644 
Rotation of Crops. 
nation of crops, the position, and the role , of the mineral consti- 
tuents must not be overlooked ; and the less can it be so, when 
their connection with the very important element — the nitrogen 
of the crops — is considered. 
As to the nitrogen : — It haslbeen seen that, although very 
characteristically benefited by nitrogenous manures, the cereal 
crops take up and retain much less nitrogen than any of the 
crops alternated with them. In fact, the root-crops may 
contain two, or more, times as much nitrogen as either of the 
cereals, and the leguminous crop, especially the clover, much 
more than the root-crops. The greater part of the nitrogen of 
the cereals is, however, sold off the farm ; but perhaps not more 
than 10 or 15 per cent, of that of either the root-crop, or the 
clover, or other forage leguminous crop, is sold off in animal 
increase or milk. Thus, most of the nitrogen of the straw of the 
cereals, and a very large proportion of that of the much more 
highly nitrogen-yielding crops, returns to the land as manure, 
for the benefit of future cereals and other crops. Indeed, it is, as 
a rule, only a comparatively small proportion of the very much 
increased amount of nitrogen obtained in rotation compared with 
that in continuous cereal-cropping (chiefly due to the interpolated 
crops), that is lost to the land in the saleable products. 
As to the source of the nitrogen of the so-called ‘ restorative 
crops,’ it has been shown that certainly in the case of the roots 
it is not, as has sometimes been assumed, that such plants take 
up nitrogen from the air by virtue of their extended leaf- 
surface. Both common expei'ience and direct experiment 
demonstrate, that they are as dependent as any crop that is 
grown, on available nitrogen within the soil, which is generally 
supplied by the direct application of nitrogenous manures — 
natural or artificial. Under such conditions of supply, however, 
the root-crops, so to speak gross feeders as they are, and 
distributing a very large amount of fibrous feeding root within 
the soil, avail themselves of a much greater quantity of the 
nitrogen supplied than the cereals would do under similar 
circumstances ; this result being partly due to their period of 
accumulation and growth extending even months after the 
period of collection by the ripening cereals has terminated, and 
at the season when nitrification within the soil is the most 
active, and the accumulation of nitrates in it is the greatest. 
Lastly, full supply of both mineral constituents and nitrogen 
being at command, these crops assimilate a very large amount 
of carbon from the atmosphere, and produce, besides nitrogenous 
food-products, a very large amount of the carbohydrate — sugar, 
as respiratory and fat-forming food for the live-stock of the farm. 
