PERMEABILITY 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 supplies of food. As before ex- 
plained, the materials of the soil are always undergoing - 
decomposition, whereby the silica, lime, phosphoric acid, 
potash, ete., 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 rapid 
the circulation of water in the soil, the more matters will 
be rendered soluble in a given time, and, other things be- 
ing equal, the less will the soil be dependent on manures 
to keep up its fertility. 
Capacity of Imbibition. Capillary Power.—No mat- 
ter how favorable the structure of the soil may be to the 
