Role of Inoculation in Establishing Subclover on California Annual Grasslands1

Abstract

AbstractSubclover (Trifolium subterraneum L.) has been difficult to establish in California annual grasslands even with approved cultural, inoculation, and fertilization practices. Symptoms indicated a Rhizobium insufficiency. The objectives of this study were to develop dependable inoculum and methods of establishing annual clovers in northern California. Rhizobia were isolated from nodules of vigorous clover plants growing among plants showing extreme N‐deficiency symptoms. These rhizobia were screened in growth chamber tests for effectiveness and competitiveness against native ineffective rhizobia. The best strains were further tested in the field with several stickers and seed coating materials, and were compared with commercial inoculants available at the time. In the field tests rhizobial strains differed in ability to compete with ineffective native rhizobia and in ability to survive the long summer drought. In composites, poor or moderately effective rhizobial strains reduced the effectiveness of the good strains. Subclover growth increased in field experiments as the amount of inoculum was increased, indicating that high rhizobial numbers were important. Peat inoculum applied with 40% gum arabic solution as a sticker and a lime coating was much superior to peat inoculum applied as a water slurry. Vacuum‐inoculation treatment gave no better results than uninoculated seed. The effectiveness of inoculation sticker materials fell generally in the following order: “PELGEL‐PELINOC” > gum arabic‐lime > methyl‐cellulose‐lime > sugar lime.