RELATION OF THE PLANT TO THE SOIL 123 _ 
The experiments in water culture show that certain sub-— 
stances, enumerated above, are necessary for the life of the 
plant ; if any are omitted the plant will not thrive. Without 
iron, for instance, the plant will not be green. Many plants, - 
however, take in other substances, for instance, silica, which 
forms a very large part of the composition of wheat. This is 
the reason that farmers plant a succession of crops in the 
same field. In Norfolk, turnips are followed by barley, barley 
by clover, and that by wheat. Clover requires a great deal 
of potash, wheat very little, so that when clover has ex- 
hausted the soil of potash, it is only wise to put in a plant 
like wheat, which does not require much. Clover, again, 
requires a great deal of lime, wheat hardly any. Farmers 
who wish to get as much as possible out of their land get 
chemists to analyse the plant they wish to grow, and also the 
soil of the field in which they propose to grow it, and if the 
soil is found to be deficient in any substance necessary for 
that particular plant, they get manure which is specially made 
to supply the deficiencies of the soil, and to provide the plant 
with the food it requires. During the last twenty or thirty 
years, landin Barbados has been made to yield crops of sugar- 
cane nearly half as large again as it used to do before chemical 
manure was applied to it. 
Absorption of | Nitrogen is taken in from the soi in the form 
Nitrogen. of nitrates (compounds of nitrogen), and all 
higher plants must take in nitrogen in this way, for they _ 
cannot absorb the free nitrogen of the atmosphere. Legu- - 
minous plants, however, appear to get their nitrogen indirectly 
from the free nitrogen of the air. The facts in support of 
this statement are: (1) These plants will flourish in soil from 
which all traces of nitrogenous compounds have been removed ; 
(2) it is found that soil in which leguminous plants have been 
grown is richer in nitrates at the end of a crop than it was 
before. It follows, therefore, that somehow or other these 
plants must get free nitrogen from the atmosphere—how is 
not yet positively known. 
These plants have little swellings on the roots, which are 
