Rotation of Crops. 
643 
minous crops, is supposed to be retained on the farm, excepting 
the small amount exported in meat and milk. 
Of potash , each of the crops takes up very much more than of 
phosphoric acid. But much less potash than phosphoric acid is 
exported in the cereal grains, much more being retained in the 
straw ; whilst the other products of the rotation — the roots and 
the Leguininosae — which are also supposed to be retained on the 
farm, contain very much more potash than the cereals, and com- 
paratively little of it is exported in meat and milk. The general 
result is, that the whole of the crops of rotation take up very 
much more of potash than of phosphoric acid, whilst probably 
even less of it is eventually lost to the land. 
Of lime, very little is taken up by the cereal crops, and by 
the roots much less than of potash ; more by the Leguminosae than 
by the other crops, and, by the clover especially, sometimes much 
more than by all the other crops of the rotation put together. 
Of the lime of the crops, however, very little goes in the saleable 
products of the farm under the conditions supposed of a self- 
supporting rotation. There is, however, frequently a considerable 
loss of lime in land-drainage. 
Although the facts relating to other mineral constituents of 
the crops are not without significance, reference can be made 
here to only one other of these constituents — namely, the silica. 
The interpolated crops of rotation— the Roots and the Legu- 
minosae — take up scarcely any silica ; but the cereals take up 
a very large amount of it. Indeed, the large amount of silica 
taken up by these crops when grown under ordinary conditions, 
is as characteristic a chemical phenomenon of rotation as is the 
very large amount of lime taken up by clover and other Legu- 
minosae. Very little silica, however, is lost to the land in the 
assumed saleable products. 
Thus, then, although different, and sometimes very large, 
amounts of these typical mineral constituents are taken up by 
the various crops constituting the rotation, there is no material 
export of any in the saleable products, excepting of phosphoric 
acid and of potash ; and, so far at least as phosphoric acid is 
concerned, experience has shown that it may be advantageously 
supplied in purchased manures. 
But, although the eventual loss to the land of mineral con- 
stituents is, in a self-supporting rotation, comparatively so small, 
the very fact that the different crops require for their growth, 
not only very different amounts of individual constituents, but 
require these to be available within the soil in very different con- 
ditions, both of combination and of distribution, points to the 
conclusion that, in any explanation of the benefits of an alter- 
