642 ANNUAL. REPORT OF THE Off. Doc. 



WATER IN THE SOIL. 



Bv H. II. UILTON, Port Allnjaiui, I'a- 



FULL. SATURATION. 



The soil is fully saturated when all the spaces between the soil 

 particles are filled with water. This occurs in the surface soil only 

 during heavy showers when the rain is falling as fast or faster than 

 the water can drain away underneath. Under these conditions the 

 movement of water is all in one direction — downward — and acting 

 under one force — gravity. The water that temporarily fills all the 

 spaces between the soil particles is called "gravitation" water. Its 

 rate of flow is determined by the size of the spaces. In the coarse 

 soils with large spaces the friction is least and water flows down- 

 ward with great freedom. In the fine textured soils with small 

 spaces friction is increased and the water movement is retarded. 



HALF SATURATION. 



The force of gravity is able to remove about one-half the water 

 from the fully saturated surface soil when it is met by an oppofiing 

 force that holds the other half against the exposed surfaces of the 

 soil particles. It is this half that remains in the surface soil after 

 drainage of the surplus water has practically ceased that the farmer 

 is vitally interested in. If he would exercise control over it, he must 

 know something of the laws that govern its movements in the soil. 



Every child watches with delight the first scattering raindroj)s 

 that fall against the window-pane. At first the drops trickle down 

 the glass slowly, leaving a film of moisture behind which gradually 

 reduces its volume and weight till it reaches a point where the 

 force of adhesion overcomes the force of gravity. There it resr^ 

 until another rain drop finds its trail, follows more quickly, and 

 adds weight that carries it still farther down the glass. In tJie 

 same way that water enters the soil, leaving a thin film of watei' 

 spread over all the exposed surfaces and, as it were, a water-skin 

 or jacket around every soil grain. 



CAPILLARY ATTRACTION. 



This force that opposes gravitation is generally known as capillars 

 attraction, and exerts its greatest power around the points of con- 

 tact where the soil grains rest against each other because in thest- 

 acute angles the capillary spaces are smallest. As the water drained 

 out is called "gravitation" water, so the water held back is called 

 "capillary" water. 



AN ILLUSTRATION. 



It is much easier to comprehend the capillary rise of water from a 

 free water or ground water level than to understand the movement 



