164 PHARMACEUTICAL BACTERIOLOGY 



wick, piece of rope or string, short piece of wood, etc.) is placed in water, 

 the moisture in the porous substance gradually rises above the level of the 

 water. This is due to what is known as capillary action. Soil is a porous 

 substance and the particles composing it (sand particles, bacteria, organic 

 matter, etc.) contain capillary moistuh derived from the hydrostatic 

 water just mentioned. While hydrostatic water invariably moves down- 

 ward under the influence of gravity, capillary moisture (or water) moves 

 exactly in the opposite direction, namely upward, or, more correctly in all 

 directions of the capillary influence. This capillary water or moisture 

 forms a continuous film over the particles composing the soil and it is in 

 this moisture that the principal bacterial development takes place. The 

 pores of the soil in which the capillary moisture exists are saturated with 

 water vapor. As the capillary moisture evaporates into the air at the 

 surface of the soil, new supplies are drawn upon from below (the hydro- 

 static water), until that supply is exhausted, and very soon thereafter the 

 capillary moisture also disappears through evaporation into the air and 

 into the soil pore spaces. From this it becomes evident that the cultural 

 operations should be directed toward the reduction in the loss (by evapora- 

 tion) of the capillary moisture, which in turn means the conservation of 

 the hydrostatic water supply of the soil. It only need be stated, for the 

 purpose of explaining moisture conservation, that the loss of capillary 

 moisture is completely checked as soon as the air spaces about the capillary 

 or porous substance is saturated with moisture and the escape of moisture 

 from such spaces is prevented. Thus a wick dipped into water, in a tightly 

 corked bottle, will hold its capillary moisture indefinitely. It must 

 however be borne in mind that a complete check in the upward movement 

 of capillary soil moisture would mean a serious interference with the 

 bacterial development as these organisms require continually renewed 

 water supplies in order to take therefrom the soluble substances which are 

 necessary as food materials. The cultural operations should therefore be 

 such as to give rise to an optimum (rather than a maximum or minimum) 

 upward movement of capillary water. 



Soils which have lost both hydrostatic and capillary water, still 

 contain some moisture derived from the air and this moisture is known as 

 hygroscopic moisture. This varies in amount and is contained in the 

 organic particles of the soil. Hygroscopic moisture is generally not 

 suflScient to permit the development and multipHcation of soil bacteria, 

 but it is sufficient to keep bacteria alive as may readily be proven by 

 making plate cultures of long kept air dry soils. In some locaUties, the 

 hygroscopic moisture may indeed be sufficient to permit some bacterial 

 development in the surface soils, but this moisture alone is inadequate 

 to permit the continued growth of crop plants or of other larger vegetation. 



