SOIL WATER 87 



the plant food dissolved in this water, is likely to 

 be largest. 



The loss of soil water by seepage can be prevented, 

 in part, by judicious farm practice. If a leachy 

 soil is filled with humus, either from manure or 

 from decaying vegetation, the large spaces between 

 the grains are clogged and water sinks through the 

 soil less rapidly. An open, porous soil may also 

 be compacted by rolling, which reduces the size 

 of the spaces by crushing the grains together. 

 Liming a sandy soil may have a slight effect in the 

 same direction. If the soil is not left bare during 

 the winter when a crop is not growing upon it, but is 

 kept covered with a catch crop, as rye, the roots and 

 herbage of this crop hold much of the water that 

 otherwise would be lost. These operations are 

 discussed at length in succeeding chapters. 



THE MOVEMENT OF FILM WATER 



In Chapter II it was stated that by far the most 

 important kind of water in the soil is that which 

 surrounds the soil grains like a film; because it is 

 this, not free water, which the roots of plants use. 

 This water is held to the surface of the soil grains by 

 tension or adhesion, as a film of water adheres to 

 a pebble dipped into the brook. There is also 

 more or less water in the spaces between the grains. 

 These films of water are not all of the same thick- 

 ness. Some grains have more water on them than 

 others; therefore parts of the soil are dryer than 

 others. The dryness of some parts of the soil may 

 be due to the fact that they have received less water 

 from rainfall. It may also be caused by the roots 

 of thirsty plants. 



The movement of film water takes place in this 



