Ur § L> 



thus maintained in a given soil depended on the nature 

 of the soil, the temperature (as described below), and 

 the heighth of tho mercury column, but not at all on the 

 rate or amount of water intaie by the plant. When nec- 

 essary the water content of the soil could be varied by re- 

 moving or adding mercury and thus changing the heighth 

 of the eolumji. 



It will be noted that, over considerable time 

 periods, the water intake by the plant must be balanced 

 by the water lost from the auto-irrigator. This loss 

 may be measured on the scale of the water-reservoir. A 

 means is thus available for measuring the water intake 

 by the plant, but one disturbing factor must be taken 

 into account. It was noted above that the capillary 

 tension on the water- film system in the soil is due to 

 surface tension. The amount of the capillary tension 

 depends, therefore, on the intensity of this surface 

 tension (per-unit of surface) as well as on the nature 

 of the soil and its wetness. The spcjifio surface ten- 

 sion of water varies v/ith two factors which may come 

 into play in the case under examination; the tempera- 

 ture and the presence of dissolved substances. The pres- 

 ence of dissolved substances is probably not impor- 

 tant practically since the substances so present in the 

 soil solution of any particular soil are presumably con- 

 stant and of constant effect on surface tension. Hence 

 they do not affect the precision of the balance betwe- 



