SOIL PHYSICS 465 



and pressure are known to be moving, on the average, at the 

 rate of about J4 mile per second. But the particles are so very 

 small and so very numerous that even at this rate of motion no 

 particle moves more than a very small portion of an inch before 

 striking another particle and being turned off in some new 

 direction. Every gas and every vapor is thus made up of mil- 

 lions of particles to each cubic inch and each of these particles is 

 moving with great speed. When we know this to be true, it' is 

 not wonderful that the vapor from the musk or oil of pepper- 

 mint quickly passes across the room, even though there be no 

 apparent movement of the air in the room. The mixing of 

 gases and vapors in this manner is known as the DIFFUSION OF 

 GASES. 



546. Diffusion of Soil-air. It has been shown that this same 

 DIFFUSION OF GASES applies to soil-air. It is known that this 

 diffusion of the soil-air causes a constant exchange to be taking 

 place between the soil-air and the air above the ground. 

 Particles of soil-air are constantly escaping into the atmosphere 

 and particles of fresh air from the atmosphere are constantly 

 entering the soil. This diffusion of the soil-air causes more of 

 an exchange of the impure, moisture-laden air for fresh air 

 from the atmosphere than all other known causes put together. 

 The whole process of change of soil-air is of great interest to 

 agriculturalists and is called the AERATION OF THE SOIL, or 



AIRING OF THE SOIL. 



The soil-air removes considerable moisture from the soil 

 above the water table. The soil-air is constantly more or less 

 saturated with moisture which it has taken up from the soil 

 particles. With each exhalation, as well as by the constant 

 process of diffusion, this moist air is being replaced by the 

 drier air from the atmosphere. The effect of soil aeration, 

 then, is to reduce materially the amount of soil moisture. 



CONSERVATION OF SOIL MOISTURE 



547. Plant Life and Moisture. All plants must be supplied 

 with moisture in order that they may live and grow. The 



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