Potassium dynamics and exchange equilibria in Loess-derived soils

A lack of cotton response to the recommended application of fertilizer potassium (K) has been well documented, prompting a renewed interest in the evaluation of soil and plant analysis. A study was conducted to evaluate varying rates and residual effects of fertilizer K on no-till cotton, in conjunction with the evaluation of cation exchange studies to establish the dynamics of K behavior in the loess-derived soils of west Tennessee. The soils were comprised of kaolinite, mica, and HIV-vermiculite. The effect of four rates of fertilizer K (0, 67, 134, and 269 kg K ha-1) , on yield and plant and soil K levels was assessed. Total vs. acetic acid-extractable plant tissue content was studied as a function of tissue type (leaf and petiole) and sampling time (at bloom and 2 after bloom). Mehlich 1 (M1) vs. Mehlich 3 (M3) soil K was determined on samples collected biannually, prior to fertilization and post-harvest, as a function of soil sampling position (in-row (IR) and between-row (BR)) and soil depth (0-7.5 and 7.5-15 cm). In preparation for cation exchange, the soils were initially saturated with Ca2+, Mg2+, or K+ and equilibrated at times varying from 2 h to 4 w. Exchange isotherms were generated as a function of equilibration times and total normality (TN). The results from the field study showed that soil moisture had a pronounced impact on the plant and soil K content and subsequently on cotton yield, as precipitation in June 1997 exceeded that received in June 1998. The plant K digestion and extraction methods and the M1 and M3 soil tests were highly correlated suggesting either test be used for the determination of K levels. The plant tissue type and time of sampling were also significantly correlated, although petiole collected at bloom may prove to be a more sensitive K indicator. The benefit of residual fertilizer K increased with increasing fertilizer levels. The plant and soil test data were able to adequately evaluate crop response to fertilizer K and illustrated the effect of moisture stress. The results from the cation exchange study showed that K+ was preferred over Ca2+ and Mg2+ by the exchanger for heterovalent exchange and Ca2+ was preferred over Mg2+ by the exchanger for homovalent exchange. There were no effects of K fertilization on the exchange isotherms. However, a decrease in the CEC of the soils as a result of K fertilization indicated the collapse of the HIV-vermiculite layers. Exchange isotherms for the heterovalent systems varied as a function of equilibration time and as a function of initial cation saturation. The data suggested a slow equilibrium followed an initial rapid K+ fixation. In general, heterovalent cation exchange was hysteretic; while homovalent was reversible. The In Kv values of the heterovalent exchange systems decreased with an increased K+ loading, with the exception of the K+-saturated soils at 0.01 1N, suggesting decreasing selectivity for K+ by the exchanger as the amount of K+ adsorbed increased. The Ca2+ -Mg2+ exchange behaved ideally and reversibly as indicated by constant In Kv values. It can be concluded that the soils used in this study fixed relatively large quantities of fertilizer K, which became available for plant use over time