26o 



SOILS AND SOIL WATER RELATIONS 



osmotic pressures and the percentage of water which had been imbibed when 

 an equilibrium was attained in each solution was first determined (Chap. IX). 

 Once the percentages of water held by these seeds when in equilibrium 

 with solutions of different osmotic pressures had been determined, the diffusion 

 pressure deficit of any medium in which they might be immersed could be 

 measured. For example, if some of these seeds were immersed in a medium 

 of unknown osmotic pressure (diffusion pressure deficit) until a dynamic 

 equilibrium was established, after which it was determined that their moisture 

 content was 32.8 per cent, a diffusion pressure deficit of 26.6 atmos. was 

 indicated for the medium (Table 17). This principle was applied to the 

 measurement of the diffusion pressure deficit of soils at different water con- 

 tents. A sample of soil at a known water content was shaken with some of 

 the seeds until an equilibrium was attained, after which the water content of 

 the seeds was determined. 



TABLE 26 — DIFFUSION PRESSURE DEFICIT VALUES OF AN " OSWEGO SILT LOAM SOIl" AT DIF- 

 FERENT WATER CONTENTS, AS INDICATED BY THE ABSORPTION OF WATER BY COCKLEBUR 

 SEEDS (data OF SHULL, I916) 



As the data in Table 26 indicate the diffusion pressure deficit of the 

 water in an air-dry soil is of the order of magnitude of 1,000 atmospheres. 

 With an increase in the soil water content its diffusion pressure deficit de- 

 creases. In the range of low water contents a small increase in water con- 

 tent corresponds to a very large decrease in the diffusion pressure deficit of 

 the soil. As the water content of the soil is increased further the diffusion 

 pressure deficit continues to decrease but at a progressively slower rate. 



Diffusion pressure deficit values for soil water may be calculated from 

 the results of a number of other types of determinations. Among these are 

 vapor pressure measurements (Thomas, 1924), determinations of the freez- 



