120 



BACTERIA IN THE SOIL 



from the decomposition of the water which permeates every part of 

 the plant, and is derived by it from the soil and from the aqueous 

 vapour in the atmosphere. The chief inorganic salts of which proto- 

 plasm is constituted are composed in part of potassium, magnesium, 

 calcium, iron, phosphorus, or sulphur. These inorganic elements do 

 not enter the plant as such, but combined with other substances or 

 dissolved in water. Potassium, calcium, and magnesium are absorbed 

 chiefly as nitrates, phosphates, and carbonates. Iron contributes to 

 the formation of the green colouring-matter of plants, indeed, is 

 essential to it, and is also derived from the soil. Phosphorus, one of 

 the chief constituents of seeds, generally occurs as nucleo-albumin. 



A SCHEME SHOWING THE PLACE AND FUNCTION OF THE 

 ECONOMIC MICRO-ORGANISMS FOUND IN SOIL 



DECOMPOSITION AND DENITRIFYING BACTERIA. 

 I 



Free Nitrogen. Gases [CO 2 ]. Water. Ammonia, [Nitrites]. 



and other elements 

 of broken-down 

 complex bodies. 



NITRIFYING BACTERIA. 



Nitrites [ = Nitrous organism 

 | (Nitrosomonas)]. 



Nitrates [ Nitric organism 

 (Nitromonas, or 

 Nitrobacter)]. 

 [In soil and 

 available for 

 plant life. ] 



NlTROGEN-FlXING 



BACTERIA. 



[In soil and in the nodules 

 on the rootlets of Legu- 

 minosce. ] 



