680 



SCIENCE. 



[N. S. Vol. XX. No. 516. 



AN AETIFICIAL ROOT FOR INDUCING CAPILLARY 

 MOVEMENT OF SOIL MOISTURE. 



To THE Editor op Science: The article in 

 Science of October 27, 1904, page 566, under 

 the above title calls for some comment on 

 account of the fact that it does not, clearly 

 and certainly follow, either from the evidence 

 presented in the paper or from the conditions 

 of the experiment, as stated, that the move- 

 ment of soil moisture observed v^as in any 

 vcay the result of capillary action. The sub- 

 ject of the paper is an important one, and 

 perhaps a fuller statement of the conditions 

 and observations will make the position taken 

 by the authors tenable; but, from the evidence 

 presented, it appears that the flow of water 

 observed may have resulted solely from per- 

 colation, induced by the suction developed by 

 the partial vacuum maintained in the filter 

 chamber. 



The authors state that ' When a tube of 

 this kind is moistened, so that the pores are 

 filled with water, and the tube is protected 

 from evaporation, it can be exhausted to a 

 pressure equal to the vapor pressure of water, 

 and, if connected to a two-liter receiver, it 

 will maintain that pressure for a day or more 

 without sensible loss.' They also state that 

 such ' A tube in good condition, when im- 

 mersed in water and exhausted, will take up 

 water at the rate of 50 grams per minute.' 

 From these two statements it appears that 

 the texture of the filter was suiEciently close 

 so that, when wet, it was air-tight against a 

 pressure of one atmosphere; but that when 

 the capillary power or surface tension of the 

 outer layer of the filter was rendered nil by 

 immersion in water the flow into it was 50 

 grams per minute under the pressure which 

 before gave no movement. It appears, there- 

 fore, that an essential condition of flow is the 

 supersaturation of the outer wall of the filter, 

 which, in effect, is equivalent to immersing 

 it in water. It can hardly be assumed, how- 

 ever, from the evidence of the paper, that 

 capillarity within the soil unaided is capable 

 of supersaturating the wall of the filter where 

 the soil is brought in contact with it and of 

 maintaining it in this condition for days to- 

 gether. It is to be expected, however, that 



the placing of the outer wall of the filter in 

 close contact with the soil did have the effect 

 of practically augmenting the thickness of the 

 wall, causing the filter to stand in the soil as 

 a porous curbing to a well. When so placed, 

 the capillary soil moisture may be expected to 

 join with that of the wall of the filter, thus 

 reducing, within the areas of good contact, 

 the power of surface tension to withstand the 

 suction from within; hence, along those lines, 

 could be established a flow exactly as if the 

 filter had been immersed in water, but purely 

 through atmospheric pressure, capillarity tak- 

 ing little or no part in the movement. As 

 the water escaped from the soil into the filter 

 the reduced pressure would spread outward, 

 and this would permit the soil air to sweep 

 more water toward the filter and thus main- 

 tain the supersaturated condition about its 

 wall until the soil moisture was so much re- 

 duced as to leave the soil open enough to 

 permit air to come in contact with the -filter ; 

 thus restoring, in effect, the condition of the 

 filter described in the first quotation, with 

 the wall only capillarily saturated and under 

 which no flow took place. We do not feel, 

 therefore, that the evidence which the authors 

 present in the article referred to is suffi- 

 ciently conclusive to warrant the views there 

 expressed, or that they have succeeded in de- 

 vising an artificial root which, in any essen- 

 tial way, can be said to represent or measure 

 the natural movement of soil moisture in a 

 soil toward an active root. 



There is no doubt that if a method could 

 be devised which would enable the rate of 

 movement of soil moisture in the different 

 soil types to be measured, under field condi- 

 tions, an important advance could be made; 

 but it is important to recognize that, even if 

 it shall appear that the flow of water in the 

 cases cited was due to capillarity, unless the 

 method can be made capable of reducing the 

 water lower than 17 per cent., in a soil whose 

 maximum is 20 to 22 per cent., investigations 

 made with it can have but a limited value. 

 The force of this remark will be seen when it 

 is pointed out that the lowest limit of moist- 

 ure, in the soil experimented with, at which 

 the flow ceased, was at least not lower than 



