24 SOILS AND FERTILIZERS 



at a general level, and fills all the spaces between the 

 soil particles. Its distance from the surface can be 

 told in a general way by the depth of surface wells. 

 Bottom water is of service to growing crops when it is 

 at such a depth that it can be brought to the plant 

 roots by capillarity, but when too near the surface so 

 that the roots are immersed, very poor conditions for 

 crop growth exist. When the bottom water can be 

 brought within reach of the roots by capillarity, a crop 

 has an almost inexhaustible supply. In many soils 

 known as old lake bottoms, such conditions exist. 



Fig. ii. Water films surrounding soil particles. 



20. Capillary Water. The water held in the 

 minute spaces above the bottom water is known as the 

 capillary water. The capillary spaces of the soil are 

 the small spaces between the soil particles in which 

 water is held by surface-tension ; that is, the force 

 acting between the soil and the water is greater than 

 the force of gravity. If a series of glass tubes of dif- 

 ferent diameters be placed in water it will be observed 

 that in the smaller tubes water rises much higher than 

 in the larger. The water rises in all of the tubes 

 until a point is reached where the force of gravity is 

 equal to the force of surface-tension. In the smaller 

 tubes surface-tension is greater than the force of 



