This study was undertaken to observe the influence of seasonal fluctuation on native peanut rhizobia populations in Coastal Plain soils in cultivated fields with or without a peanut history, in pasture, and in forest environments. Soil samples were collected from Ap horizons every 30 days during a 16-month period and rhizobia populations were enumerated using a most probable number (MPN) technique with the peanut (Arachis hypogaea L.) as the host for plant infectivity tests.

Cultivated fields with a peanut production history had higher peanut rhizobia populations than cultivated fields without a peanut history, pastures, or forest environments. Seasonal fluctuation was evident in cultivated fields with a peanut history; peanut rhizobia populations were greatest in July whether peanuts or another crop was being grown that year and very low from September to May. Peak peanut rhizobia population reached 10,000 per gram of soil in peanut fields in July. Populations were generally less than 30 per gram of soil in cotton, corn, soybean, clover pasture, and alfalfa hay fields, and less than 10 per gram of soil in orchard and forest environments. Peanut rhizobia always constituted less than 0.01% of the total aerobic bacteria in all fields sampled.