SOIL WATER RELATIONS UNDER FIELD CONDITIONS 253 



the water table is so far below the soil surface that it has no effect on soil 

 water conditions in the upper layers of the soil. Even an "air dry" soil is 

 not entirely devoid of moisture. Depending upon the tj'pe of soil the hygro- 

 scopic ?noisture, i.e. the water in an air dry soil, ranges from less than i per 

 cent of the dry weight of the soil in fine sands to 5 per cent in clay loams. 

 The hygroscopic moisture represents almost entirely water held by imbibition 

 in the cell colloids, or water strongly adsorbed on the surfaces of the soil 

 particles, hence no capillary movement of this fraction of the soil water is 

 possible and it is completely unavailable to plants. 



Let us now assume that two inches of rain falls on this soil. After an 

 equilibrium has been attained the soil will be moistened to a certain depth 

 which will vary greatly depending upon the porosity and other properties of 

 the soil. In general the depth of this moist blanket of soil would lie within a 

 range of a few inches in heavy clay soils to perhaps as much as two feet in 

 very sandy soils. In this hj-pothetical example let us assume that the soil has 

 been moistened to a depth of one foot. The rain which enters the soil through 

 its surface layer increases its water content sufficiently so that capillary distribu- 

 tion of water within the soil begins, capillary movement under such condi- 

 tions being in the downward direction. The force of gravity also has an influ- 

 ence, but under the conditions as postulated it is so small in comparison with 

 the capillary forces at work that it may be disregarded. As the water be- 

 comes distributed through a soil the water films gradually become attenuated 

 until eventually capillary movement of water ceases. When this attenuation 

 of the soil water has reached its limit, water is believed to occupy only the 

 smaller interstices in the soil. 



The depth of this moist blanket of surface soil will depend therefore upon 

 the limit to which water can be transferred in the downward direction by 

 capillary movement. Furthermore the water content of this moist layer of 

 soil will be approximately uniform and the boundary between it and the zone 

 of drier soil below will be fairly sharp (Fig. 67, A). The attainment of this 

 condition has required the movement of water until it comes to equilibrium 

 with the capillary and gravitational forces influencing its translocation through 

 the soil (Veihmeyer and Hendrickson, 1927). 



Following Veihmeyer and Hendrickson (1931) we will use the term field 

 capacity as a name for the water content of the moist layer of a soil after 

 an equilibrium has been attained as just described. The field capacity is a 

 critical point in the soil water relations, representing as it does the water 

 content at which further downward capillary movement of water through 

 the soil becomes negligible. Generally speaking the field capacity will range 

 from about 5 per cent in very sandy soils to about 35 per cent in clay loams. 



