lo Sept., 1912.] ]V//i'ai and Its Ciiltivalioii. 545 



Perhaps the most imptjrtant grou]) of organisms are the nitrifying 

 bacteria. The nitrogen required by the wheat plant can only Ixi taken 

 in bv the root in the form of nitrates. These nitrates are formed from 

 thf organic matter of soil bv, three distinct processes, each process involving 

 a distinct set of bacteria. The process by which this organic nitrogen is 

 converted into nitrates is called nitrification. Detailed experiments relating 

 to this process have alieady appeared in this Journal* It is interesting 

 to note, in this connexion, that these nitrifying organisms require for their 

 full development — {a) air ; {h) warmth — they thrive best at 99 degrees F.. 

 and are ten times more acti\e at that temperature than at 57 degrees F.f ; 

 (c) moisture ; (d) pre.sence of lime ; {c) organic matter. 



Opposed to these nitrifying organisms are the denitrifxing bacteria, 

 which undo the valuable work of the nitrifying organisms, and are most 

 active when the soil is deficient in air. as a result of bad drainage or want 

 of tillage. When these bacteria are active, the s(;hible nitrates quickly 

 disappear, and even dissipate to free nitrogen, and crop yields are reduced. 

 Everv effort .should be made, therefore, to reduce the activity of these 

 organisms to a minimum. 



Again, in well-tilled well-drained soils, the organic matter of the soil be- 

 comes slowly oxidized h\ a group of putrefactive bacteria to carbon dioxide 

 and other products. It is through the direct action of ..several form of bacteria 

 and through the indirect effects of the products of these organisms, that a 

 large portion of the insoluble mineral plant foods of the soil are rendered 

 available for the use of crops. 



In badly drained .soils, where the organic matter decomposes in the 

 absence of air. marsh gas and organic acids are formed, and sourness and 

 acidity in the soil thereby result. 



On the roots of well-grown sjfecimens of the Lt\:^niiiiiiosu — the pea 

 family of plant.s — colonies of bacteria of great economic importance exist. 

 The curious nodules or wart-like growths on the roots of clover, peas, 

 beans, and lucerne, are the dwelling-places of teeming millions of organisms. 

 They live in a .sort of partnership with the legume, and ha\-e the power 

 of fixing free nitrogen from the air and making it available for the plant 

 in return for a supply of carbonaceous food given by the plant. Thus, 

 they enable a farmer to supply his soil with its most expensive plant food 

 free of cost. This inteiartion is called symbiosis, and may be de.scribed as a 

 kind of partner.ship in which two organisms, differing widely in character, 

 live together for the advantage of both. 



Organisms have been i.solated by WinogradskV and Beyerinck wh.ich 

 are able to fix the nitrogen of the air without associating themselves with 

 anv leguminous plant. The.se organisms .secure the food necessary for 

 their energy from the decaying organic m.atter of the .soil. 



Finallv, recent research has revealed the presence of numerous 

 organisms called protozoa, which prey on the various bacteria in the soil. 

 It would appear that the.se organism.s — the natural en'-mies of the bacteria 

 — are destroyed by exposure to moderately high temperatuies, leaving a 

 free field for the de\elopment of the bacteria. 



The tendency of this teeming bacterial population of the .soil is to arrive 

 at a state of equilibrium appropriate to the nature of the soil, and its 

 physical condition. In well-tilled soils, in good '" heart "" and condition, 

 containing adequate supplies of carbonate of lime, the useful types of 

 bacteria predominate. In l)adly-tilled soils, sour through want of drainage,. 



* Journal of Aqricmture. Victoria, pp. 275 and 393. 

 t Schloesing and Muntz. 



