ON THE ACTION OF MANURES. 
439 
no effect in increasing the corn crop ; whilst, on the other hand, 
by manuring with the same phosphates a turnip field, a much 
larger crop of roots was produced than on a field unmanured. 
In manuring with salts of ammonia the result was reversed ; 
the corn crop being increased by them, but no perceptible 
effect being produced on turnips. 
A so-called practical doctrine was derived from these facts ; 
they proved, it was said, that nitrogen (ammonia, nitric acid) 
was an especially efficacious ingredient in manure for corn, and 
phosphoric acid equally so for turnips. 
The specific efficacy of ammoniacal salts on corn was further 
shown by the fact, that a field manured for several years with 
them alone produced equally large crops of corn during that 
time, and likewise, in a very striking manner, ceased on the 
third year to yield a crop of turnips. The great efficacy of 
super-phosphate of lime on turnips was shown by the other 
fact, that a field which, unmanured, gave no more turnips on 
the third year, produced nine crops, one after another, when 
this manure only was applied. 
Now, a very slight reflection is sufficient, I think, to show 
us that this mode of testing the efficacy of the several nutri- 
tious elements of plants is idle and useless ; for if it be once 
proved, as an empirical lav/, that ammonia, phosphoric acid, 
potash, lime, &c., are nutritious elements, and as such indis- 
pensable for all plants, there can be no further doubt of their 
efficacy in any single case, and no additional proof of their 
utility or value will therefore be needed. If, by manuring a 
corn field with salts of ammonia, or a turnip field with super- 
phosphate, the crops of corn or of turnip-roots be increased, 
the fact is not in the least wanted to prove the efficacy of those 
manures, which is already known and undisputed ; nor if, by 
manuring a field with potash or lime, &c., no increase of crop 
be observed, does it therefore follow that these substances are 
not in themselves efficacious. 
It is easy to understand that phosphoric acid, or ammonia, 
may not, either alone or together, exert the slightest influence 
on the growth of a plant. Supposing phosphoric acid, potash, 
lime, magnesia, silicic acid, &c., to be indispensable conditions 
for the efficacy of ammonia, and supposing ammonia, potash, 
lime, silicic acid, to be indispensable conditions for the efficacy 
of phosphoric acid, it follows that if a wheat field manured with 
ammonia alone gives a higher return than it does without it, 
all that is proved is that this field contained an excess of 
phosphoric acid, potash, lime, &c., inefficacious because a 
certain proportion of ammonia was wanting’, but rendered 
efficacious by increasing the quantity of ammonia in the soil ; 
while without this excess of available but not efficacious 
