SOIL WATER AND SOIL AIR 17 



the water near the surface evaporates or drains off, the water 

 table is lowered leaving in the upper part of the soil only 

 capillary water, the presence of which is favorable for seed 

 germination and plant growth. 



The free water acts as a reservoir to be used later by the 

 plants in the growing season when there may be less rainfall. 

 It becomes important, therefore, to get a sufficient supply of 

 free water into the soil during the time of heavy rains. We 

 need to know in this connection something about how water 

 moves downward in the soil, in order that measures may be 

 taken to secure the greatest benefits from water that reaches 

 the soil. 



The downward movement of water through the soil is 

 caused by gravity. The rate, or rapidity of movement, 

 depends upon the size of the soil particles. When one lamp 

 chimney is filled with sand and another with clay, and water 

 poured into each, it will be noticed that the water runs through 

 the sand very quickly but through the clay very slowly. 

 The rate of percolation in heavy soils is so slow that when 

 rain falls on them, much of the water runs off instead of 

 entering. This run-off not only results in the loss of water 

 that might otherwise be stored up in the soil for future use, 

 but takes with it some of the soil, another considerable loss. 



Sandy soils need no attention in this respect, since the 

 water readily enters and rapidly sinks to lower levels. 



How water moves upward. The oil in a lamp constantly 

 moves along the wick and up to the flame. In a similar way 

 water passes through a column of soil. This may be illustrated 

 by filling a lamp chimney with sand and placing one end in a 

 glass of water. Immediately the water will begin to rise in 

 the sand, soon reaching the top. This action is known as 

 capillarity. The soil particles or grains of sand nearest the 



