BIOLOGY OF THE SOIL. 79 



destructive to plant food, and some highly injurious to animal 

 life if they once gain admission to the proper portion of their 

 victims (e.g., the bacterium of tetanus or lock-jaw). Of the 

 work of the nitrifying and denitrifying organisms a brief de- 

 scription has already been given ; it remains here only to say a 

 word or two about their appearance. 



Of organisms possessing the power of converting organic 

 nitrogenous substances into ammonium compounds a con- 

 siderable number are known, as has already been stated, some 

 being moulds (active especially where the quantity of organic 

 matter is large), while others are bacilli, e.g., B. myco'ides 

 (p. 65) and B . fluorescens , and micrococci, e.g., Micrococcus urea. 



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 

 particular soil. 



The pure nitrous organism is described by Warington* as 

 consisting of two forms of micrococcus. One form is nearly 

 spherical in shape and varies in diameter from the very minute 

 up to 1 micromillimetre ('001mm.). The other is oval shaped 

 and larger, its greatest length being more than -001mm. 



The nitric organism isolated by Winogradski in 1890 is of 

 very minute size, consisting of rods not more than -0005mm. 

 in length and from -00017 to -00025mm. in thickness. 



The conditions favourable for the growth and development 

 of these organisms have already been described. With refe- 

 rence to their distribution W T arington found that nitrifying 

 organisms did not exist at greater depths than 18 inches and 

 were most abundant within a short distance from the surface. 

 In later experiments he found that a few nitrifying organisms 

 were present even at a depth of several feet. Winogradskif in 

 1893 obtained from soil a large bacillus which, when culti- 

 vated in a solution 'containing suitable mineral ingredients and 

 pure dextrose and supplied with air purified by means of 

 potash and sulphuric acid, caused the destruction of the dex- 

 trose (C fi H 10 O 6 ), the formation of butyric acid (C H H 7 COOH), 

 the evolution of carbon dioxide and free hydrogen, and the 



* J.S.S. 1891, Trans. 484. 

 t Coiupt. Rend. 11(5, 1385; J.C.S. 1893, abat. ii., 482. 



