SOILS FERTILIZERS. 721 



Azotobaeter can utilize in its frrowtli sccdiulary calciuni phosphate as well as 

 calcium salts of the organic acids, such as lactic acid and citric acid, in addition 

 to calciuni carbonate, but it can not utilize in this way tribasic calcium phos- 

 phate, calciuni chlorid, and calcium sulphate. Thomas slag as well as second- 

 ary calcium plios])liates are easily assimilated by Azotobaeter, while iron and 

 aluminum phosphates, bone ash and raw phosphates, and bone meal are almost 

 unassimilable. The author believes that these facts encourage the hope that 

 it may be possible to work out a biological method, for determining in a general 

 way the available plant food of soils. 



The author's experiments show very positively that a certain amount of phos- 

 phoric acid and also of lime in the soil is an essential condition to the decom- 

 position of mannite. He reports experiments with a soil wiiich had been con- 

 tinuously fertilized for 12 years with nitrate of soda without liming and in 

 this way exhausted of phosphoric acid, which when used to inoculate a culture 

 solution of maiuiitc. calcium carbonate, and jjotassium chlorid gave no reduc- 

 tion of mannite. 



The author urges great care in the taking of samples for the determination 

 of the occurrence and distribution of Azotobaeter to prevent foreign infection, 

 and points out that it is necessary to inoculate the solutions as quickly after 

 the taking of the sample as possible. However, for biological reactions relat- 

 ing to the lime and phosphoric acid content such precautions are not of so 

 much importance. 



Some observations on the assimilation of atmospheric nitrogen by a free 

 living soil organism — Azotobaeter chroococcum of Beijerinck, S. F. Ashby 

 (./niir. Agr. .S'cl, 2 (1!)07), Xo. 1, pii. 33-.JI). — This article reviews briefly the 

 history of investigations on nitrogen fixation, describes the preparation of cul- 

 tures of Azotobaeter ehrooeoecnm and the appearance of this oi'ganism, and re- 

 ports tests of the rate of fixation of nitrogen by pure and impure cultures of 

 this organism obtained from soil from Mombasa, East Africa; Cairo, Egypt, and 

 Kothamsted, as well as in culture solutions Inoculated with soil from different 

 sources, some of which contained Azotobaeter, and others were free from the 

 organism. 



The rate of fixation in pure cultures varied from less than 5 to more than 7 

 mg. of nitrogen for each milligram of cai'bohydrnte used, the rate of fixation be- 

 ing apparently somewhat greater in the case of mannite than of glucose. 



In the experinuMits in which the culture solutions were inoculated with soils 

 from different sources the rate of fixation was ().95 mg. of nitrogen for each 

 gram of mannite in case of the soils containing Azotobaeter and 3.22 mg. in case 

 of soils which did not contain the organism. 



The author states as conditions fa voidable for fixation of nitrogen (1) an 

 abundant supply of air, (2) the presence of a base — calcium carbonate or, pref- 

 erably, according to the author's investigations, magnesium carbonate, which 

 !K)t only more effectually neutralizes acidity due to foreign organisms in the 

 early stages of culture, but also prevents butyric fermentation. The magne- 

 sium carbonate delays the development of the nitrogen-fixing organisni some- 

 what, but the final yield of nitrogen is larger than with calcium carbonate. 



Th(( organism obt.-iined from East African soils was in every way similar to 

 that olitained from Kothamsted soil, oxceiit that fixation with it was more rapid 

 and greater. It also possesse<I the projierty of turning brown and finally black 

 with age. even in pure cultures. 



It was observed that while Azotobaeter eliroococcum produces no .spores, yet it 

 can resist drying up in the air for a long time. " Old cultures of the organism 

 on agar which had dried down to a leathery consistency after many months 

 still showed abundant growth after pouring a fresh culture solution over them. 



