Different forms of soil potassium and clay mineralogy as influenced by the lemon tree rhizospheric environment

Lemon (Citrus limon), a species of the genus Citrus, is an economically significant crop and extensively cultivated in Iran, especially in Fars Province. Very little information is currently available on the changes in the different forms of soil potassium under the influence of lemon tree plantations or the associated mineralogical changes. The objective of this study was to investigate the status of different forms of K and the transformation of K-bearing minerals in the rhizosphere in lemon orchards of varying ages. Results demonstrated that the amounts of soluble, available, exchangeable, and nonexchangeable K in the control were higher than those in the lemon orchards of all the age range studied. The highest reduction in the different forms of soil K was measured for the 50-year-old orchard. The nonexchangeable K was significantly lower in the 20- and 50-year-old lemon orchards as compared with the control soil. Soil potassium concentration was observed to decrease with increasing soil depth. Our findings also indicated that vermiculitization of illitic clay minerals occurred due to both root potassium uptake and root activities in the rhizosphere of lemon trees. Shallower soil depths in the orchards were found to contain a higher illite content, while the vermiculite and interstratified I/S content increased with depth, except in the case of the 5-year-old orchard. Our findings also show that soils under lemon orchards of different ages exhibit differences in their K availability and, further, that vermiculitization might take place as a result of long-term cultivation despite the application of potassium fertilizers.