190 Causes of Fertility or Barrenness of Soils. 
but in such a condition as to become slowly available for vege- 
tation ; limestone soils, at least the poorer varieties, are deficient 
in these substances, consequently their frequent application in 
small quantities is desirable ; they should be applied as a top 
dressing in spring, so that the roots may take them up before 
they pass away into the subsoil ; guano and all other animal 
manures owe a portion of tlieir effect to the alkalies present in 
them. The power ol substitution supposed to exist in certain 
plants, has not been very clearly proved ; it is said that sea-side 
plants which contain a large percentage of soda when grown 
inland, and upon soils rich in potash, have the power of absorbing 
the latter, and vice versa, but further experiments are required 
to determine this interesting question, which is of great practical 
importance, inasmuch as soda in the form of chloride exists 
much more abundantly in nature than any salt of polassa, and 
could therefore be applied much more economically. 
Chlorine is the last mineral constituent of soils we shall men- 
tion. It occurs in most plants, and, where deficient in the soil, 
should be added in the form of common salt. The application of 
this substance has much increased of late years, and the successful 
results obtained would warrant our believing, that besides its 
food value, common salt may possess some peculiar chemical 
power in assisting to make other matters available as food for 
plants. 
The assimilation of the above mineral matters by the vege- 
table kingdom appears to depend upon the presence in suit- 
able proportions of the organic elements, carbon, hydrogen, 
oxygen, and nitrogen. The three first are very abundant, and 
can be obtained both by the roots and leaves of plants ; the 
nitrogen occurs more sparingly, and fertility appears to depend 
mainly upon its presence in the soil. Small quantities existing 
in the atmosphere as ammonia and nitric acid are carried down 
into the soil with every shower, or absorbed by the surface soil. 
This property of absorption is enjoyed in a different degree by 
different soils. The experiments of Professor Way, though not 
yet completed, have gone further to throw light on this important 
subject and to determine the real causes of barrenness and fertility 
of soils, than the investigations of all his coteniporaries or pre- 
decessors. Not content with ascertaining that certain soils pos- 
sessed a power of absorbing and retaining ammonia, of which 
others were almost destitute, alone a very important discovery, 
he has most successfully demonstrated that this property is owing 
to the presence of a class of mineral compounds, termed double 
silicates, that is, silicate of alumina united with silicate of some 
other base, such as lime, soda, potash, or magnesia. The am- 
monia, possessing the power of displacing any of the above, takes 
