One of the potassium (K) sources for plant growth are the soil clay minerals (such as vermiculite) that have K element in their structure. The purpose of this research was the investigation of non-exchangeable K release from vermiculite clay soil to citric acid and calcium chloride extractants. Treatments included two vermiculite clay soil samples (with carbonatic (CV) and non-carbonatic (NCV)) and two extractants (0.01 M CaCl2 and citric acid). Thirteen successive extractions of K were done during 1, 2, 4, 6, 16, 24, 48, 72, 120, 168, 312, 384 and 552 h. Cumulative K release was determined and data was fitted to five kinetic equations. Results showed that there was a significant difference between treatments and extractants in K release. Cumulative K release to CaCl2 for CV and NCV was 3161 and 2935 mg kg−1, respectively while this was for citric acid 2172 and 1503 mgkg−1, respectively. The best kinetic models describing K release to CaCl2 and citric acid were Elovich (r = 0.98 and 0.80, p < .01) and power function equations (r = 0.96 and 0.76, p < .01). Maximum K release was observed in carbonatic vermiculite extracted with CaCl2 (1873 mgkg−1). Results of X-ray diffraction analysis showed that the peak intensity of vermiculite reduced with extraction time and no complete conversion were observed but its peaks showed the presence of mixed minerals such as mica-vermiculite and chlorite-vermiculite. Scanning electron microscopy analysis revealed that vermiculite clay soil structure was changed and more voids were observed in the mineral structure with extraction time. It’s recommended that release experiment is done in longer time for better monitoring of vermiculite structural changes. The present study showed that vermiculite clay soil taken from Jiroft mine can be used as a natural source of potash fertilizer in sustainable agriculture.
