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Study of the behaviour of representative aerobic freeliving dinitrogen-fixers, other than azotobacters, with respect to different carbon sources, showed that a broad range of compounds is accessible to them. This group of nitrogen-fixers can assimilate molecular nitrogen as a result of utilization of carbohydrates, alcohols, and organic acid salts. The highest nitrogen-fixation productivity, reaching 20.6-27.3 mg of nitrogen per g of carbon utilized, was observed on medium containing glucose as the carbon and energy source. Supplementary growth factors added to the N-deficient medium in the form of small amounts of yeast extract stimulated their growth and fixation of atmospheric nitrogen. This stimulation is particularly evident in mixed cultures. The nature of the mutual influences of micro-organisms in these cultures remains fairly obscure and further inquiry is necessary to elucidate this problem. A medium containing 1% glucose and Difco yeast extract to produce a bound nitrogen concentration of 10 ppm appeared to be the most favourable for the cultivation and N2-fixation of this group of microorganisms.

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The association between A. vinelandii and either Agrobacterium sp. or Micrococcus sp., which are usually found as contaminants in Azotobacter cultures, was investigated. In comparison with pure cultures, association increased the microbiol counts in addition to increasing nitrogen fixation rates and efficiency. In liquid cultures higher Azotobacter densities were observed in the top 5 cm of the column concomitant with lowering the economic coefficient of utilization of carbonaceous compounds, which resulted in low efficiency of nitrogen fixation. In deep layers, lower amounts of nitrogen gain were obtained, but higher efficiencies of N2-fixation were recorded. In sand cultures, the biggest amounts of fixed nitrogen were in the 5--15 cm layer of the soil column and in deeper layers economic utilization of sugars occurred, but nitrogen gain sharply decreased.

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Sixty isolates, representing the different types of azotobacters in Egyptian soils, were studied for their N2-fixing capacity. When A. chroococcum and A. vinelandii were grown in shallow layers of N-deficient liquid medium, nitrogen gains ranged from 48 to 92 and from 20 to 120 ppm with efficiencies of N2-fixation from 15.8 to 50.0 and from 16.6 to 21.2 mg N fixed/g C oxidized, respectively. Culturing in deep layers generally lowered the nitrogen gains and the oxidation of carbon, but at different rates, resulting in the increase of the efficiency of N2-fixation by 2--78%, indicating that the depth of the liquid medium has a definite effect on the outcome of N2-fixation. In another experiment, A. vinelandii was grown in increasing depths of liquid or solid substrates and consequently at variable ratios of surface area: depth. In liquid medium, maximum counts, sugar consumption, and nitrogen gains were detected in the widest surface area: depth ratio, but the efficiency of N2-fixation increased with the narrowing of the ratio. In sand cultures an opposite trend was observed.

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Soil samples from different locations in Egypt, representing various types of soils, different degrees of fertility and under various standing crops, were used for isolating Azotobacter strains. Sixty Azotobacter isolates were obtained, and A. chroococcum was found to be the most predominant species (56 isolates), while A. vinclandii was sporadically found (4 isolates). Other species of the genus Azotobacter were not detected in the examined soil samples. Isolates were studied for their morphological, cultural, and physiological properties. Thirteen isolates, differing in morphological features, were selected for studying their pleomorphic character.

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Twenty isolates of micro-organisms capable of growing on nitrogen-deficient medium and found as contaminants in Azotobacter cultures were isolated from Egyptian soils and studied for their morphological, cultural, and physiological properties. These micro-organisms s are members of Rhizobiaceae, Pseudomonadaceae, Achromobacteriaceae, Enterobacteriaceae, Micrococcaceae, Bacillaceae, and Streptomycetaceae as well as some yeasts. In nitrogen-free medium the micro-organism fixed only small amounts of atmospheric nitrogen, hardly exceeding 3 ppm and because of their low sugar consumption rates, efficiences of N2-fixation sometimes reaching 18 mg nitrogen fixed/g carbon oxidized were recorded. Addition of 15 ppm combined nitrogen to the medium increased the amounts of nitrogen fixed to 3--9 ppm.

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