350 
composition ; the difference being the greatest between plants which are 
not nearly related to each other. When a plant grows in a healthy con¬ 
dition and matures seeds, it has been found that the nature and pro¬ 
portions of its mineral matters are nearly alike, whatever may be the 
character of the soil on which it was grown. If, then, two plants of 
different genera are grown in a soil of known composition, and the ana¬ 
lysis of their ashes proves that one had taken from the soil some sub¬ 
stance in much greater proportion than the other, and that this difference 
in the composition of the ash invariably occurs ; the conclusion to be 
arrived at is, that these substances in these relative proportions exercise 
some important function in the economy of the plant, and that they are 
essential to its healthy development. 
It is evident, that plants must have the power of selecting from the 
inorganic matter of the soil those materials which they specially require; 
if, therefore, one plant, the same kind of crop is frequently repeated on 
the same soil without any thing being added to it in the shape of manure, 
that the soil must ^>oner or later, according to circumstances, become 
exhausted of one or more substances required by that particular crop, 
and the plants will, in consequence, gradually decline in vigor and their 
produce diminish. The effects of growing plants in the same soil a 
number of years in succession, and of changing the soil each year, has 
been put to the test of experiment by Professor Daubeny, who obtained 
the following results": 
Average of five years. 
Fotatoes _In tbe same plot. 72.9 lbs. of tubers. 
In different plots. 92.8 lbs. of tubers. 
Flax .Same. 15.0 lbs. 
Different. 19.9 lbs. 
Deans 
Barley 
.Same. 32.8 lbs. 
Different. 34.8 lbs. 
_Same. 30.0 lbs. 
Different. 46.5 lbs. 
'Turnips.. .. _Same... 101.0 lbs. 
Different. 173.0 lbs. 
Oats .Same. 28.0 
Different... 32.0 
/ 
