ROTATION OF CROPS. 251 



Of phosphoric acid, the cereal crops take up as much as, or Phosphoric 

 more than, any of the other crops of the rotation, excepting acid " 

 clover ; and the greater portion of what they take up is lost 

 to the farm in the saleable product — the grain. The remainder, 

 that in the straw, as well as that in the roots and the legu- 

 minous crops, is supposed to be retained on the farm, except- 

 ing the small amount exported in meat and milk. 



Of potash, each of the crops takes up very much more than Potash. 

 of phosphoric acid. But much less potash than phosphoric 

 acid is exported in the cereal grains, much more being re- 

 tained in the straw ; whilst the other products of the rotation 

 — the roots and the Leguminosa? — which are also supposed to 

 be retained on the farm, contain very much more potash than 

 the cereals, and comparatively 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 phos- 

 phoric 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 Lime. 

 the roots much less than of potash ; more by the Leguminosse 

 than by the other crops, and, by the clover especially, some- 

 times 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- silica. 

 minosse — 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 condi- 

 tions, is as characteristic a chemical phenomenon of rotation 

 as is the very large amount of lime taken up by clover and 

 other Leguminosse. Very little silica, however, is lost to the 

 land in the assumed saleable products. 



Thus, then, although different, and sometimes very large, Loss and 

 amounts of these typical mineral constituents are taken up by r ^ e ^ a / 

 the various crops constituting the rotation, there is no mate- constitu- 

 rial export of any in the saleable products, excepting of phos- ents - 

 phoric acid and of potash ; and, so far at least as phosphoric 

 acid is concerned, experience has shown that it may be ad- 

 vantageously supplied in purchased manures. 



But, although the eventual loss to the land of mineral con- 

 stituents is, in a self-supporting rotation, comparatively so 



