Effects of Potassium Topdressing a Low Fertility Silt Loam Soil on Alfalfa Herbage Yields and Composition and on Soil K Values1

Alfalfa (Medicago sativa L.) uses large amounts of soil K. The purpose of this study was to determine the amount of topdressed K needed to obtain maximum herbage yields from alfalfa under presently recommended harvest schedules. ‘Ranger’ alfalfa was established on a low K Typic Argiudoll (Plano silt loam) and topdressed the first two of three harvest years with zero, 56, 112, 224, 448, 672, 896, and 1120 kg/ha K as KCI. Two annual harvest schedules were used; (a) three cuts at first flower and (b) same as (a) plus an early October cut (4 cuts). Yield responses were similar for the two harvest schedules, but 3‐year average yields were highest with three cuts annually. Residual yields and stands showed that the detrimental effect of an early October cut (4 cuts) was partially alleviated by high K topdressing rates. The 3‐year average yields were highest with 672 kg/ha of topdressed K, and then decreased at higher K rates. Herbage K percentage at 672 kg/ha K averaged 2.8% for the 3 harvest years, and 56% of the fertilizer K was recovered in the herbage. Analysis of total herbage from the third harvest year showed that Mn concentration, as well as K, increased as K topdressing rate increased, whereas concentrations of N, P, S, Ca, Mg, Na, Cu, Zn, and B decreased. There was no effect on IVDDM, Fe, and AI concentrations. It is concluded that higher rates of applied K are needed than presently are being used in order to obtain and maintain high herbage yields and persistence from alfalfa stands.