III.] 



HYGROSCOPIC MOISTURE 



97 



In each field the upper layer of soil possesses a 

 higher capacity for retaining hygroscopic moisture than 

 does the lower layer, but in field C this upper layer is 

 thicker than in field B. It will be seen from the table 

 that as regards total water there is no great difference 

 between the two fields, but when the hygroscopic 

 moisture is deducted, B contains 3-4 per cent, more 

 water available for the plant. This difference was 

 manifested in the following season in the yield, which 

 was only 2160 lbs. of grain and straw on C and 9200 

 lbs. on B. Had the really free water been determined, 

 that which is in excess of the amount held by the soil 

 when wilting sets in, these differences would have been 

 even more pronounced, as is evident from the fact that 

 the moisture in the soil at the wilting point is greater 

 than the hygroscopic moisture but is in proportion to it. 

 A relationship has been established between the wilting 

 coefficient and the hygroscopic moisture ; as both are 

 points on the same curve, of which the determining 

 factors are the surface of the particles and the amount 

 of the colloids, their connection is intelligible enough. 



Though the hygroscopic moisture gathered by the 

 surface soil in the cooler and damper periods of the 



H 



