14 
Research Bulletin No. 5 
thickened films around the soil grains, and in the small pore 
spaces between the soil particles — called capillary water. 
3. That which is being drawn downward thru the soil by the 
pull of gravity — called gravitational water. 
HYGROSCOPIC WATER. 
Hygroscopic water is that which an absolutely dry soil will 
absorb when exposed to a moist atmosphere. The amount of 
water a soil will absorb from a saturated atmosphere depends 
mainly upon the temperature and the type of the soil. The finer 
the soil particles, the more water they will absorb. This water 
is held in the soil as thin films around the soil particles. It is 
of no value in furnishing water to agricultural plants. It prob- 
ably has a beneficial action in bringing into solution some of the 
plant food material held in the soil, on account of its close con- 
tact with the soil particles. From determinations that have been 
made in the laboratory, and from studies made under field con- 
ditions, the conclusion has been reached that plants are not 
able to use this moisture for growth. 
CAPILLARY WATER. 
Capillary water, or that which is held against gravity in the 
small pore spaces and adhering as thickened films to the soil 
particles, is the source of supply to growing vegetation. This 
water may move in any direction in the soil, but its movement is 
slow. It moves thru the soil much the same as oil moves along a 
lamp wick. As some portion of the soil becomes drier, the films 
of water around these soil particles become thinner, exert a 
greater force, and consequently draw some water from the ad- 
joining particles of soil. The films around these particles in turn 
draw from the next and so on, moving the water slowly from the 
more moist to the drier portion of the soil. If the loss of water is 
checked, the movement of water will cease as soon as the soil 
particles at the drier portion have received sufficient water so 
that they can no longer exert a stronger force than the adjoin- 
ing particles. 
From the standpoint of erop production we are interested 
almost wholly with the water that is held by capillary force 
within the soil zone occupied by the plant roots. The movement 
of water thru the soil by capillarity is so slow that it is practi- 
cally useless in bringing water from a lower soil area for the 
use of a growing crop. The rapid use of water by crops, espe- 
cially during seasons of drouth and increased transpiration, re- 
