SURFACE TENSION AND CAPILLARITY 159 



the capillary moisture capacity of a soil, one other is to 

 be considered. The surface tension, or cohesiveness, 

 of the moisture was described as one of the forces which 

 acts in conjunction with the texture, structure and or- 

 ganic content, to retain water. The surface tension 

 of any liquid is not a constant quantity, and the soil 

 water is no exception to this rule. Anything which in- 

 creases surface tension increases moisture retention, 

 and likewise anything which decreases surface tension 

 decreases the moisture retention. Soil moisture is sub- 

 ject to considerable variation in surface tension. Two 

 things are most active to change this tension, or co- 

 hesiveness. These are: (1) Materials in solution in 

 the water. Lime and many other salts increase the 

 tension, some substances decrease it below the normal 

 for pure water. (2) Changes in temperature alter the 

 surface tension. 



Whitney and others have determined the surface 

 tension of a number of salt and soil solutions, some 

 of which are given in the table on the following page. 

 The concentrations are not uniform. 



The figures show that many salts increase the surface 

 tension of the soil moisture above that for pure water, 

 and that certain other substances decrease the surface 

 tension. Among the latter are some of the most common 

 constituents of manures which greatly decrease surface 

 tension. All oily or fatty substances reduce the tension, 

 and, since both these latter are present in nearly all 

 soils, the average surface tension of the soil moisture 

 is less than that of pure water. Consequently, the 

 tendency is to retain less of such a solution than of pure 



