SOIL PHYSICS 461 



AIR. The upper portion of soil, down to the water table, is 

 saturated with air. The spaces between the soil particles are 

 completely filled with this GROUND-AIR, or SOIL-AIR, as it is 

 often called. Each soil particle above the water table is sur- 

 rounded by a thin film of water, FILM WATER, which can be 

 removed only by the roots of plants and by evaporation 

 (Art. 13). Since this moisture can be removed by evapora- 

 tion, the soil-air will constantly be nearly saturated with mois- 

 ture. Plant life is largely dependent upon this moist soil-air 

 and upon the film water for its very existence (see Art. 547). 

 The chief purpose of artificial drainage is so to lower the water 

 table that the soil-air may reach the plant roots. 



539. Movements of Soil-air. The soil- water moves through 

 the soil chiefly through the influence of its own weight ; we say 

 it is moved by gravity. It is constantly moving down hill to- 

 ward the sea level or toward the level of some lake or river. 

 The movements of soil-air are strikingly different. Soil-air 

 does not move, to any considerable extent, in response to the 

 pull of gravity. Its motion is controlled almost entirely by: 

 (1) Changes in the temperature of the soil, (2) changes in at- 

 mospheric pressure (see Chap. IV, Sec. II), and (3) by diffusion. 



540. How Heat Causes Movements of Soil-air. We know 

 that when air is heated it expands. In fact, we have learned 

 that when air is heated 1C. it expands M?3 part of its 

 volume at 0C. (Art. 131, Ex. 35); if it is heated 10 it will 

 expand l %is of its volume. Now, if the soil temperature 

 should rise 5C., it is evident that 1 cu. ft. of air out of every 55 

 cu. ft. would be forced out of the soil, i.e., the earth would ex- 

 hale, or breathe out, 1 cu. ft. to every 2 cu. yd., of soil-air which 

 it contained. Prove that this is so. 



The earth receives its supply of energy from the sun. The 

 sun sends down a greater amount of energy at noon than at any 

 other hour of the day. Why is this so (Chap. V, Sec. I)? 

 But if we were to take the temperature each hour of the day 

 for a few days with the thermometer hanging on the north 

 side of a tree or a building, we should find that the atmosphere 



