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

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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 SO4 ions can injure alfalfa when high rates of KCl and K2SO4 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 K2SO4. 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 K2SO4. Highest K concentrations for both KCl and K2SO4‐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 K2SO4‐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 K2SO4 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 K2SO4 may have to be substituted for KCl as all or part of the K fertilizer.