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The Influence of Soil Water Potential and Soil Texture on the Survival of Rhizobium Japonicum and Rhizobium Leguminosarum Isolates in the Soil¹

ABSTRACT

Water stress and differences in soil texture are thought to be primary factors in inoculation failure in the southeastern United States. There is, however, a lack of quantitative data relating the effect of soil water potential on rhizobia survival in soils of differing textures.

The objectives of this study were to investigate the effect of soil water potential on the survival of slow- and fast-growing rhizobia in soils varying in texture. Rhizobium japonicum serogroup isolates 122 and 123 and Rhizobium leguminosarum serogroup isolates WA-01 and WA-02 were incubated in seven different soils maintained at water potentials of –0.3, –1.0, –5.0, and –15.0 bars for periods to 7 weeks. The most probable number (MPN) technique and plant infectivity tests were used to enumerate rhizobia.

Soil water potential and soil texture were both found to play important roles in the survival of the four rhizobia isolates. Rhizobia populations were always greatest at –0.3 bars. Populations were proportional to decreasing water potentials to –15.0 bars. Populations were lowest in the sands and clay loam soils. Conversely, populations were usually highest in the sandy loams, silt loams, and sandy clay loam soils.

Differences between the four isolates were also observed. Rhizobium japonicum serogroup isolates 122 and 123 survived in greater numbers than the R. leguminosarium isolates during the initial incubation period. R. leguminosarum isolate WA-02, however, performed as well as or better than the R. japonicum isolates after 1 week for most soil textures and water potentials. Rhizobia populations declined during the first week of incubation and increased, in most cases, by the seventh week.

¹ Contribution from the N. C. Agric. Res. Serv., Raleigh, NC 27650. Journal Series Paper no. 6636.

² Formerly Research Assistant, now Assistant Professor, Dep. of Plant and Soil Sciences, Univ. of Idaho, Moscow, ID 83843; and Professor of Soil Science, Soil Science Dep., N. C. State Univ., Raleigh, NC 27650.

Received for publication October 8, 1980. Accepted for publication February 6, 1981.