TRANSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 



699 



MOISTURE POTENTIAL 



Figure 3. Idealized curves of the effects of texture and moisture potential 

 on the specific yield. 



for the same values of the potential, depending on the direction from 

 which the equilibrium is approached. Little progress has been made in 

 handling this perplexing problem, and most investigators have avoided 

 the issue by restricting their analyses to systems that undergo either 

 continuously increasing or continuously decreasing changes of poten- 

 tial. Water flow in such a dynamic system as the soil-plant-atmosphere 

 continuum exhibits diurnal cycles of desorption and rehydration at 

 several points along the flow path. At present no satisfactory mathe- 

 matical technique has been developed to handle adequately the type 

 of complexity introduced into such problems by hysteresis. 



Flow of water in the soil-plant system 



The movement of water through the soil, into and through the 

 living plant, and into the atmosphere involves a series of interdepend- 

 ent flow phenomena. Despite the serious limitations mentioned in the 

 preceding section, the entire system, as well as each of its component 

 parts, may be analyzed, at least formally, in terms of the General 

 Transport Law (Honert, 1948; Philip, 1957). Figure 4 serves to illus- 

 trate some of the important characteristics of the flow of water through 

 the soil-plant-atmosphere system. 



Water flows in series through each part of the system, and be- 

 cause the amount of water transpired daily greatly exceeds any diurnal 



