Yield and Chemical Composition of Alfalfa as Influenced by High Rates of K Topdressed as KC1 and K2SO4¹

Abstract

Abstract Alfalfa ( Medicago sativa L.) uses large amounts of soil K. The usual K source for fertilizing alfalfa is KCl. The purpose of the current study was to determine whether Cl and SO 4 ions can injure alfalfa when high rates of KCl and K 2 SO 4 are applied. A field stand of ‘Vernal’ alfalfa growing on low fertility Miami silt loam soil (Typic Hapludalf fine‐loamy mixed, mesic) was topdressed for 2 years with 0, 448, 896, 1,344, and 1,792 kg K/ha/year as either KCl or K 2 SO 4 . Four harvests were taken in 1972 and in 1973, and one in spring of 1974. Highest 2‐year total herbage yield occurred in KCl‐fertilized plots where 448 kg K/ha/year were applied; yields decreased significantly at rates higher than 896 kg K/ha/year. High yields were obtained at all rates of K 2 SO 4 . Highest K concentrations for both KCl and K 2 SO 4 ‐fertilized herbage occurred where 1,792 kg K/ha/year were applied in 1972 (3.70 and 3.77%, respectively) and in 1973 (3.04 and 3.41%, respectively). Chlorine concentrations in herbage fertilized with KCl ranged from 1.50 to 2.21% in 1972 and 0.82 to 1.15% in 1973. Chlorine concentrations in K 2 SO 4 ‐fertilized herbage were lower than in KCl‐fertilized herbage; less than 0.77% in 1972 and 0.42% in 1973. Sulfur concentrations in herbage fertilized with K 2 SO 4 ranged from 0.31 to 0.36% in 1972 and from 0.30 to 0.33% in 1973. Where KCl was applied, S concentrations were lower than the control (0.23% in 1972 and 0.26% in 1973). All applications of K lowered N concentrations below the control. It is evident that fertilizer rates higher than 896 kg K/ha/year as KCl can cause significant yield reductions. Therefore, when KCl fertilizer is topdressed at these rates, applications may have to be split among harvests or K 2 SO 4 may have to be substituted for KCl as all or part of the K fertilizer.