Experiments on the Growth of Wheat. 
43 
of straw was, however, in (ivcry case exceptinfi;- that of the farm- 
yard manure, less than formerly, and jjenerally very much less. 
Thus, the amounts of increase obtained for two years alter the 
application of the manures had been stopped, further show that 
the condition of the land was too high for the full action of the 
manures in the years of their application. They also show that 
their influence was not even then exhausted ; and further evidence 
of this is to be found in the fact, that calculation leads to the 
conclusion that, in these Rodmersham experiments, there was a 
less proportion of the nitrogen supplied in the manures in the 
four years, recovered in the increase of the six years, and in 
some cases much less, than is sometimes recovered in the crop 
immediately succeeding the application of a rritrogenous manure. 
Under favourable circumstances, from 40 to 50 per cent, of the 
nitrogen supplied in an artificial manure for wheat may be 
recovered in the increase of a first crop. But it is estimated that, 
in the cases of the rape-cake and of the ammonia-salts alone, 
there was only about one-fourth, and in those of the mineral 
manure and ammonia-salts, and of the guano, under 40 per cent, 
of the nitrogen supplied in the manure of the four years recovered 
in the increase of the six years. 
It will still be useful to give an estimate of the value of the 
increase so far obtained, by the side of the cost of the manures 
applied in one or two of the experiments. 
The mixed mineral manures of plot 2 were far too expensive 
in proportion to the amount of increase they yielded, for it to be 
at all worth while to reckon the cost against the increase in their 
case. Looking to the objects in view, it was still quite essential 
to have the evidence of direct experiment as to their effect. 
Ammonia-salts are, generally, neither so cheap a source of 
nitrogen, nor are they, when used alone, so good a manure for 
corn-crops as Peruvian guano, which contains a large proportion 
of phosphates as well as nitrogen. Rape-cake, though a recog- 
nised manure in the market for wheat, acts somewhat more 
slowly for the amount of nitrogen it contains than guano. It 
will be well, for the sake of comparison, to show the cost of the 
manure, and the value of the increase of the three manures — 
rape-cake, ammonia-salts, and Peruvian guano. This is done in 
the following table (p. 44). 
Reckoning the value of the increase against the cost of the 
manures, there is a considerable margin in favour both of the 
ammonia-salts and the guano, but particularly of the guano. The 
evidence further goes to show that these active nitrogenous 
manures are by no means fully exhausted in the first year of 
their application. The quantity of guano used — nearly 5 cwts. 
to the acre — was, however, much more than is usually applied ; 
indeed 
