MOVEMENTS OF SOIL WATER 85 



capillary water may contain air in the intervening spaces. This 

 allows the constant work of bacteria and other soil organisms. 

 This condition is affected by tillage. If the soil be puddled by 

 plowing it when it is too wet the particles may be pressed so firmly 

 together that much air is excluded. If conditions are favorable 

 at the time, the operation of plowing or harrowing will allow more 

 air to enter. 



Hygroscopic Moisture. This may be termed fine film moisture. 

 The film about the soil grains is so thin that it does not move by 

 capillary action from one part to ariother, and from the grains to 

 the root surface. If a sample of air-dry soil which has the appear- 

 ance of dust be heated in a test tube over a flame, moisture will 

 be seen to collect above the dust on the sides of the tube. This 

 water had existed in the form of hygroscopic moisture on the soil 

 grains. It is thus seen that water in such fine films can be caused 

 to move from place to place through the action of heat. Air in 

 the soil may become saturated with such moisture when the soil 

 is very warm. At night, as the surface cools, this soil air may 

 deposit some of its moisture upon the cold grains near the surface. 



Hygroscopic moisture is of value to the plants by keeping the 

 plant food in solution during dry weather. It helps to maintain 

 the life of plants in arid regions, and during dry seasons in humid 

 regions. If the amount of soil moisture be reduced to this condi- 

 tion the plants cannot thrive. Its presence in the soil will cause the 

 soil to take up water more readily when rains occur. A soil which 

 is absolutely dry will not take up water quickly. As soon as each 

 particle has a thin film about it this may be increased through 

 capillary action as soon as water is supplied. 



Movements of Soil Water. Free water moves downward 

 through soils by the force of gravity. For this reason it is some- 

 times called gravitational water. The process may be called 

 percolation. When such water carries with it certain plant food, 

 or other ingredients in solution, these substances are said to be 

 carried away through the pores by ' 'leaching. " 



It is evident, from what has already been said, that capillary 

 water moves by capillary action. This may be in any direction 

 in the soil. It is natural, however, to think of it as being chiefly 

 upward or laterally. But it may be downward also, as when a 

 light rain occurs upon a rather dry field, the water near or at the 

 surface is distributed among the soil grains below, through the 

 capillary process. 



