mg²⁺ cations with potassium. Clay content showed a strong positive association (R = 0.85, p < 0.05) with specific surface area, confirming the predominance of clay mineral composition over bulk total potassium in controlling availability of potassium status.