1916] SOILS FERTILIZERS. 321 



" Many different forms of Azotobacter were observed in those soils possessing 

 that group of organisms. Very frequently one soil showed the presence of two 

 or three different species of Azotobacter. A. chroococcum, however, was the 

 most prominent of all the species and was found most widely distributed in 

 the several soils. 



" In a number of cases the amount of pigment produced by the Azotobacter 

 forms was most marked. The organism surpassing all others studied in this 

 respect was a form of A. chroococcum in the poor soil from Sinaloa, Mexico. 



" Only about half the soils tested showed notable or vigorous gas formation 

 in mannite solution. Only three of these contained Azotobacter organisms. 

 Sixteen of them were the highly ferruginous and humus soils obtained from 

 various portions of the Hawaiian Islands. Gas formation in mannite solutions 

 inoculated with soil would, therefore, seem to be largely accomplished by 

 Clostridium and other rod forms and not by Azotobacter. . . . 



" Pigment production by cultures ran almost entirely parallel with Azoto- 

 bacter development in them. Thus the total number of cultures producing pig- 

 ment was 20, only slightly in excess of the number showing Azotobacter 

 organisms. Of these 20 all but two gave a brown to black pigment. The other 

 two gave a yellow to orange pigment. Twenty-five of the mixed cultures ex- 

 hibited more or less membrane formation. In nearly all cases the presence 

 of membranes was due to Azotobacter development." 



In pure culture studies in mannite solution and sandy soil it was found that 

 on the whole the sandy soil was far superior to the solution as a medium 

 for nitrogen fixation by the several forms of Azotobacter studied, since 17 out 

 of 20 organisms added to the soil in every case more than 3 mg. of nitrogen. 

 The latter was accomplished by only 11 organisms in the case of the mannite 

 solutions. Sixteen out of 20 organisms fixed in every case more than 5 mg. 

 of nitrogen per gram of mannite in the soil as a medium, whereas there were 

 but four such in the case of the mannite solution cultures. There were nearly 

 five times as many of the same organisms which fix 6 mg. or more of nitrogen 

 in the soil culture as there were in the case of the solution cultures. 



Studies of the relation of soil type to nitrogen fixation by pure cultures of 

 two types of Azotobacter showed that the fixation of nitrogen by the two 

 organisms tested was notably smaller in the soils which were their natural 

 habitat than in a totally different type of soil. It is concluded " that the soil 

 type ... is the determinant above all other factors of an organism's power to 

 fix atmospheric nitrogen." 



It was further found that considerable difference existed between the powers 

 of different organisms to use certain carbon compounds in nitrogen fixation. 



A final study with 56 soils showed the absence of any general law regulating 

 the ratio of nitrogen present in soils to nitrogen fixation by those soils. 



Nitrification and total nitrogen as affected by crops, fertilizers, and copper 

 sulphate, C. A. Jensen (Jour. Amer. Soc. Agron., 8 (1916), No. 1, pp. 10-22; 

 abs. in Chem. Abs., 10 (1916), No. 6, pp. 795, 796).— Studies on the changes in 

 nitrification and total nitrogen content of soils under field conditions at Rocky 

 Ford, Colo., during the summers of 1911 and 1912, and on the influence of 

 different cropping and fertilizer treatments on excessive nitrate accumulation 

 in these soils, are reported. 



It was found that " the accumulation of nitrates in the soils in the Arkansas 

 River Valley in Colorado ... in 1910 and 1911 . . . became less in 1912 and 

 1913. In the work in 1911 mustard appeared to have some effect in checking 

 the accumulation of nitrate in the field. Copper sulphate at the rate of 100 

 lbs. per acre on fallow was also effective in checking nitrification, reducing the 

 average seasonal accumulation to about 60 per cent of the amount found in 



