262 SOILS AND SOIL WATER RELATIONS 



cells in the absorbing zone of the root. Except in halophytes the diffusion 

 pressure deficit of root cells probably seldom exceeds 25 atmos. and is usually 

 considerably less. Inspection of the curve in Fig. 68 shows that the diffusion 

 pressure deficit of the water in a soil will attain an equilibrium with a plant 

 in which a diffusion pressure deficit of 25 atmos. has developed in the root 

 cells at only a slightly lower soil water content than with a plant in which 

 the diffusion pressure deficit of the root cells is only 5 atmos. Hence wilting 

 percentages as determined with different species of plants are all approxi- 

 mately the same since the actual differences in wilting percentages usually do 

 not exceed the experimental errors inherent in the method of determining 

 this quantity. 



To the right of the point corresponding to the wilting percentage the 

 slope of the curve is very gradual. The field capacity corresponds to a dif- 

 fusion pressure deficit of about one atmosphere, indicating that plants have 

 very little difficulty in absorbing water at soil moisture contents corresponding 

 to this value. 



The preceding discussion has been based on the assumption that the soluble 

 salt concentration of the soil is not sufficient to have an appreciable effect 

 on the diffusion pressure deficit of the soil water. In any soil heavily charged 

 with soluble salts, as for example "alkali soils," the diffusion pressure deficit 

 of the water may range up to 100 atmos. or even higher, due to the osmotic 

 effect of the dissolved salts. The curve expressing the soil water content-dif- 

 fusion pressure deficit relationship in such a soil would not approach very 

 closely to the zero axis, even in soils which are nearly or entirely saturated, 

 but would level off at a value corresponding to the osmotic pressure of the 

 soil solution. 



Discussion" Questions 



1. Why Is the water content of a clay soil much greater than that of a sandy 



soil when both are at the wilting percentage? 



2. Why are percentage soil water contents expressed on a dry weight basis, but 



the moisture percentage of plant tissues usually on a fresh weight basis? 



3. List some local habitats in which plants obtain most or all of their water 



from a water table. Some in which only a part of the water is obtained 

 from a water table. Some in which little or none of the water is obtained 

 from a water table. 



4. How could the wilting percentage of a soil be determined if a cactus or 



other succulent is used as the test plant? 



5. What would be some of the effects of tiling a field on the water relations 



of the soil? 



6. A waterproof pot is filled with a known weight of air dry soil with a moisture 



equivalent of 20 per cent. Water equal in weight to 10 per cent of the 

 dry weight of the entire soil mass is poured on the surface of the soil. 

 How would you expect to find this water distributed in the soil? 



