642 
Eolation of Crops. 
other demand — proximity to towns, easy railway or other com- 
munication, and so on — the products which would otherwise be 
retained on the farm are exported from it. the import of town 
or other manures is generally an essential condition of such 
practice. Indeed, this system of free sale very frequently in- 
volves full compensation by purchased manures of some kind. In 
our own country, such deviations from the practice of merely sell- 
ing grain and meat have been much developed in recent years ; 
and they will doubtless continue to increase under the altered 
conditions of our agriculture, dependent on very large imports 
of grain, increasing imports of meat and other products of feed- 
ing, and very large imports of cattle-food and other agricultural 
produce. Already much more attention is being devoted to 
dairy products, not only on grass farms, but on those that are 
mainly arable ; and there will doubtless be some, but probably 
by no means so great an extension as some suppose, in the 
production of other smaller articles required by town popula- 
tions. 
It is further true, though the remark applies in a very limited 
degree to our own country, that there are other deviations which 
have more the character of exceptions to the general rule of 
rotation, such as the introduction of flax, hemp, tobacco, or 
other so-called industrial crops. But, in these cases, as with 
potatoes, the growth involves special expenditure for manure 
instead of conservation of it. Indeed, the inducement is the 
high price of the product, rather than the maintenance, or the 
improvement, of the condition of the land for future crops. 
Still, as such deviations from regular rotation practice as 
have been referred to, do, as has been said, generally involve 
more or less, and frequently full, compensation by manure from 
external sources, we may, in endeavouring to explain the bene- 
fits which accrue from the practice of rotation, confine attention, 
for the purposes of illustration, to what may be called the self- 
supporting system, and to the simple four-course. one which has 
been selected for investigation at Rothamsted, and from the 
results relating to which the illustrations which have been 
brought forward have been drawn. 
It will be well first briefly to refer to the evidence relating 
to some of the more important mineral constituents found in the 
different crops of the four-course rotation. 
Of phosphoric acid , the cereal crops take up as much as, or 
more than, any of the other crops of the rotation, excepting 
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- 
