lis AiSTNUAL REPORT OF THE Off. Doc. 



at all points. In other words, when the soil is less than half sat- 

 urated, the moA'ement of the film water may be in any direction, up, 

 down, or laterally, but always from the point in the soil where the 

 films are thickest to the points where it is thinnest. This move- 

 ment is most free and rapid when the soil is nearest half saturation, 

 or in the averai2;e soil when it contains three inches of water to one 

 foot in depth of soil, and the movement practically ceases when the 

 water content has been reduced to quarter saturation, or one and 

 one-half inches per foot of depth, so that while there may be three 

 inches of water left in every foot depth of soil after drainage has 

 carried off the surplus of the shower, if any, there is never more 

 than one and one-half inches of water available to stimulate plant 

 growth. This would equal about two gallons of water to each 

 cubic foot of soil. 



"a." "'b," 



Fig. 4. 



The three spheres at ''a" in Figure 4 represent three soil particles, 

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

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

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

 equally distributed between all three soil particles, as shown at 

 •b" in the same diagram. 



HOW THE RAIN GETS IN. 



When the supply of water in the soil is reduced to quarter sat- 

 uration, its movement practically ceases. In a soil capable of retain- 

 ing 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 one and one-half inches per foot, plants may live, 

 but vigorous, profitable growth is checked. 



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 over- 

 come. If the rainfall is excessive, there will be a tendency to 

 puddle in fine texture 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 underneath, 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 operatiou. The water films in the moist 

 soil that are the first to feel the contact with the rainwater become 

 thickened. Those directly under having thinner films and greater 

 tension, commence pulling from or robbing their thicker skinned 



