Abstract

Free-living diazotrophic isolates L1 (Bacillus cereus Frankland) and Br2 (Bacillus subtilisEhrenberg Cohn) were evaluated in greenhouse trials for their optimum dose and frequency of application on seedlings of a single lettuce variety (Great Lakes) in a composted pine bark medium. Bacterial isolates were grown in Burke’s broth and were applied as a drench at four different doses (10⁵, 10⁶, 10⁷ and 10⁸ colony forming units (cfu) ml^-1) and at different frequencies of application (1, 7, 14 and 21 days). Two months later, wet weight, dry weight and plant N levels were measured. Lettuce growth and plant N level responded positively to both bacterial isolates inoculated at 10⁶cfuml^-1 weekly and fortnightly. Isolate Br2 inoculated at 10⁶cfuml^-1 fixed 32.4% plant N when applied weekly and 26.7% when applied every two weeks. Isolate L1, at the same dose, applied weekly or every two weeks, fixed 27.7 and 29.1% of plant N requirement, respectively. The lettuce seedlings responded less well to a higher dose at 10⁸cfu ml^-1 of either isolate applied weekly and every two weeks. The response to dosage depended on frequency of application. Doses of 10⁶ cfu ml^-1 of both isolates applied weekly or every two weeks had the best effect on lettuce seedling growth and plant N.

Key words: Free-living diazotrophs, lettuce, optimum dosage, frequency of application.