64 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



the third foot-section with 13.5 per cent of water is reduced to its 

 wilting coefficient, while if the sixth foot-section had 13.5 per cent 

 of water it would contain 6 per cent of water available for growth. 



OTHER INDIRECT MEASUREMENTS OF THE WILTING COEFFICIENT. 



The relationship established between the wilting coefficient and 

 the moisture equivalent led to the belief that a similar relationship 

 might be found for some of the other physical measurements of 

 soil-moisture retentivity. Accordingly similar comparisons were 

 made of the wilting coefficient with the hygroscopic coefficient, the 

 moisture-holding capacity, and the soil texture, as expressed by 

 mechanical analysis. The last-mentioned determination does not 

 measure moisture retentivity, but it does measure certain properties of 

 the soil which determine the moisture retentivity to a large extent. 

 The results of those comparisons will now be considered. 



RELATION OP THE "WILTING COEFFICIENT TO THE HYGROSCOPIC COEFFICIENT. 



When a dry soil is placed in a saturated atmosphere it will absorb 

 water vapor until a condition of approximate equilibrium is attained. 

 The moisture content of a soil under such conditions is known as the 

 hygroscopic coefficient of that soil. 



The determination of the hygroscopic coefficient, unless carried 

 out with special precautions, is not very exact. It is influenced by 

 variation in temperature and by any departure from a condition of 

 complete saturation of the surrounding air. 1 The time element is 

 also an important factor, since the soil absorbs water very slowly, 

 particularly near the point of equilibrium. In fact, equilibrium, 

 theoretically, would not be obtained until the interstitial spaces 

 of the soil were practically filled with water. The method thus has 

 certain inherent disadvantages which are not encountered in moisture- 

 equivalent determinations. The hygroscopic-moisture determina- 

 tions given in this paper were carried out in a double-walled ice chest 

 kept in a subterranean room, where the temperature was approxi- 

 mately 20° C. 2 The bottom of the chest was covered with water and 

 the zinc walls were lined with blotting paper, which was kept 

 saturated. 



A comparison of the hygroscopic coefficient and the wilting 

 coefficient for a number of soils is given in Table XX. The soils 

 used are the same as those employed in the preceding experiments, 

 being arranged in the order of increasing moisture equivalents. 



i Hilgard, E. W. Soils, New York, 1906, p. 196. 2 Determinations by Mr. J. W. McLane. 



230 



