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. 
