474 
Report of Experiments on the Groieth of Wheat. 
of lime alone in each of the 12, and in the 4 preceding years, and 
previously with Peruvian guano or the mixture of the surplus of 
the other manures. Under these conditions the average annual 
produce during the last 12 years was 184; bushels of dressed corn, 
and 1846 lbs., equal about 1GJ cwts. of straw; equivalent to an 
average annual increase over the unmanured plot of only 2'^ 
bushels of corn, and 184 lbs. of straw, and to a deficiency com- 
pared with the produce by farmyard manure of about 17 bushels 
of corn, and 18 cwts. of straw. Since a portion of the small 
increase was, doubtless, due to the unexhausted residue of previous 
nitrogenous manuring, it is obvious that but little remains to be 
attributed to the subsequent annual supply of superphosphate of 
lime. It is obvious, too, that if it were in mineral constituents 
that the soil had become relatively deficient by the previous 
cropping, it was not for want of phosphoric acid alone that the 
plot yielded so little more produce than was obtained without 
manure, and so much less than was obtained with farmyard 
manure. 
Nor was it in potass, soda, magnesia, and sulphuric acid alone, 
that the soil had become relatively exhausted ; for on plot 1 these 
constituents were supplied liberally every year for 15 consecutive 
years, and the average annual produce and increase over the 12 
years were even less than on plot 0 with superphosphate of lime. 
The increase over the unmanured produce was, in fact, not quite 
1 bushel of dressed corn, and not quite 1 cwt. of straw, per acre 
per annum. 
Plot 5, was manured with a mixture of both superphosphate of 
lime, and the sulphates of potass, soda, and magnesia, supplying 
to the soil much more of probably every mineral constituent, except 
silica, than was taken off in the crops. But this mixture gave 
scarcely any more increase than the superphosphate of lime 
alone, and part of that which it did give has been shown to be 
most probably due to the unexhausted residue of previous nitro- 
genous manuring. 
It was not, therefore, for want of mineral constituents, unless 
of available silica, that the soil had become, by the previous 
cropping, incapable of producing full wheat crops. The defective 
result, as compared with that by farmyard manure, was obviously 
due to the want of some constituent which was supplied by it, 
but not in either of the artificial mineral manures. The questions 
arise — was the wanting constituent available silica? — was it 
organic matter yielding carbon to the plant ? — or was it nitrogen 
in some available form of combination? These questions will 
be pretty satisfactorily answered by the results considered in the 
next Section. 
