Wadleigh and Richards 413 



plant growth moisture range and for a wide range of soil textures. 

 The combination of a porous cup and a vacuum gage, which has 

 been called a tensiometer, can be used for measuring soil moisture 

 tension over the range from zero to about 0.85 atmosphere. The rela- 

 tion of soil moisture content to soil moisture tension over the whole 

 plant growth range has been studied by means of porous membrane 



8 15 



SOIL MOISTURE TENSION — ATMOSPHERES 



Figure i. Curves showing the relation between the soil moisture tension 

 and the moisture content of the soil. 



apparatus {102). Recent improvements have been made in measuring 

 methods so that it is now conveniently possible, by the freezing point 

 procedure, to estimate soil moisture tension in nonsaline soils from 

 freezing point depression values (104). Soil moisture tension is thus an 

 experimentally usable scale for expressing the security or tenacity with 

 which water is held in the soil by surface force action. If appreciable 

 quantities of soluble salt are present in the soil, then an osmotic force 

 also resists the absorption of water by plants and must also be taken 

 into consideration. The sum of the soil moisture tension and the 



