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contact with the capillary film- system of the soil. 

 The tension of the latter then draws water from the 

 auto-irrigator and will a raw it against a considerable 

 opposing tension, as, for instance when the water reser- 

 voir is placed at a lower level and water must be drawn 

 against a gravity head due to this difference in levels. 

 Another method of providing an opposing tension is to 

 insert a mercury column, as was done in the connecting 

 U-tube used in these experiments and described above. 

 Water is then drawn against a head equal, with allowance j*or- 

 differing densities, to the heighth of the column 

 mercury. 



The amount of this opposing head furnished by 

 water or mercury columns, regulates the amount of water 

 in the soil. It was noted that the capillary tension 

 of the water-film system in theJSoil decreases as the 

 soil becomes wetter. If, therefore, an auto-irrigator 

 cup be placed in a relatively dry soil and allowed to 

 establish connection with the capillary film-system, 

 water will be drawn by the soil from the cup until the 

 soil becomes wet enough for its capillary tension just 

 to balance the opposing tension, or head, of the water 

 in the auto-irrigator. In these experiments the water 

 content of the soil in the pots was kept nearly con- 

 stant automatically in this way. As water was talren 

 from the soil by the plant roots, more water was drawn 



by the soil from the auto-irrigator. The actual water c»nfrn 



