DIFFUSION PRESSURE DEFICIT OF WATER IN SOILS 261 



140 



ing point depression of the soil, and measurements of the amount of water 

 retained by a soil in equilibrium with tensions of a known magnitude (Grad- 

 mann, 1928). Each of these methods is suitable only for a certain range of 

 soil water contents. The first 

 can be used satisfactorily only 

 with relatively dry soils, the last 

 with relatively wet soils, and the 

 second only over an intermediate 

 range of soil water contents. 



From data obtained by one 

 or more of these methods it is 

 possible to construct for any soil 5 



. . -^ w 80 



a water content-diffusion pressure 



in 

 \ii 

 a. 



gil20t- 

 O 



I- 

 < 



z 



tt 100 



UJ 



;— 



I 



I 



60 



d: 

 ^'»0|- 



WILTING 



PERCENTAGE 



deficit curve (Fig. 68). Most of 

 the important soil water relations 

 can be interpreted in terms of 

 such curves. 



Such a curve is characterized 

 by an abrupt rise in diffusion 

 pressure deficit in the region of % 20 

 low soil water contents. As al- 

 ready shown by the results pre- 

 sented in Table 26, in this zone 

 of soil moisture contents a very 

 small decrease in soil water con- 

 tent corresponds to a very con- 

 siderable increase in diffusion 

 pressure deficit. 



The position of the wilting percentage is just to the right of the sharp 

 inflection which occurs in the cune. The diffusion pressure deficit of the soil 

 water at the wilting percentage lies within a range of 4-12 atmos. Ex- 

 perimentally determined values for this quantity vary slightly depending upon 

 the method used for their measurement. The position of the wilting percent- 

 age makes understandable the fact that all species of plants seem to be able 

 to reduce the soil water content of any given soil to approximately the same 

 percentage before they pass into a state of permanent wilting. As shown 

 by this curve reduction of the water content of the soil by only a very small 

 percentage below that of the wilting percentage results in a very rapid rise in 

 the diffusion pressure deficit of the soil. Plants can only obtain water when 

 the diffusion pressure deficit of the soil water is less than that of the peripheral 



10 15 20 25 30 35 



WATER CONTENT, PER CENT OF DRY WEIGHT OF SOIL 



Fig. 68. Relation between soil water con- 

 tent and diffusion pressure deficit of the soil 

 water in a silty clay loam soil. Data of 

 Magistad and Breazeale (1929). 



