IX, A, 1 Cox and Argilelles: Soils of Luzon 7 



excreta may be destroyed or their injurious effect neutralized 

 by proper cultivation, the action of microorganisms in the soil, 

 the oxidizing ^ower of roots themselves, and by the action of 

 fertilizers. Schreiner and his associates^" have recently carried 

 on many interesting experiments which bear upon the com- 

 plicated question of soil fertility, but their results v^ill need 

 to be thoroughly tested under actual field conditions. 



BACTERIA IN THE SOIL 



It is a recognized fact that certain bacteria play a conspicuous 

 part in the growth of plants. Bacteria are greatest in number 

 near the surface, decrease with the depth, and practically dis- 

 appear at 2 meters more or less, depending on the porosity of 

 the soil, the moisture, temperature conditions, and the physical 

 and chemical nature of the soil. Hilgard^^ found that a high 

 humus content in a soil is favorable to bacterial development, 

 confirming the fact that organic matter in the soil is conducive 

 to increased bacterial activity. A gram of moist surface soil 

 may contain fifteen million bacteria. Only aerobic bacteria play 

 an important role in soil fertility. Their function is not well 

 established, but it is probable that they take part in breaking 

 down the inorganic constituents into available form for the 

 plants and in improving the tilth of the soil. 



The ammonia-forming, nitrifying, denitrifying, and nitrogen- 

 fixing bacteria are important groups of microorganisms. A 

 conspicuous function is their part in the degradation process of 

 nitrogenous organic matter. The chemical reactions involved in 

 this degradation vary with the conditions involved. ^^ If it takes 

 place in the presence of a supply of oxygen (the process of decay) , 

 such as in a light well-aerated soil, it differs in speed and other es- 

 sentials from that which takes place when oxygen is conspicu- 

 ously absent, as in heavy clay or tight loam soil (the process of pu- 

 trefaction), but in both cases the process is the simplification of 

 nitrogenous organic substances, a product of which is ammonia. 



Ammonia-forming bacteria. — It has been shown by Muntz^' 

 and Goudon that ammonification does not take place in sterile 



''Bull. U. S. Dept. Agr., Bur. Soils (1908), 53; (1910), 74; (1911), 

 77; (1911), 80; (1911), 83. 



"Soils, New York (1907), 144. 



"Cf. Bull. U. S. Dept. Agr., Off. Exp. Sta. (1907), 194, 48-49; Wellny, 

 Die Zersetzung der organischen Stoffe (1897), 2; Fliigge, Die Mikro- 

 organismen, 3d ed. (1896), 1, 254. 



"Compt. rend. Acad. sci. (1893), 116, 395; Ann. Agron. (1893), 19, 

 209; Bull. U. S. Dept. Agr., Off. Exp. Sta. (1907), 194, 50. 



