MICROORGANISMS AND SOIL FERTILITY 73 



of the soil. Evaporation of moisture is constantly taking 

 place at the surface. To replace this loss by evaporation, 

 through capillary action of the soil particles, the water 

 moves upward toward the surface. Ultimately a point is 

 reached at which the water films surrounding the soil par- 

 ticles become so thin that this upward movement does not 

 continue to replace the loss due to evaporation. Such a 

 soil is said to be air-dry, and contains only hygroscopic 

 moisture. It is doubtful whether bacteria can grow in such 

 a soil. It is probable that the soil of humid regions never 

 becomes so dry as to destroy any large portion of the non- 

 spore-forming bacteria by desiccation. 



In coarse-grained soils the extent of surface of the parti- 

 cles in a unit volume of the soil is much less than in a finer 

 soil. It is thus evident that in a fine soil a given amount 

 of water will form a thinner film, and be more tightly held 

 by the soil particles, than will be the case with a coarse soil. 

 A state of physiological dryness will therefore be reached 

 at a higher water content in fine soil than in a coarse soil. 



The air supply of the soil is in an inverse ratio to its 

 water content. As the water is removed by drainage or 

 evaporation, the pore spaces become filled with air. In 

 coarse sandy soils the air supply is usually sufficient to favor 

 rapid bacterial action. Indeed, often the farmer is de- 

 sirous of limiting the air supply by packing and rolling the 

 soil in order to retard decomposition. The ventilation of 

 close-textured soils, owing to variations in atmospheric pres- 

 sure and to the effect of wind, is not sufficient to cause any 

 rapid replacement of the soil air laden with carbon-dioxide 

 by atmospheric air. Under the most favorable natural con- 

 ditions, the aeration of such soils is insufficient to permit 

 the rapid growth of such pronounced aerobic organisms as 

 the nitrifying bacteria, except at the immediate surface. 

 In the close-textured soils the decomposition of organic mat- 



