No. 7. DEPARTMENT OF AGRICULTURE. 643 



of the water that takes place in th(i soil when it is less than half 

 saturated and beyond the influence of a ground water level. Per- 

 haps the following illustration will be helpful: If a broad rubber 

 band is slipped OA^er a marble and pulled with a gentle pressure, 

 the marble will represent the soil grain and the rubber band the 

 film of moisture surrounding it when the soil is half-saturated. 

 Stretch the rubber band to its fullest limit; its thickness is dimin- 

 ished, its tension increased; stretching the rubber band does not 

 change the volume, but it vastly changes the tension. When the 

 rubber band is thickest it has the least grip on the marble; as it 

 becomes thinner by stretching, its tension or grip on the marble is 

 increased. Although controlled by a ditferent law, yet m a simihir 

 way the water adheres to the soil grains with the least force when 

 the film is thickest and the surface exposed to the surrounding air 

 is least, and with greatest force when the film is thinnest and the sur- 

 face exposed to the air is greatest. As the film becomes thinner its 

 strain or tension is increased and it is this strain of force that moves 

 the water from the point in the soil where the films are thickest, to 

 the point in the soil where these are thinnest, till the differences are 

 adjusted. 



Let us imagine three spheres representing three soil particles 

 surrounded by thick, medium and thin films of water. The greater 

 tension or strain of the thin film pulls from the medium, and the 

 medium pulls from the thick one till the total amount of water is 

 equally divided between all three soil particles. This movement of 

 water when the soil is half saturated or less can be in any direction 

 — up, down or laterally, within certain limits. The thick films (half 

 saturated) are more elastic than the thin ones (quarter saturated) 

 and will move with greater freedom, i. e., the movement from a soil 

 twenty-five per cent, moist into an adjoining soil twenty per cent, 

 moist will be more free and rapid than when the differences are 

 twenty and fifteen per cent. When the supply of water in the soil 

 is reduced to quarter saturation its movement practically ceases. 

 In a soil capable of retaining three inches of capillary water in each 

 foot of depth, it is the third inch that sets in motion all the energies 

 and activities in the soil that feed and force plant growth. When 

 the water content is reduced to two inches per foot plants may live, 

 but vigorous, profitable growth is checked. 



HOW THE RAIN GETS IN. 



When the rain falls on a dry soil that has been pulverized to a 

 fine state of division the tendency is to fully saturate as it goes. It 

 descends like the rain-drops on the window-pane, leaving a film 

 spread over all the soil particles it passes, but with a bead at the 

 end. Its motion is slower because it has more friction to overcome. 

 If the rainfall is excessive, there will be a tendency to puddle in 

 fine textured soils and make a mud blanket that causes some of the 

 rain water to flow off on the surface because the soil cannot take it 

 in fast enough. The mud blanket also imprisons the air under- 

 neath and this captured air resists the further descent of the water. 

 But when the falling rain comes in contact with moist soil, new 

 forces are in operation. The water films in the moist soil that are the 

 first to feel the contact with the rain water become thickened. Those 

 directly under having thinner films and greater tension commence 



