Effect of Hiiniiis on Nitrogen Fixation 205 



colloid has also bci-n astrihcd to the (listribution ol the phosphorus 

 and to the buffering effect upon the rcact'ioii ol the nieihum. 



Hurk conchided that the favoral)le effect of hunuis upon the nitro- 

 gen-fixation process is chie entireU to its iron content. Accorch'ng to 

 Hirch-Hirschfeld, soil extract contains se\'eral components that have 

 a fa\'orable effect upon growth and nitrogen fixation by Azotohacter. 

 These components are of botli an organic and an inorganic nature, 

 the growth stimulation being due to the organic complex. Molybde- 

 num cannot take the place of soil extract, although both favor about 

 alike the amount of nitrogen fixed per unit of sugar consumed. 



Burema and Wieringa demonstrated that the role of molybdenum 

 in the fixation of nitrogen is that of a reducing agent. Less molybde- 

 num is required for the reduction of nitrate than for the reduction 

 of free nitrogen. Jensen reported that Az. indicum requires molyb- 

 denum for nitrogen fixation; molybdenum could not be replaced by 

 vanadium. 



The nitrogen fixed through nonsymbiotic processes can at best 

 restore only a part of the losses of nitrogen from soil by crop re- 

 moval and by leaching. The common estimates are 20-50 pounds 

 fixed per acre annually, but the actual amounts may be much smaller. 



The frequently expressed opinion that soils from arid climates 

 have an extraordinary nitrogen-fixing power and may be employed, 

 by the use of crop residues by nonsymbiotic nitrogen-fixing organ- 

 isms, for cereal cultivation without depletion of nitrogen, has been 

 denied by Jensen. In Australian wheat soils no gain at all is usually 

 expected, and only under exceptionally favorable circumstances was 

 a fixation obtained corresponding to one-third of the nitrogen re- 

 quirements of the crops on wheat land worked on the usual wheat- 

 fallow rotation. The activity of nonsymbiotic nitrogen fixation in 

 nature appears to be largely confined to uncultivated soils where no 

 crops are carried away and the vegetable debris is allowed to de- 

 compose. 



Jensen further concluded that the practice of growing wheat 

 alternating with fallow and without use of nitrogenous fertilizers 

 is to be regarded as a gradual consumption of the nitrogen reserves 

 of the soil, from which some nitrate is produced during fallowing. 

 The nonsymbiotic nitrogen fixation and the effect of the rain will 

 compensate for this loss only incompletely. If continued, it must 

 in time lead to permanent loss of fertility. The growth of leguminous 

 crops in the rotations and the judicious application of nitrogenous 

 fertilizers are the logical correctives. 



