Evaluating Rhizobium viability in commercial inoculum

Abstract

Commercial inoculum prepared with a peat base is subject to many factors that can reduce viability prior to and after retail purchase. No standards of quality are available to protect the consumer. Quality control of inoculum viability would be helpful but existing test procedures are time consuming and details have not been clearly defined for routine use. The objective of this paper is to describe a method for testing commercial inoculum quality by determining the number of viable rhizobial cells per gram of peat-base carrier. These procedures can be used by extension personnel to assure high quality inoculum sources. Also, students in college and high school crop production courses can use the procedures as a learning aid through laboratory exercises. These procedures can be used to test viability of nodulating bacteria on seed sold with preinoculated coatings. Evaluation procedures are based on a plant infection test. Seedlings of the species compatible to the inoculum are produced from sterilized seed in plastic growth pouches. Rhizobial cells in the peat-base carrier are suspended in water. Ten-fold serial dilutions are made until there is theoretically one Rhizobium cell per 100 ml. Five pouches of each of the five greatest dilutions are inoculated. Presence of nodules is scored after several weeks of growth. Numbers of rhizobial cells in the commercial inoculum are obtained from a Most-Probable-Number table. A minimum of 107 bacteria per gram of inoculum is suggested as a standard of good quality. Several packages of commercial inoculum we tested failed to meet this standard.