1. Free-living nitrogen-fixing bacteria in Egyptian soils and their possible contribution to soil fertility;Abd-el-Malek;Plant and Soil, Special volume,1971
2. American Public Health Association and Am. Wt. Wks. Assoc. (A.P.H.A.): Standard Methods for the Examination of Water, Sewage, and Industrial Wastes. 10th ed., New York 1955.
3. Nitrogen-fixing bacteria of the genus Beijerinckia in South African soils;Becking;Plant and Soil,1959
4. Studies on nitrogen-fixing bacteria of the genus Beijerinckia. I. Geographical and ecological distribution in soils;Becking;Plant and Soil,1961
5. Family Azotobacteriaceae;Becking,1974