64 THE ESSENTIALS OF AGRICULTURE 



68. The supply of soil water. The supply of water in the 

 soil is the most important of the factors controlling the yield of 

 crops. Water is not only the chief food of plants but it is the 

 carrier of other plant foods which come from the soil. The 

 proper control of soil water, therefore, is one of the most impor- 

 tant considerations in profitable soil management. It is neces- 

 sary to have a soil in such condition that the excess water will 

 readily drain away, while the soil retains the amount necessary 

 to the proper growth of crops. 



The pore space in the soil is from 30 per cent to 60 per cent 

 of the volume. About 50 per cent of the volume of loam soil, 

 if in good physical condition, is pore space. When rain falls, 

 the water penetrates first into the large openings, such as cracks 

 and wormholes, then into the finer openings between the par- 

 ticles. When these openings are filled with water, the soil is 

 said to be saturated. The air is thus forced out of the soil, and 

 until a large part of this water drains into the subsoil, so that 

 the air can again enter the soil openings, agricultural plants will 

 not grow. Plants on a saturated soil soon turn yellow, and if 

 the water remains long enough, they will die. 



After heavy rains in the spring, water will often rise in a 

 post hole to a point only a few inches below the surface. This 

 shows that the soil is saturated with water almost to its surface. 

 The upper limit of this standing water in the soil is known as 

 the water table, and it is only when this water table sinks to a 

 depth of three or more feet below the surface that the soil is in 

 the best condition for crops. 



69. How water is retained in the soil. After the standing 

 water drains downward in the soil, considerable water is still 

 held in the fine openings asjilm, or capillary, water. If a marble 

 is dipped in water and then removed, it will be covered with a 

 film of water. This represents the manner in which film water 

 is held around the particles of soil. If one end of a very small 

 glass tube is dipped in water, the water will rise to a considera- 

 ble height in the tube. This is what is known as the capillary rise 

 of water, and the finer the opening, the higher the water will rise. 



