CnAPTEE V. 

 SOIL MOISTURE. 



151. Occurrence of Moisture in the Soil. For purposes of 

 discussing the cultural relations of soil moisture water may 

 be said to occur in the soil under three conditions: 



(1) That which fills the pore spaces between the soil 

 grains and is free to move under gravitational or hydro- 

 static pressure and may be called gravitational or hydro- 

 static water. 



(2) That which adheres to the surfaces of soil grains 

 and to the roots of plants in films thick enough to allow 

 surface tension to move it slowly from place to place, and 

 which may be called capillary water. 



(3) That still retained on the surfaces of soil grains 

 when they become air-dry; whose chief movements are 

 those of evaporation and condensation and which has been 

 designated hygroscopic moisture. 



152. Gravitational Water When water in a soil in- 

 creases in quantity sufficiently to move readily under the 

 pull of gravity it may be harmful in three ways: (1) by 

 washing out the soluble plant foods, thus leaving the soil 

 poor; (2) by excluding the air and thus causing suffocation 

 of the roots of plants and micro-organisms living in the 

 soil ; ( 3 ) by preventing surface tension and by dissolving 

 cementing materials, thus destroying or reducing the gran- 

 ulation of soils, injuring their texture. It may be helpful 

 in two ways: (1) by replenishing the capillary moisture 

 when this has become too small to enable crops to supply 

 themselves, and (2) by washing out and carrying away sol- 

 uble substances which, if allowed to accumulate, become in- 



