TK\NSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 701 



in soils having a portion of their pores filled with air. In such unsatu- 

 rated soils the movement of water to the root surface occurs through 

 the water-filled voids; hence the transmission characteristics of such 

 media are highly dependent upon their degree of saturation, and the 

 potential gradient needed to cause a given rate of water flow to the 

 root surface will be dependent upon the moisture content of the soil 

 surrounding the absorbing root. During steady-state water flow to the 

 root, the equipotential surfaces may be represented as concentric 

 circles around the root and the flow lines as radii which converge as 

 they approach the root surface. As the root surface is approached, the 

 flow velocity per unit transmitting area increases sharply and is ac- 

 companied by a corresponding increase in the potential gradient. This 

 fact accounts for the shape of the moisture-potential function in the 

 soil section of Figure 4. If the flow into the root from the cylindrical 

 layer of soil adjacent to the root exceeds the rate at which water moves 

 into that layer from the surrounding soil, a reduction in moisture con- 

 tent and in the moisture potential will occur in accordance with the 

 C == f(P) relationship shown in Figure 1. The reduction of the mois- 

 ture potential in the adjacent soil will reduce the gradient between the 

 root and the soil but will increase the gradient between that region 

 and the surrounding soil. The lower limit of potential in the soil adja- 

 cent to the root is thus set as being the minimum potential that can be 

 created in the absorbing root. The reduction in moisture potential is 

 accompanied, however, by a marked increase in the resistance to water 

 flow, as illustrated by the great reduction in k depicted in Figure 2. 

 The pattern of water extraction from the soil adjacent to an absorbing 

 plant root therefore is dependent on the moisture-yield characteristics 

 of the soil as expressed in the S =: f(P) relationship, on the velocity of 

 flow per unit absorbing area of the root, on the initial moisture content 

 of the soil, and on the geometry of the absorbing surface ( Ogata, Rich- 

 ards, and Gardner, 1960 ) . 



The rate of water flow through the soil-plant-atmosphere system 

 varies with time. Diurnal fluctuations, largely controlled by the varia- 

 tion in the amount of energy available at the leaf surface for the vapori- 

 zation of water from the substomatal cellular surfaces, are superim- 

 posed on longer-term changes, which are influenced not only by the 

 energy supply but also by the changing morphology of the plant, par- 

 ticularly the ratio of active absorbing root surface to active transpiring 

 leaf surface. Short periods of high transpiration may result in some de- 

 pletion of moisture content of the plant tissues, as well as that of the 

 soil immediately adjacent to the absorbing root. Following periods of 

 high water demand, readjustments of moisture occur, both in the plant 

 and in the soil immediately adjacent to the absorbing roots. Conse- 

 quently, both the plant and the soil adjacent to the active roots undergo 



