TRANSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 



697 



grain size, specific surface, and pore-size distribution have met with 

 only limited success. It is usually necessary to determine k experiment- 

 ally, either in the laboratory on suitably taken samples or in situ, be- 

 cause of the heterogeneity commonly encountered in the field. For the 

 flow of incompressible fluids such as water through saturated porous 

 media, the specific yield is zero, and the transmission coefficient k is 

 independent of B, the potential, if the flow remains non-turbulent. The 

 fact that C, S, and k are not dependent on the potential greatly sim- 

 plifies the mathematical forms of the flow equations used to describe 

 steady-state flow in fully saturated porous media. The analysis of the 

 flow of water or oil to a well through a saturated aquifer or oil-bearing 

 strata and the movement of water into tile drains are examples of the 

 successful use of the General Transport Law for the complete analyti- 

 cal description of flow phenomena. 



The flow of water in unsaturated soil is a phenomenon of great 

 agricultural importance. It also obeys the General Transport Law. Al- 

 though basically similar to the flow through a saturated soil, flow in 

 unsaturated soil differs from the former in that C ( and hence S ) and 

 k are both highly dependent on the potential. Typical curves of the 

 C = f(P),k=rf(P), and S = f(P) relations for soils having different 

 textures are shown in Figures 1, 2, and 3. The potential is negative for 

 unsaturated soils. The reference state, which is taken as a free, flat- 

 water surface at the same temperature, is given a potential of zero. 



MOISTURE CONTENT— ► 



Figure L Idealized moisture-retention curves for three soils. 



