1 6 PRINCIPLES OF FARM PRACTICE 



strated by filling two tin cans of the same size, having perfo- 

 rated bottoms, one with dry sand, the other with dry, clay 

 soil. After weighing, each can is saturated with water, and 

 then allowed to drain. The water held in each can after 

 draining will be mostly film water. When weighed again, 

 the amount of film water in each may be determined by 

 subtracting the first weight from the second. It will be found 

 that the clay or fine soil has held more film water than the 

 coarse soil. Sandy soil has a low water-holding capacity 

 compared with clay or clay loam; its capacity may be in- 

 creased by adding organic matter, such as manure, which 

 retains a great deal of water. 



Clay and clay loam, because of the fineness of their parti- 

 cles, retain relatively a large amount of film water, but they 

 have a tendency to bake and become hard and cloddy after 

 a rain. This tendency makes such soils difficult to handle. 

 They are sometimes called heavy soils, not because they are 

 really heavy but because they are hard to work. This is a 

 serious difficulty, but one that may be overcome, in a large 

 measure, by modifying the soil structure in such a way as 

 to make it retain its capacity to hold water and, at the same 

 time, make it more easily handled. 



How water moves downward in the soil. When rain falls 

 or snow melts the water has a tendency to move downward 

 or percolate through the soil. This movement proceeds until 

 the level of the free water below, known as the water table, 

 is reached. As the free water accumulates, the water table 

 rises until finally the upper surface of the soil is reached. The 

 soil is then saturated, all the spaces being filled with water. 

 When more rain falls, the water either runs off or stands in 

 puddles. This condition frequently occurs in early spring 

 after the spring rains or melting of the winter snows. As 



