618 EXPERIMENT STATION RECORD. [Vol 40 



and of the effect of growing four of the strains on both agar and sand in 

 large flasks with and without the presence of growing plants. The organisms used 

 included four large celled nitrogen-fixing strains of bacteria giving all of the 

 staining reactions of the Azotobacter type and isolated from the humus plats at 

 the Iowa Experiment Station, and pure cultures of A. vinclandii, A. chroococ- 

 cum, A. beijerinckii, and A. rhroococcum (ECM). A nitrogen-free medium was 

 used throughout. As a result of the laboratory tests, it was concluded that 

 transfers made on a nitrogen-free dextrose agar more often than once a week 

 were detrimental to the nitrogen fixing power of the organisms studied ; that 

 transfers made once each week into a pure sand medium containing some car- 

 bonaceous material were beneficial to the nitrogen fixing power of Azotobacter 

 in general, but were detrimental to A. beijerinckii; that the nitrogen fixinc: 

 power of A. vinebindil was markedly stimulated when the organism \v;is 

 grown in large flasks for five weeks in the presence of red clover and oats on 

 both agar and sand, and in the presence of algse when grown on agar but not 

 on sand; that the nitrogen fixing power of .4. rhroococcum was also markedly 

 stimulated when the organism was grown on agar for five weeks in the pres- 

 ence of oats and red clover but to a less extent when grown with these plants 

 In sand, while the greatest stimulation for this organism was produced in the 

 presence of alga? in either sand or agar; and that the" nitrogen fixing power of 

 A. beijerinckii was stimulated by the presence of red clover when the organism 

 was grown on either sand or agar, and by oats when grown in sand, while n 1 tr:i' 

 in either agar or sand appeared to have a depressing effect on this organism. 



In the pot experiments the eight typos usod in the laboratory tests were 

 inoculated into Miami silt loam soil said to be free from Azotobacter or similar 

 forms. Ground oat straw or ground clover hay was added to the soil at the 

 rate of five tons per acre, and t lie nitrogen fixing nbillty of the different organ- 

 Isms both in fallow soils and in the presence of growing oat plants determined. 

 The length of the growing period of the oats was determined by the appearance 

 of the seed-bearing spike when the crop was harvested, the Soil sampled, and 

 immediately reseoded. The experiments were continued through three growing 

 periods, with results as follows : 



When three crops of oats were grown continuously on this soil the nitrogen 

 content of the soil increased during the first period, decreased during the 

 second, and increased slightly during the third, while the nitrogen fixing powers 

 of the bacteria and the crop responses paralleled the total nitrogen content of 

 the soil. The nitrogen fixing powers of A. beljerincbU and A. vinelandii were 

 stimulated to a greater extent by decaying oat straw than by clover hay, es- 

 pecially during the earlier stages of decomposition, while some other types of 

 Azotobacter appeared to be stimulated to a greater extent by the presence of 

 decaying clover hay. The nitrogen fixing power of all the organisms eventually 

 became greater in fallow than in cropped soils, while all the organisms were 

 also eventually Influenced in their activities in the same manner and by the 

 same materials. It is concluded that soils may be profitably inoculated with 

 Azotobacter and similar forms, the best results in these tests having been se- 

 cured with A. beijerinckii or A. vinclandii. The conditions essential for maxi- 

 mum nitrogen fixation are said to be good environmental factors such as tillage, 

 drainage, etc., the presence of rapidly decaying organic matter containing 

 small amounts of nitrogen, and freedom from growing plants. 



Those soils inoculated with pure cultures of .4. chroncoccitm, A. beijerinckii, 

 and A. Vinelandii in the greenhouse tests were further examined for acid 

 extract, amino, nonprotein, and polypeptld nitrogen in order to ascertain 

 whether bacterial action had any effect on the accumulation or disappearance 

 of nitrogen in these forms. The methods employed in making the various 



