206 PRACTICAL AGRICULTURE. 



is more fully supplied with water than the soil below. 

 A part of the water will slowly find its way down- 

 ward, in order to equalize itself throughout. When 

 it has attained something^ like an equilibrium, its tend- 

 ency would be to remain nearly stationary, if there were 

 a damp atmosphere and no sun. But if the sun shine, 

 the air in contact with the soil becomes heated ; it takes 

 moisture from the soil ; the surface becomes dry, and 

 the water below moves upward. 



RELATIONS OF SOIL TO THE ATMOSPHERE. 



392. Soils not only require, in order to be produc- 

 tive, that thfe air should permeate them, but they have 

 the power of absorbing from the air various gases, 

 and of retaining them for the use of plants. Among 

 these gases are oxygen and nitrogen, the principal 

 constituents of the atmosphere ; also, ammonia, car- 

 bonic acid, and various other gases, which are per- 

 manently or incidentally floating in the atmosphere. 



893. Peaty soils have this power of absorbing nu- 

 tritious gases from the air in the highest degree. 

 Hence while peat, or swamp nud, is in process of cuV' 

 ing^ before being used in composts, it is continually 

 growing better, not only by losing its coldness and 

 sourness, while exposed to sun, air, and rain, but by 

 the absorption of nutritious gases from the air. 



394. Clay, next after peat, possesses this power in 

 a high degree. Loams possess it in a greater or less 



