Response of Azotobacter to Molybdenum Addition to Soil

Abstract

The population of Azotobacter in the soil increased sharply upon addition of simple organic compounds such as sucrose at a rate of 1%. The most-probable number of cells per gram of soil was determined as 5.4 x 107 after three weeks of incubation in both molybdenum amended and non-amended samples compared to 3.3 x 102 in the control. Although addition of molybdenum appeared to have no significant effect on the numbers of Azotobacter, its activity in terms of nitrogen fixation was highly influenced by the availability of this element in the soil. The percent total nitrogen increased from 0.034 in the control to 0.063 in sucrose amended soil and reached 0.068 in samples receiving both sucrose and molybdenum. In terms of milligrams of nitrogen fixed per gram of sucrose metabolized, an increase of 2 to 5 milligrams of nitrogen fixed were observed by addition of molybdenum at a rate of 0.1 to 5.0 ppm.