Mechanisms of potassium release from calcareous soils to different salt, organic acid and inorganic acid solutions M. Najaﬁ-Ghiri A,D , M. Niazi B , M. Khodabakhshi B , H. R. Boostani A , and H. R. Owliaie C A Department of Soil Science, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Islamic Republic of Iran. B Department of Agroecology, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Islamic Republic of Iran. C College of Agriculture, Yasouj University, Yasouj, Islamic Republic of Iran. D Corresponding author. Email: mnajaﬁghiri@yahoo.com Abstract. Mechanisms of potassium (K) release and ﬁxation in calcareous soils may differ from non-calcareous soils. In the current investigation, four soils with different properties were extracted three and 10 times (45 and 150 min, deﬁned as short- and long-term respectively) with 0.025 mol L 1 CaCl 2 , HCl and oxalic and citric acid solutions and the contents of calcium (Ca), magnesium (Mg), aluminium (Al) and K in the extracts were determined. Immediately after K extraction, 500 mg kg –1 of K (as KCl) was added to the remaining soils and shaken for 24 h and K-ﬁxation capacity was determined. No signiﬁcant difference was observed between CaCl 2 and HCl solutions in K extraction from soils in the short- and long-term experiments; the concentrations of Ca, Mg and Al in the extractants showed that HCl was not able to dissolve soil minerals and the soils released K via exchange reaction of Ca (originated from CaCO 3 dissolution by HCl) with interlayer K. The organic acids had similar behaviour to each other in the short- term experiment; however, partial dissolution of minerals by citric acid was observed. Oxalic acid extracted less K than other solutions in the short-term experiment due to precipitation of soluble Ca as oxalate salt and thereby less exchange of soluble Ca with non-exchangeable K. Oxalic and citric acids dissolved K-bearing minerals and released K in the long-term experiment, but oxalic acid was more effective. The K-ﬁxation capacity of soils was lower when treated with organic acids than with CaCl 2 and HCl, possibly due to the dissolution of K-ﬁxing minerals like smectite and illite by oxalate and citrate. Long-term treatment of soils with different solutions decreased K-ﬁxation capacity of soils due to dissolution of K-ﬁxing minerals or ﬂocculation of minerals by Ca and slower K diffusion to the interlayer. With respect to the highly calcareous nature of the studied soils, the soluble cations in calcareous soils (predominantly Ca and Mg) may have been exchanged with non-exchangeable K of clay minerals and buffered soluble K as well as organic acids produced by plant roots and microorganisms. Additional keywords: CaCO 3 , citric acid, exchange reaction, mineral dissolution, oxalic acid. Received 8 October 2018, accepted 23 January 2019, published online 1 March 2019 Introduction Potassium (K) is an important and necessary nutrient for plant growth and development and its behaviour in soil is of great interest to soil researchers. Soil K exists as soluble, exchangeable, non-exchangeable (NEK) and mineral forms (Sparks 2000). The mean contents of these K forms in calcareous soils of Iran are 20, 244, 763 and 5300 mg kg –1 respectively, with a total content of 6300 mg kg –1 (Najaﬁ Ghiri et al. 2011c). Although soil available K in calcareous soils of Iran is sufﬁcient, some researchers recently showed that its content has decreased due to extensive cropping with no K fertiliser application (Balali and Malakouti 1998). Potassium release from K-bearing minerals and its ﬁxation are two important processes of soil K in calcareous soils. The release of NEK from illite, vermiculite and smectite may increase the availability of K, while ﬁxation of K ions by smectite, vermiculite and illite decreases its availability (Najaﬁ Ghiri et al. 2011a; Najaﬁ Ghiri and Abtahi 2012). Many researchers have studied the release and ﬁxation of K and factors affecting these processes in calcareous soils (Jalali 2006; Najaﬁ Ghiri et al. 2011b; Hosseinpur et al. 2012). Potassium released to different solutions including salt solutions (e.g. dilute CaCl 2 ), inorganic acids (e.g. HCl, H 2 SO 4 and HNO 3 ), organic acids (e.g. oxalic, citric, acetic and malic) and sodium tetraphenyl boron may vary widely and depends on different factors such as content and type of clay minerals, mineral size, calcium carbonate (CaCO 3 ) and quartz contents, soil development and weathering status (Gil-Sotres and Rubio 1992; Cox and Joern 1997; Rao et al. 1999; Tu et al. 2007; Najaﬁ-Ghiri and Jaberi 2013). Journal compilation Ó CSIRO 2019 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2019, 57, 301–309 https://doi.org/10.1071/SR18301