Non-symbiotic nitrogen-fixing bacteria in soil. II. Studies on Azotobacter.

Abstract

Forty-two soils from the Western Australian wheat belt were examined for the presence of Azotobacter by microscopic examination of enrichment cultures, and by plate identification on three different selective agar media. Judged microscopically, most of the soils appeared to contain Azotobacter; culturally, distribution was sporadic and varied greatly according to the medium used. Distribution was not correlated with soil type. Colony production on the selective media was not improved by the addition of straw extracts, amino acids, or vitamins, or when the plates were incubated at lowered oxygen partial pressures. A freshly isolated strain of A. chroococcum fixed nitrogen more efficiently when grown in association with another soil bacterium; in pure culture it showed adaptation to a nitrogen-free medium. Viable counts on different agar media with different treatments indicated the presence in one medium of an inhibitor which greatly reduced the number of colonies formed. It is concluded that nitrogen-free selective media may give an unreliable estimate of the distribution of Azotobacter in soils.