CHARACTERISTICS OF SOILS 25 



itational water is surface water before it enters the soil. 

 Surface water is that which stands upon or moves off 

 over the surface of the ground. It must very often be 

 taken into account in planning a system of drainage. 



38. Capillary water. The water that remains cling- 

 ing to the walls of the soil particles with sufficient force 

 to withstand the pull of its own weight is apillary water. 

 It is often defined as the water that remains clinging to 

 the walls of the soil particles after all gravitational water 

 has moved away. It covers the walls in very thin layers. 

 Just as the outer molecules of the water of a dew-drop 

 hold the mass in a spherical shape, so the outer molecules 

 of the capillary water covering the soil grains hold the 

 water about the grains. 



39. Hygroscopic water. The water that is found in 

 soil after it becomes air-dry is hygroscopic water. The 

 amount of hygroscopic water in a given weight of soil 

 usually depends upon three things, viz. the fineness of 

 the soil particles, the temperature of the soil, and the 

 humidity of the air in contact with the soil particles. 

 The amount of organic matter in the soil becomes a fourth 

 factor if it is not included in the first named. The humid- 

 ity of the air may be roughly defined as the readiness 

 with which it gives up its moisture. Some days the 

 pitcher and the pump-spout " sweat " ; some days they 

 do not, depending upon the humidity of the air and the 

 temperature of pitcher and spout. 



The dust that rises from the road on a hot, sunshiny 

 July day at noon, dry as it may appear, carries a measur- 

 able amount of hygroscopic moisture. 



Opinions are not agreed as to the value of hygroscopic 

 water in agriculture. It is probable that its presence 

 results only in good. 



