Wadleigh and Richards 415 



moisture tensions of the order of 10 to 20 atmospheres. The high ten- 

 sion in the soil moisture in the vicinity of the root sets up a force action 

 in the soil-water system that tends to move water toward the root. This 

 tendency of water to move toward plant roots in response to tension 

 gradients must be of considerable importance, particularly for perennial 

 plants with large developed root systems, because a small distance of 

 movement over a considerable combined length of root system would 

 account for an appreciable volume of water. However, for young plants 

 with a newly developing root system, this movement is so slow that 

 sufficient water for normal growth would not be supplied unless the 

 plant roots are able to extend themselves outward into a fresh soil mois- 

 ture supply. Therefore, as has been described by Davis (^5), when a 

 new corn plant is developing, the available moisture is extracted in the 

 vicinity of the base of the plant and the soil approaches the wilting 

 condition whereas just a few inches further away from the plant the 

 soil may be at or near field capacity. The roots of the newly developing 

 plant must extend themselves outward in order to maintain a continu- 

 ous supply of available water. When the roots of the plant have per- 

 meated the soil region in which they can grow well, the soil moisture 

 tension throughout the soil region occupied by roots will approach that 

 corresponding to the wilting percentage, and, unless additional mois- 

 ture is supplied by rain or irrigation, vegetative growth will cease and 

 the plant will show symptoms of wilting. The depletion of soil mois- 

 ture under perennial plants with developed root systems has been 

 described in detail by Veihmeyer and Hendrickson (12]). 



During this process of moisture extraction by roots the soil water is 

 withdrawn successively from the large, medium, and fine pores and 

 is replaced by a gaseous phase. Thus, aeration processes are expedited 

 and become freer as the moisture content of the soil is depleted. Soils 

 having favorable structure for the growth of plants tend to have an 

 appreciable fraction of the pore system made up of large pores. These 

 pores are the first to empty during moisture depletion, thus promoting 

 good drainage and favorable aeration. 



From saturation to the field capacity, moisture depletion is accounted 

 for by downward drainage. Leaching and loss of soluble material from 

 the profile occurs during this process. After the field-capacity condition 



