566 



SCIENCE. 



[N. S. Vol. XX. No. 513. 



SPECIAL ARTICLES. 



AN ARTIFICIAL BOOT FOR INDUCING CAPILLARY 

 MOVEMENT OF SOIL MOISTURE. 



The rate at which a plant is able to secure 

 water from a soil, under any given condi- 

 tions, depends upon two factors : (1) The 

 pulling force which a plant is able to exert 

 Upon the water in the soil, and (2) the capil- 

 lary force with which the soil holds the water 

 which it contains. Under a condition of 

 equilibrium, this latter force could be ex- 

 pressed in terms of the curvature of the cap- 

 illary surfaces. When the water in the soil 

 begins to move, however, there is introduced, 

 in addition to the static pull of the curved 

 capillary surfaces, a resistance to the move- 

 ment or translocation of the water over the 

 surface of the soil particles. This resistance, 

 which obviously depends upon the thickness 

 of the film, must be taken into consideration 

 in all questions relating to the rate of move- 

 ment of the capillary moisture in soils. 



In order to determine the rate at which a 

 given soil with a given moisture content is 

 able to supply moisture to the roots of a 

 plant, one must create in the soil at some 

 point a pulling force analogous to the action 

 of the plant root. This may be done in sev- 

 eral different ways : 



1. The surface of the soil may be exposed 

 to evaporation, thus producing an upward 

 movement of the water from the lower por- 

 tions of the soil. This method is open to 

 serious objections. The surface soil soon 

 becomes air-dry, unless the water content is 

 kept abnormally high, so that evaporation 

 must take place more or less from within the 

 soil mass, which leads to indeterminate con- 

 ditions. Furthermore, previous experiments 

 have shown that the distance through which 

 water will rise in a dry sand is only one 

 fourth the distance through which a vertical 

 movement will take place in moist sand.* 

 From these facts it is evident that the capil- 

 lary movement induced by a dry surface soil 

 is not representative of the capillary move- 

 ment in the soil surrounding the active roots 



* Briggs and Lapham, Bull. 19, Division of 

 Soils, U. S. Dept. of Agri., 1900. 



of a plant, where the soil grains are covered 

 with water films. 



2. An osmotic cell buried in the soil may be 

 used to produce an inward movement of the 

 soil moisture towards the cell,* provided the 

 osmotic pressure of the cell solution is greater 

 than the capillary pressiire of the soil moist- 

 ure, f Unfortunately, the rate at which 

 water diffuses into a cell of this kind is so 

 slow as to preclude the possibility of taxing 

 the soil by this method, except under condi- 

 tions approximating drouth. This, com- 

 bined with the changes in concentration which 

 take place at the inner wall of the cell as 

 water enters, and the difficulty experienced in 

 preparing cells capable of withstanding high 

 pressures, makes the method in general un- 

 suitable for investigating the rate of capillary 

 movement. When the moisture content of a 

 soil is reduced to such an extent that the rate 

 of movement is extremely slow, the osmotic 

 cell furnishes a very beautiful means of pro- 

 ducing in the soil a capillary pull of known 

 magnitude. 



3. The method which we are about to de- 

 scribe avoids the errors and difficulties inci- 

 dent to the two methods outlined above. The 

 apparatus consists of a close-grained unglazed 

 porcelain tube, closed at one end, and provided 

 at the other with a tubulure, by which it can 



* Cameron ( Bull. 22, Bureau of Soils, Dept. of 

 Agri., 1903) has shown experimentally that when 

 an osmotic cell, having a calculated osmotic pres- 

 sure of 36 atmospheres, is buried in a soil, there 

 is an inward or outward movement of water de- 

 pending upon the moisture content of the soil. 



t The capillary pressure of the soil moisture is 

 due to the existence of curved water-air surfaces 

 within the soil which tend to contract and thus 

 produce a pressure outward along the normal to 

 the water-air surface. This surface can actually 

 move outward only by drawing in additional 

 water from svirrounding capillarj' spaces. If 

 the moisture in the soil is in equilibrium the 

 curvature of all the water surfaces at a uniform 

 level is the same. The pressure exerted by the 

 capillarj' surfaces depends upon the curvature 

 and the surface tension. When the moisture 

 content of the soil is diminished, the curvature, 

 and consequently the capillary pressure, is in- 

 creased. See Bulletin 10, Div. of Soils, U. S. 

 Dept. Agri., 1898. 



