Potassium Availability in Tea Plantations of Different Ages Grown on Alfisols: Content, Dynamics, Release, and Composition of Potassium-Bearing Minerals

Potassium (K) deficiency is widespread in Alfisols on which tea is grown, and quantifying the availability of K to the tea plants is important in designing fertilizer regimes and in ensuring that tea production systems remain sustainable. In this study, the contents of available K, its release characteristics, and the composition of K-bearing minerals were determined for Alfisols on which tea had been grown for 5, 10, 15, or 30 years. The contents of exchangeable K (EK), non-exchangeable K (NEK), K extracted with sodium tetraphenylboron, release threshold values of total labile K, and equilibrium K concentration increased as the age of the plantation increased. However, the content of EK even in the oldest plantations was far below the critical value for K deficiency (80 mg kg). The concentration of total labile K was higher than that of EK irrespective of the age of the plantation, which points to serious exhaustion of NEK reserves. Furthermore, compared to older plantations, the younger plantations contained less of illite and chlorite and more of kaolinite and vermiculite in the clay fraction of the soils. Chlorite and illite was significantly positive correlated to available K either in the surface layer (0–15 cm) or in the deeper layer (15–30 cm), whereas kaolinite was significantly and negatively correlated to available K at both depths. Although tea plantations increased the reserves of soil available K, the increase was not enough either to overcome the deficiency of available K—which continues to be extremely severe—or to prevent the distortion of clay minerals in Alfisols. Adequate inputs of K are therefore essential for increasing the inherent capacity of the Alfisols to supply K and, in turn, to obtain high yields and superior-quality tea.