534 
Value of Artificial Manures. 
refer them to it. As it is, however, I must very shortly state 
whence it comes that, with abundant knowledge of the composi- 
tion of different manures, and of the prices at which their ingre- 
dients are to be purchased in the market, the chemist yet hesitates 
to fix with any degree of exactness the value of a compound 
manure from the analysis which he has made of it. 
It will be found indeed that the greater part of the difficulty 
arises from the imperfect state of our knowledge Avith respect to 
vegetable nutrition, and consequently to the action of manures. 
The obstacles to an exact valuation of manures appear to be as 
follows : — 
1st. The agricultural value of different substances has not 
yet in all cases been clearly ascertained. 
2nd. Where this value is ascertained the same substance is 
of different value in different soils, and especially under 
the influence of variation of climate. 
3rd. The value is not the same for all crops. 
4th. The mechanical condition of a manure materially 
affects its action. 
5th. The commercial value of the same substance varies 
with the source ; or, in other words, the same substance 
has a different value in the arts according to its origin 
or form of combination. 
6th. The price of the same substance in the same form 
varies continually from a variety of causes. 
I will shortly illustrate these different points. 
1. " The agricultural value of different substances has not yet 
been clearly ascertained." Thus, of the very important ele- 
ment, nitroo-en, we have three verv different forms : in the salts 
of ammonia, where it is in combination with hydrogen ; in 
the nitrates, where nitrogen is combined with oxygen ; and 
in undecomposed animal matter, such as blood, flesh, &c., 
where it is united with carbon, oxygen, and hydrogen. We 
know that all these forms are very valuable, but we cannot yet 
be said to know the exact relative value of a given quantity of 
nitrogen in any one of them compared with another. So much is 
certain, — that they differ in immediate availability to vegetation ; 
that salts of ammonia and nitrates produce a more rapid effect 
than dried blood or animal matter ; and for this reason, if we 
require to use either of them for an immediate effect, as for 
instance to topdress wheat, we should choose the two former in 
preference. But we are not justified in saying that in the long 
run animal matters supplving a given amount of nitrogen may 
not be as valuable, or even more so, than an equivalent quantity 
of ammoniacal salts or nitrates. 
Again, the relative value of manure, especially of liquid 
