CLIMATE 283 



vated soil. This was the case even where the soil was incubated 

 without carbohydrates and the nitrogen determined directly. The 

 average results for many determinations were as follows: 



Mgm. of 

 nitrogen fluid. 



Virgin soil 6.99 



Cultivated 14.28 



Wheat 11.83 



Alfalfa 12.24 



Fallow 22.81 



The fallow soil had received considerable manure, hence these 

 results are undoubtedly high. It would, however, be possible to 

 fallow or crop soils so continuously that extremely small quantities 

 of plant residues would be returned to the soil, under which condi- 

 tions there might be a decrease in nitrogen-fixation. The conditions 

 of moisture and aeration are much more nearly ideal in a fallow soil 

 than in a cropped soil. It is just possible that the high fixation 

 noted where wheat is grown continuously may be due to the method 

 in vogue in the arid districts of leaving the greater part of the straw 

 on the soil. This would act as readily assimilable carbonaceous 

 material for the Azotobacter. Welbel and Winkler have found that 

 fallowing not only increases the assimilable nitrogen but also the 

 available phosphorus of the soil, a liberal supply of which causes the 

 Azotobacter to utilize its energy more economically. That the 

 increased nitrogen-fixation noted when soils are cultivated is not 

 confined to the arid soil, is seen from the recent work of Reed 

 and Williams. Brown's work indicates that crop rotation increases 

 the nitrogen-fixing powers of a soil. 



Climate. It has been maintained for a long time that there is a 

 close correlation between the chemical, physical, and biological 

 transformations going on in a soil and the climatic conditions, but 

 there was nothing definite on this subject until the highly interesting 

 work of Lipman and Waynick appeared. They found a definite 

 relationship between climate and the nitrogen-fixing powers of a soil. 

 Removal of California soil to Kansas increased the vigor of the 

 Awtobacter flora and especially that of A. chroococcum. It increased 

 the nitrogen-fixation by 50 per cent, over that attained by the same 

 soil in California. Similar results were obtained in California soils 

 removed from Maryland. Kansas soil taken to California lost its 

 power to produce a membrane in mannite solution, the Azotobacter 

 flora became rather feeble, and the nitrogen-fixing powers of the soil 

 were greatly reduced. The removal of the Kansas soil to Maryland 

 increased the vigor of the Azotobacter and induced a higher fixation 

 of nitrogen. The Maryland soil in California diminishes in nitrogen- 

 fixing powers, but not in so great a degree as does the Kansas soil. 

 This also happened when the Maryland soil was taken to Kansas. 



