SOIL WATER RELATIOxNS UxNDER FIELD CONDITIONS 255 



flows through the moist blanket of soil, after which it is distributed to the dry 

 soil layer underneath by capillarity (Shantz, 1927). 



Successive rainfalls would thus continue to deepen the layer of soil which 

 has been moistened up to its field capacity. If the water table is not too 

 deep, and if sufficient rainfall penetrates the soil, not too much of which is 

 utilized by plants growing thereon, eventually the entire soil from its surface 

 down to the water table will be moistened up to its field capacity. A zone 

 several feet in height just above the water table may be enriched somewhat 

 above field capacity by upward capillary movement from the water table 

 (Fig. 67, C). If additional water is applied to the soil, after the entire soil 

 mass down to the water table has attained its field capacity, this water per- 

 colates down through the soil under the influence of gravity and becomes a 

 part of the ground water. Such conditions obtain in many of the soils of 

 more humid regions at least during the wetter seasons of the year, provided 

 that the water table is not located at too great a depth. This downward per- 

 colation of water to the water table is an important factor in influencing 

 its depth below the soil surface, which usually fluctuates considerably from 

 season to season. 



In the preceding discussion we have assumed, in the interests of simplicity, 

 a homogeneous soil. However, most soils consist of a vertical succession of 

 several distinct horizons, each with more or less distinctive physical and chemi- 

 cal properties. Even after long continued disturbance of a soil by plowing or 

 other cultural activities at least some semblance of the original soil stratifica- 

 tion usually persists. In the horizons below those reached by the plow the 

 original structural organization of the soil is usually maintained practically 

 intact. Although the individual horizons of a soil are often fairly homo- 

 geneous within themselves, each such horizon in a given soil may have a 

 distinct field capacity of its own. The water contents of the different hori- 

 zons, even after an equilibrium in the capillary distribution of water has been 

 attained, may therefore be very different. 



A somewhat erratic distribution of rainfall to an underlying soil often 

 results from cracks or channels which are opened into the soil by one agency 

 or another. Many soils crack upon drying, sometimes to depths of several 

 feet. Such cracks often facilitate a mass flow of rainwater to a considerable 

 depth in the soil. Similarly in forest soils the decomposition of roots may leave 

 channels through which water flows down into the deeper layers of the soil. 

 The burrows of animals may also facilitate the entry of water into a soil. 

 The presence of rock strata, or of relatively impervious layers of soil close 

 to the surface, will also modify very markedly the simple picture which has 

 been presented of soil water conditions. 



