THE WATER OF SOILS. 213 



Loughridge during several drought years in California; so ar- 

 ranged as to show the differences of moisture content for the 

 same crop in different soils. It will be observed that in all 

 cases where a crop growing on a clay soil could be compared 

 with the same on a lighter soil, the moisture required to keep 

 the crop in good condition was very much greater in the clay 

 than in the loam or sandy soils. In the case of apples, e. g., 

 8.3% of water was abundant to keep the trees in excellent con- 

 dition on a loam soil, while on a clay soil holding 12.3% the 

 condition was very poor. That this difference is due in the 

 main to the difference in the hygroscopic-moisture coefficient 

 of the respective soils, is plainly apparent in several cases. It 

 is therefore not the total moisture content, but the free mois- 

 ture present in excess of what is held by hygroscopic absorp' 

 tion, that determines the welfare of the plant. 



By determining, first, the total moisture in the soils, as taken 

 in the field, then, after allowing them to become air-dry, deter- 

 mining the maximum of hygroscopic moisture they would ab- 

 sorb (see p. 198), Loughridge found by difference the amount 

 of free moisture, or liquid water which must be present in the 

 soil to prevent the crops from suffering. An exceptionally 

 good opportunity for these observations was offered by the 

 dry season of 1898, during which crops suffering and not 

 suffering, on identical lands, could easily be found. The de- 

 terminations were always made for each foot of the upper 

 four feet of the land in the immediate neighborhood of the 

 trees or among the field crops. The first table exemplifies the 

 method of procedure; the second gives the summary of results 

 for the several crops and trees, as calculated from observations 

 made during the season. 



