termkability of soils to liquid water. 179 



water often causes an accumulation of salts on the surface 

 of the ground. Thus in Bengal many soils which in the 

 wet season produce the most luxuriant crops, during the 

 rainless portion of the year become covered with white 

 crusts of saltpeter. The beds of nitrate of soda that are 

 found in Peru, and the carbonate of soda and other salts 

 which incrust the deserts of Utah, and often fill the air 

 with alkaline dust, have accumulated in the same manner. 

 So in our western caves the earth sheltered from rains is 

 saturated with salts — epsom-salts, Glauber's-salts, and salt- 

 peter, or mixtures of these. Often the rich soil of gardens 

 is slightly incrusted in this manner in our summer weather ; 

 but the saline matters are carried into the soil with the 

 next rain. 



It is easy to see how, in a good soil, capillarity thus 

 acts in keeping the roots of plants constantly immersed in 

 a stream of water or moisture that is now ascending, now 

 descending, but never at rest, and how the food of the 

 plant is thus made to circulate around the organs fitted 

 for absorbing it. 



The same causes that maintain this perpetual supply of 

 water and food to the plant are also efficacious in con- 

 stantly preparing new sup])lies of food. As before ex- 

 plained, the materials of the soil are always undergoing 

 decomposition, whereby the silica, lime, phosphoric acid, 

 potash, etc., of the insoluble fragments of rock, become 

 soluble in water and accessible to the plant. Water 

 charged with carbonic acid and oxygen is the chief agent 

 in these chemical changes. The more extensive and rapi<l 

 the circulation of water in the soil, the more matters will 

 be rendered soluble in a given time, and, other things be- 

 ing equal, tlie less will the soil be dependent on manures 

 to keep up its fertility. 



Capacity of Imbibition. ( apillary Power.— No mat- 

 ter how favorable the structure of the soil may be to tho 



