MOVEMENTS OF THE SOIL AIR 439 



with certain bases to form compounds beneficial to the 

 soil. Particularly is this the case with calcium carbo- 

 nate, which is of the greatest benefit to the soil in main- 

 taining a slight alkalinity very favorable to the develop- 

 ment of beneficial bacteria and to the maintenance 

 (if good tilth. 



When combined as sodium or potassium carbonate 

 in considerable quantity, as in certain alkali soils, a very 

 injurious action upon plant-roots, and upon soil-struc- 

 ture results. Upon plants it acts as a direct poison. 

 (See page 312.) The effect upon soil structure is to de- 

 flocculate the particles producing the separate grain or 

 compact arrangement. (See page 116.) 



IV. MOVEMENT OF SOIL AIR 



There is a constant movement of the air in the inter- 

 stitial spaces of the soil, and an exchange of gases between 

 the soil atmosphere and the outside atmosphere, as well 

 as a more general but probably less effective, movement 

 of the air out of. or into the soil, as the controlling- 

 conditions may determine. 



The movement may be produced by any one or more 

 of the following phenomena: (1) Gaseous diffusion. 

 (2) Movement of water. (3) Change of atmospheric 

 pressure. (-1) Change of temperature in soil or atmos- 

 phere. (5) Suction produced by wind. 



305. Diffusion of gases. The wide difference in the 

 composition of soil ami atmospheric air gives rise to a 

 movement of gases due to a tendency for the external 

 and internal gases to come into equilibrium. According 



