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Some Characteristics of Rhizobium meliloti Isolates from Alfalfa Fields in Oregon¹

ABSTRACT

Field populations of Rhizobium meliloti were sampled from root nodules of alfalfa (Medicago sativa L.) growing in 61 sites in five regions of Oregon. In four of the regions, 60 to 92% of the sites had no previous history of alfalfa. Most probable number (MPN) determinations of R. meliloti in the soils of these regions showed that 50 to 70% of the sites possessed < 10⁴ R. meliloti/g of soil, and 17 to 50% had plants showing extremely poor nodulation. Neither preinoculation nor slurry/dusting inoculation practices had been completely satisfactory. In effectiveness studies, only 24 isolates out of 422 evaluated could be classed as superior N fixers. That is, they produced plants equal in weight to nitrate-grown plants. Twenty-eight to forty-four percent of isolates within each region were inferior N fixers, and 40 to 58% of the sites in each region contained 50% or more inferior isolates. Only in 13% of sites were inferior isolates absent. The highest percentage of inferior isolates was found in the Willamette Valley, where most sites had (i) a long history of alfalfa, (ii) the largest soil populations of R. meliloti, and (iii) suboptimum fertility levels.

¹ Oregon State Univ. Agric. Exp. Stn. Technical Paper no. 6563.

² Assistant Professor and Graduate Assistant, Dep. of Microbiology and Dep. of Soil Science, Oregon State Univ., Corvallis, OR 97331-3804.

Received for publication October 18, 1982. Accepted for publication June 9, 1983.

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