SOIL WATER RELATIONS UNDER FIELD CONDITIONS 251 



content of the soil is also favorable to the development of most species of 

 bacteria, apparently because it favors a granular structure of the soil, thus 

 improving aeration. Some species of bacteria, on the other hand, are anaerobic, 

 and thrive when aeration of the soil is deficient. The denitrifying bacteria 

 and certain of the nitrogen-fixing bacteria {Clostridium spp.) are examples 

 of anaerobes. 



Fungi are, in general, most abundant in soils of acid reaction. In such 

 soils they largely replace bacteria as agents of decomposition of organic matter. 



The soil fauna includes protozoa, nematodes, earthw^orms, insects, insect 

 larvae, and burrowing species of the higher animals. The earthworms are 

 generally credited with having the most important effects on soil structure, at 

 least in many soils. Their activities result principally in a general loosening 

 of the soil, which facilitates both aeration and distribution of water. Many 

 of the other soil animals have similar effects on the structural organization of 

 the soil. 



Soil Water Relations under Field Conditions. — In a region of moist 

 climate, if a hole be dug or bored into the ground at almost any place where 

 the soil is deep enough a level at which the soil is completely saturated with 

 water will be encountered at some depth or other. Water will stand in this 

 hole up to this level of complete saturation which is called the ivatcr table. 

 In river valleys or in close proximity to large bodies of water the water table 

 will usually be reached at a depth of only a few feet under the soil surface. 

 Even in regions of arid climate there are some local situations in which a 

 water table is present. The depth of the water table in any locality usually 

 fluctuates, sometimes very markedly, with seasonal and periodic changes in 

 the relative rates of rainfall, evaporation, transpiration and other factors. 

 Relatively impermeable soil layers sometimes impede downward percolation of 

 water sufficiently to cause the development of temporary water tables which 

 may be far above the level of the true water table. Such tem.porary water 

 tables have essentially the same effects on soil water relations as true water 

 tables, except that their influence is often only a transient one. 



For many years an important role was ascribed to the capillary rise of the 

 water from the water table into the soil above in maintaining the moisture 

 conditions within that soil. Recent investigations have shown, however, that 

 the importance of this source of water in soils has been over-emphasized. Ex- 

 periments upon the rise of water through columns of soil indicate that water 

 seldom rises through the soil by capillarity from the water table at an appre- 

 ciable rate for heights of more than a few feet (Keen, 1928). In a typical 

 loam soil the absolute maximum capillary rise of water is about 8 feet ( Shaw 

 and Smith, 1927). Such capillary rise as does occur takes place most slowly, 



