Evaluation of Non-Exchangeable Potassium Release from K-Bearing Minerals by Different Extractants

Potassium is one of essential nutrients for plants and its importance in agriculture is well known. Non-exchangeable potassium that is mainly placed with in layers of K-bearing minerals, such as K-feldspar and mica, is considered as an important source of potassium for plant growth in most soils. Regarding that low molecular weight acids (LMW) play an important role in improving the bioavailability of soil nutrients such as non-exchangeable K (NEK), and the release rate of NEK plays a significant role in supplying necessary K for plants, the purpose of this study was comparison of potassium release kinetic from K-bearing including feldspar, illite as well as phlogopite minerals and choose the best kinetic equation describing potassium release process, influenced by organic as well as mineral extractants.
Material and The experiment carried out in a completely randomized design with three replications. Experiment factors were including extractant type (0.01 mol l-1 oxalic acid, 0.01 mol l-1 calcium chloride, control (deionized water)), potassium mineral type (feldspar, illite and phlogopite) and incubation time (1, 2, 4, 8, 12, 16, 24, 32, 48, and 64 hours). Elemental composition of minerals identified by Fluorescence spectroscopy device (S4 Pioneer). Used minerals in the experiment including feldspar, phlogopite and illite were ground and filtered through a 230 mesh sieve. In order to remove exchangeable K, samples were saturated by calcium chloride solution (with a ratio of 2:1), after washing with HCl, samples were dried at 105 °C for 48 hours. 100 mg of washed minerals, was weighed carefully and transferred to centrifuge tubes. Then 20 ml of each of extractants (oxalic acid and calcium chloride 0.01M) was added to the tubes. After 15 minutes shaking, tubes containing a mixture of minerals-extractants was carried out in a controlled incubation chamber for periods of 1, 2, 4, 8, 12, 16, 24, 32, 48 and 64 hours at 25 °C. After each period, samples were centrifuged at 3000 rpm for 10 minutes and filtered using Whatman paper (No. 41). pH and potassium concentration of samples were measured by pH meter and flame photometer, respectively. Data related to potassium release was fitted by zero order, first order, second order, power function, parabolic diffusion and ellovich equations.
Results showed that the effect of extractant type was significant on kinetic of potassium release, so that potassium release amount in samples extracted with oxalic acid was 1.48 and 2.35 times higher than samples extracted with calcium chloride and control (deionized water), respectively. Also, different minerals released various amounts of potassium. K release from phlogopite was 1.99 and 2.95 times higher than feldspar and illite, respectively. The maximum potassium concentration (440 mg kg-1) was seen in phlogopite which was extracted with oxalic acid. So that, amount of potassium in this treatment was 3.15 times higher than control one. Furthermore, the effect of extraction time on K release was significant. So that, at the beginning of incubation period the release of potassium by different extractants was more, but its amount decreased over time and finally continued with a constant speed. Kinetic equation fitting showed that zero order, first order, power function, parabolic diffusion and ellovich equations are able to describe potassium release but second order model cannot justify it. Among these five equation, the power function and parabolic diffusion equations with the maximum coefficient of determination (R2) and the least standard error of estimate (SE), could reasonably describe the K release kinetics, so they are introduced as the best models for data fitting. The slope (b) and interception (a) of ellovich equation indicate interlayer and initial K release, respectively. Oxalic acid and phlogopite had the most amount of interception, it means that the impact of oxalic acid on initial and interlayer release rate of K in phlogopite, is more effective than calcium chloride.
It is concluded that different factors like mineral and extractant type influence kinetic of potassium release and organic extractant have more ability in extracting non-exchangeable potassium from minerals structure. Also, the adjustment of the results of this study with first order, parabolic diffusion and power function equations suggest that nonexchangeable potassium release from minerals can be affected by diffusion process from the surface of the study minerals, indicating that NEK release rate is controlled by K diffusion out of the mineral interlayer.