FIXATION OF FREE NITROGEN 793 



within it. The latter view appears to be more in harmony with 

 the known experimental evidence. 



Well-rotted dung applied to soils containing nitrate has little 

 denitrifying effect, and this result we should expect, for the 

 oxidisable compounds in such dung have already been changed 

 in the early fermentation processes which it has undergone. 

 These peculiarities of dung are worthy of thought and con- 

 sideration although the amounts of farmyard manure applied 

 in ordinary practice is too small to have any serious denitrifying 

 effect, even if used quite fresh : possibly in horticultural practice 

 if one hundred tons or more per acre were employed it might 

 be advisable to apply it in a well-rotted condition. There is, 

 however, such a loss of nitrogen by the fermentation processes 

 previously mentioned from mixed urine and dung when kept, 

 that it is undoubtedly best from an economical point of view 

 to feed animals on the land wherever practicable. 



II. Fixation of Free Nitrogen. — (i) By Clostridium Pasteur- 

 ianum Win. It has frequently been determined that the total 

 amount of combined nitrogen in bare uncropped soils increases 

 to a slight extent, even under conditions which preclude the 

 possibility of the addition of ammoniacal or other nitrogenous 

 compounds from the air. The result has been found to be due 

 to the growth and multiplication within the soil of minute living 

 organisms which possess the power of absorbing the free 

 nitrogen of the air and building it up into complex organic 

 nitrogenous compounds. At first the lower forms of algse were 

 supposed to be capable of thus 'fixing' and utiHsing free 

 nitrogen, but it has been demonstrated that these organisms 

 are incapable of doing so. A bacterium possessing this re- 

 markable power has, however, been isolated from the soil and 

 is known as Clostridium Pasteurianum Win. It occurs in the 

 form of rods which become spindle-shaped or swollen in the 

 middle when spore - formation takes place, and is strictly 

 anaerobic. In the soil the removal of oxygen for the pio- 



