DRYLAND FARMING. 341 



vary greatly with the nature of the soil. Broadly speaking, 

 sandy soils absorb the smallest amount, while clayey soils, 

 and those containing much humus or finely divided ferric 

 hydrate, take up the largest amounts. The quantity absorbed 

 is almost entirely a function of the total surface exposed, but 

 in no case is it sufficient to make the soil visibly moist. 



The percentage of moisture representing the full 

 condensation of water upon soil from saturated air, under 

 given conditions of temperature, is known as the Hygroscopic 

 Coefficient. 



The hygroscopicity of soils is important in plant growth, 

 because soils of high lijgroscopic power can withdraw from 

 the moist air enough moisture to be indirectly of material help 

 in sustaining the life of vegetation during droughts, in that 

 high moisture absorption prevents rapid and undue heating of 

 the surface soil to the danger point, and thus may save crops 

 that would be lost in soils of low hygroscopic power. Except, 

 perhaps, in the case of some desert plants, hygroscopic mois- 

 ture cannot maintain normal growth. 



Capillary Water. — This form of water is held in the soil 

 by capillary forces, and exists in the form of surface films 

 around the particles. It differs from hygroscopic moisture in 

 that it evaporates at ordinary temperatures, is not condensed 

 again on the soil particles, and may move from one particle 

 to another. 



Normally, it not only serves as the vehicle of all plant 

 food absorbed from the soil during the growth of the crops, 

 but also sustains the enormous evaporation by which the plant 

 maintains, during the heat of the day, a temperature suffi- 

 ciently low to permit of the proper operation of the various 

 growth processes. 



Working with fine glass tubes, it is found that the height 

 to which water rises by capillarity is inversely proportional to 

 the diameter of the tube. The rise of water by capillary 

 action, or the " capillary pull," in soils is somewhat analogous 

 to this, being greatest in the finest textured soils. Verj' fine 

 texture, however, offers considerable resistance to the move- 

 ment of water, so that the rate of movement is slower than 

 where the texture is coarser. Speaking generally, therefore, 

 the capillary pull is strong in clays and weak in sands, but the 

 rate of water movement in sands is far more rapid than in 

 clays. Granulation of clay will increase the rate of move- 

 ment, and compacting sands will strengthen the capillary 

 pull. 



