EXTENSION COURSE IN SOILS. 21 



solid soil particles and the moisture film surrounding these particles. 

 It is the space in well-drained soils which is open to the circulation 

 of air and other gases and to the growth of plant roots. The total 

 pore space in any soil depends less upon the size of soil particles than 

 upon the arrangement of these particles. From the standpoint of 

 pore space the granules in soil are similar to single soil grains. The 

 pore space in sandy soils under ordinary field conditions is about 40 

 per cent of the total. In clay loams the granulation is commonly 

 such that 55 per cent of the total volume of the soil is pore space, 

 only 35 per cent being occupied by solid matter; while in fertile 

 heavy clays granulation may be present to such an extent that 65 

 per cent of the total volume is pore space. Ample pore space in 

 soil to a depth of 4 feet or more is very essential to a thorough distri- 

 bution of plant roots, and a free circulation of air in the soil is indis- 

 pensable to the growth of farm plants. 



Circulation of air in the soil. — Some of the principal causes of the 

 circulation of air in soils are: (1) Water movements in the soil. Any 

 movement of water through the soil has an effect upon the circulation 

 of the soil air. A good example of this is seen in underground 

 drainage. Following rains, or accompanying irrigation in arid lands, 

 as the water passes downward through the soil into the drains, the 

 atmospheric pressure forces the air into the pore spaces opened by 

 the water passing out. (2) Changes in barometric pressure. Varia- 

 tions in the pressure of the atmosphere, indicated by the barometer, 

 produce currents of air, or winds, which pass over the earth's surface. 

 These causes of surface movements of air also affect subsurface 

 movements of soil air, but to a lesser degree. (3) Changes in tem- 

 perature, due to day and night. After sunset the atmosphere cools 

 more rapidly than the earth's surface. The warmer air of the soil, 

 being lighter, moves upward through the pore spaces and into the 

 atmosphere, while an equal volume of cooler air above the surface 

 moves downward into the soil to take its place. (4) Diffusion. 

 It is a physical law that when two gases are in contact they always 

 mix, or diffuse. Carbon dioxid given off in the soil from the roots 

 of plants and from vegetable decay, together with other soil gases, 

 gradually diffuses in the soil air and thereby helps to produce a 

 certain kind of circulation. 



Among the causes influencing pore space and soil-air circulation 

 those most under control in soil management are drainage and 

 granulation. If soils are filled with water there can be no circulation 

 of air therein. On the other hand, if clay soils are so managed as to 

 become puddled and baked, the lack of pore space and "granulation 

 will result in poor circulation of air through them and thus prevent 

 the successful growth of plants. 



