Report of Experiments on the Growth of Wheat. 
497 
and to the application to arable land of the solid manure result- 
ing from the consumption of the sewaged grass, for obtaining 
other produce than milk and meat by means of sewage. 
In the illustrations given above, therefore, it is sought to convey 
an approximate idea, on the one hand of the utmost extent, and 
on the other of the probable limit, of the loss to which our arable 
soils are subject by the sale of corn and meat, supposing the 
mineral constituents be not returned to the land whence they 
came. Confining attention to this object, we necessarily leave 
out of view the cases in which roots, hay, or straw, are largely 
sold, for, in such, compensation is generally made by the return 
to the land of town manures of some kind. If this be not done 
the loss of mineral constituents will, of course, be very con- 
siderable. 
In view of the facts above adduced, we think it may safely 
be concluded, that the modern practices of this country, taken as 
a whole, do not tend to the injurious exhaustion of the mineral 
constituents in anything like the degree that has been assumed 
by some. Further than this, we think the evidence is more 
in favour of the supposition that, in a great majority of our 
soils, they are, by the combined aid of progressive liberation, 
and of restoration from without, becoming, in -the course of 
cultivation, richer rather than poorer in immediately available 
mineral constituents relatively to immediately available nitrogen. 
So far as this is attained at the expense of the constituents of the 
soil itself, there is, of course, the less to fall back upon within a 
given depth from the surface. But, it surely cannot be denied, 
that if there really is an annual liberation of mineral constituents 
in available form for the growth of plants, at least a portion of 
this may, with propriety, be sold off the farm for good and all. 
The exact amount of annual loss of mineral constituents which 
any soil, with its workable subsoil, can permanently support 
without injury, cannot, indeed, be proved. But such evidence 
as is at command goes to show, that, under the conditions at 
present existing, the nature and extent of the loss to which our 
soils are subject are such, that the majority are deficient of avail- 
able nitrogen rather than of available mineral constituents, so 
far as the requirements for full crops of the cereal grains are 
concerned. 
Insisting strongly, then, as we have always done, upon the 
absolute necessity of a full supply of available mineral con- 
stituents within the soil, relatively to that of nitrogen, we still 
believe that, in the actually existing conditions of British agri- 
culture, it is not they, but the available nitrogen, that is generally 
found to be relatively deficient. 
What then, are the sources of available nitrogen within the 
