440 
POPULAR SCIENCE REVIEW. 
elements, tlie largest manuring with ammonia would have had 
no effect. In like manner it is perfectly plain, that if a field be 
deficient in potash, of which turnip roots want ten times as much 
as they do of phosphates, supposing the available quantity of 
potash be only sufficient for the formation of three crops, then 
only three would have been obtained, and the largest dressing 
with superphosphate could not produce nine. 
Experiments of this kind indicate only what elements of 
food are abundant or wanting in the soil, and it is impos- 
sible to prove by them anything about the efficacy of a manure 
element. The increase or non-increase establishes only the 
quality or nature of a field, the knowledge of which is un- 
doubtedly very useful to the owner of the land, but to him 
alone ; for it is of no avail to his next neighbour, if the 
quality of this neighbour's land happens to be different. 
Numberless experiments of the same character have been 
repeatedly made in Germany at the instance of several agricul- 
tural societies. On many corn fields manuring with ammonia 
salts had not the slightest effect, while super-phosphate of 
lime alone produced much larger corn crops than any other 
manure had afforded. It is really astonishing that farmers, 
who call themselves men of experience, can be made to believe 
that because a manure has produced on a field in a certain 
country a high return of corn or roots, it should produce an 
equal effect, and possess an equal value, on all the fields in 
Great Britain ; for, if the efficacy of a manure, A, be believed 
to depend on the presence and quantity of the manure B, C, 
D, &c., it must be assumed that all the fields in a country 
or land contain the same quantity of B, C, D, &c. Now, it is 
an indisputable fact, that there are scarcely two fields of the 
same country, often not two fields on the same farm, which 
possess the same identical geological, chemical, or mechanical 
character ; so that the quantity of the manures B, C, D, &c., 
varies in each instance. It must be plain, therefore, that the 
same quantity of manure. A, be it ammonia, phosphoric acid, 
or potash,' must necessarily have quite a different operation 
in proportion as the fields are differently constituted. Even 
stable-dung, which contains all the nutritive elements in con- 
junction, produces different effects when applied in the same 
quantity to different fields. 
With reference to practical farming, it is important to 
remark, that it follows as a corollary from the law of equality 
of nutritive value belonging to the constituents of food, that 
the element or elements, which are either wanting in the 
soil, or are contained in it in insufficient quantity, are the ones 
which will prove of preponderating value in the manures 
applied. 
