CHAPTER VII 



THE LIVING ORGANISMS OF THE SOIL 



Decay and Humification of Organic Matter in the Soil Alinit 

 The Fixation or Free Nitrogen by Fiacteria living in Sym- 

 biosis with Leguminous Plants Soil Inoculation with Nodule 

 Organisms Fixation of Nitrogen by Bacteria living free in the 

 Soil Nitrification Denitrification Protozoa Iron Bac- 

 teria Fungi of Importance in the Soil : Mycorhiza, and the 

 Slime Fungus of "Finger-and-Toe." 



The soil is the seat of a number of slow chemical 

 changes affecting the organic material it receives : 

 residues of an animal or vegetable nature, when applied 

 to the soil, are converted into the dark-coloured complex 

 known as "humus," which becomes eventually oxidised 

 to carbonic acid, water, nitric acid, and other simple 

 substances serving as food for plants. These changes, 

 at one time regarded as purely chemical, are now 

 recognised as dependent upon the vital processes 

 of certain minute organisms, universally distributed 

 throughout cultivated soil, and subject to the same 

 laws of nutrition, multiplication, life and death, as 

 hold for the higher organisms with whicii we are more 

 generally familiar. 



The microscopic life of the soil, roughly classed 

 as protozoa, fungi and bacteria, is vast, and has been very 

 inadequately explored as yet : certain types of change 



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