146 DRY-FARMING 
exists in any cultivated soil. In fact, the soil-water 
film may be viewed as being in a state of trembling 
activity, tending to place itself in full equilibrium 
with the surrounding contending forces which, them- 
selves, constantly change. Were it not that the 
water film held closely around the soil particles is 
possessed of extreme mobility, it would not be 
possible to meet the demands of the plants upon 
the water at comparatively great distances. Even 
as it is, it frequently happens that when crops are 
planted too thickly on dry-farms, the soil-moisture 
cannot move quickly enough to the absorbing 
roots to maintain plant growth, and crop failure 
results. Incidentally, this points to planting that 
shall be proportional to the moisture contained by 
the soil. See Chapter XI. 
As the temperature rises in spring, with a decrease 
in the relative humidity, and an increase in direct 
sunshine, evaporation from the soil surface increases 
greatly. However, as the topsoil becomes drier, 
that is, as the water film becomes thinner, there is 
an attempt at readjustment, and water moves up- 
ward to take the place of that lost by evaporation. 
As this continues throughout the season, the moisture 
stored eight or ten feet or more below the surface 
is gradually brought to the top and evaporated, and 
thus lost to plant use. 
