PLANT NUTRITION. 19 
which. barley has been grown for thirty years in succes- 
sion, a mineral manure with nitrogen has been applied ; 
-but in the one case lime has been added, in the other 
‘(otherwise treated exactly in the same way) no lime 
‘has been added. The plants on the plot without the 
lime are always of a darker green color, but they are rela- 
tively deficient in carbon. Under equal conditions, it is 
seen that the amount of carbon assimilated from the at- 
mosphere in the manner to be hereafter mentioned is di- 
rectly dependent on the amount of available nitrogen, 
which latter is derived from the soil—(Lawes and Gil- 
bert). 
It must not be forgotten that the substances we have 
mentioned, as well as others not alluded to, though pos- 
‘sibly not directly concerned in the nutrition of the plant, 
yet are so indirectly by causing changes in the soil, by 
rendering some matters soluble and capable of osmotic ab- 
sorption which would not be so without their aid, by 
storing up and preventing the waste of ingredients useful 
as plant food, and so forth ; but these matters pertain 
rather to the physical and chemical history of the soil, on 
which account they may be passed over here without 
‘further mention. 
Principles of Manuring.—The few remarks we have 
thought it right to make as to the nature of the substances 
absorbed with the water from the soil have an impor- 
tant bearing on the theory and practice of manuring. 
The nitrogenous and the saline substances are taken 
from the soil, used up in the plant, and removed in the 
crop. The annual produce of hay on unmanured land at 
Rothamsted, has been found to be about two thousand 
five hundred and seventy-six pounds per acre, over an 
average of twenty-five years, the range of variation ac- 
cording to season having been from nine hundred to four 
thousand three hundred and sixty-eight pounds. Qn the 
