ROTATION OF CROPS. 
213 
fixes and stores them up in its substance. This amount, whatever it may be, is the annual sup¬ 
ply of nitrogen. The succeeding year wheat is cultivated upon the clover; and hence the 
plant is in the way of obtaining what was stored up in the clover plant, and also of taking di¬ 
rectly from the soil the two annual supplies of the same substance. It has, therefore, or may 
be supposed to have, a double source for this important substance. Adopting this view of the 
value of clover, we can not fail to see the importance of clover as one of the essential crops in 
our husbandry. Whatever hypothesis we may adopt as to its modus operandi, there can be 
no doubt of its great value, as a member of a rotation of crops; its large growth, both above 
and below the surface; its vigor and strength; its universal adaptation to soils by means of 
plaster, are among its admirable qualities, to say nothing of it as a fodder. It may be well to 
repeat that we should by no means regard the relation which clover sustains to nitrogen as its 
main recommendation, or as the foundation upon which its cultivation rests. Indeed no single 
relation should ever be regarded as the sum of utility. There are other unities which must 
come in for consideration. It is of little utility to discuss the question which is of the most 
value, nitrogen, the alkalies or phosphates ; for it is true that a seed is imperfect and unsound, 
and valueless, when either is absent; even though one is required in greater quantities than 
another, still the smaller is necessary to the perfection of the whole. We may talk of the im¬ 
portance of the keystone of an arch ; yet it will be found true that the arch will fall if a single 
small stone is left out, or we may rather say it will be no arch at all, until every stone is in its 
place. 
The forgoing remarks lead me to go farther in the same train of thought. We may observe 
that there is no crop which feeds exclusively on a single element, or that exhausts a soil of a single 
element. Every plant, on analysis, exhibits the same elements, as silica, sulphates, phosphates, 
the alkalies, potash, soda, lime and magnesia—the difference being simply in amount; but 
there is no greater difference in the composition of two species of plants than between the parts 
of the same plant. So also, there are variations of composition in the different stages of 
growth. An inference of importance follows legitimately from the foregoing facts, viz. 
that a rotation founded and pursued on the principle that the plant feeds upon one kind of 
food, or in other words, exhausts the soil of one element mainly, is wrong ; it is wrong from 
the nature of the facts, and the relation of the composition of the plant to the elements of the 
soil. It is true there will be, in such a rotation, an approximation to truth : there is a plausi¬ 
bility which at first recommends to favor. Take a rotation which has been carried out, as the 
following: manure, roots, tobacco, oats with clover, wheat, beans.* The first crop, corn, 
exhausts the soil of phosphates and potash; the tobacco, of potash especially; the third of 
both phosphates of the alkaline earths and of potash; a clover crop intervenes, but it can not 
restore those important elements I have named; but it may collect and bring to the surface 
what remains in the soil; wheat, an exhausting crop then follows; and lastly, beans, which is 
both a foliage and a seed crop, and is exhausting to a great amount. It would be difficult to 
* Transactions of the American Agricultural Association, Gardner paper, page 6. 
