62 NITRIFICATION 



Ordinary soils contain large numbers of different bacteria, some 

 fulfilling useful functions in agriculture, some being destructive to plant 

 food, and some highly injurious to animal life if they once gain ad- 

 mission to the proper portion of their victims (e.g., the bacterium of 

 tetanus or lock-jaw). 



Of organisms possessing the power of converting organic nitrogen- 

 ous substances into ammonium compounds, a considerable number is 

 known, some being moulds (active especially where the quantity of or- 

 ganic matter is large), while others are bacilli, e.g., B. mycoides and B. 

 fluorescens, and micrococci, e.g., Micrococcus urea. 



Nitrification. Of organisms capable of effecting the oxidation of 

 ammonia to nitrous acid there appears to be possibly more than one, 

 but according to Winogradski only one is usually present in any par- 

 ticular soil. 



The pure nitrous organism is described by Warington a as consist- 

 ing of two forms of micrococcus. One form is nearly spherical in 

 shape and varies in diameter from the very minute up to 1 micromilli- 

 metre (-001mm.). The other is oval shaped and larger, its greatest 

 length being more than -001 millimetre. 



The nitric organism isolated by Winogradski in 1890 is of very 

 minute size, consisting of rods not more than '0005 millimetre in 

 length and from '00017 to '00025 millimetres in thickness. 



The conversion of nitrogen existing as organic matter or ammonium 

 compounds into nitric acid, is a most important one and has received 

 an immense amount of attention of late years. It is known as ni- 

 trification and is effected, as indeed are probably all the changes of 

 organic matter, through the action of micro-organisms. 



The process of nitrification is an extremely important one, since in 

 the case of the majority of plants, it is mainly in the form of nitrates 

 that nitrogen is assimilated. The organic nitrogenous matter existing 

 in the soil is present in various forms, which appear to differ in their 

 susceptibility of undergoing nitrification and also in their behaviour 

 towards alkalies and acids. 2 All animal and vegetable refuse contains 

 combined nitrogen, probably mainly in the form of proteid substances ; 

 as these undergo putrefaction in the soil and become converted into 

 humus, carbon dioxide is evolved, and the nitrogen is probably con- 

 verted into bodies of simpler constitution, amides, 3 ammonium carbon- 

 ate, and in some cases free nitrogen. As has been already shown, the 

 form in which nitrogen occurs in humus is not known. It probably 

 exists mainly as somewhat complex compounds, possibly partly of an 

 amide character, or as ami no-acids, 4 but of this little knowledge has 

 been acquired. These organic compounds are probably broken down 

 in the soil, yielding first, ammonium compounds, e.g. : 



1 Jour. Chem. Soc., 1891, Trans., 484. 



2 Andre, Compt. Rend., 1898, 414. 



3 An amide is a substance derived from an organic acid by the replacement of 



/ITT 



the OH group by NH 2 . Thus from acetic acid, | COOH' is derived acetamide, 



IcONH . f rom car k mc acid, CO I Q- j carbamide (urea), CO 

 4 Sestini, Jour. Chem. Soc., 1899, Abstracts, ii. 120. 



