length the seed has reached maturity, the skele- 
ton merely of the plant remains, and this, on being 
given back to the soil, returns to it but a very 
inconsiderable portion of what it had previously 
taken from it.” Different groups of species, too, 
abstract different inorganic principles from the 
soil; and in proportion to the general rareness 
of any peculiar principles which some species ab- 
stract, or to the comparative voracity with which 
they abstract them, these species are exhausting. 
Some of the rarer principles, likewise, particu- 
larly the phosphatic and the nitrogenous, are far 
more plentifully abstracted from the soil by the 
seeds of any plants, especially by those of the 
cereal grasses, than by any or sometimes all of 
the previously formed organisms; so that the 
exhaustion during the formation and maturing 
of seeds is often greater from this cause than 
even from the seminal exsiccation of the stems 
and roots,—and the aggregate amount of exhaus- 
tion caused by any particular species of plants, as 
compared to that caused by any other species, 
may, in general, be pretty closely estimated by 
the proportion of these principles in the seeds. 
This topic, however, will be resumed and fol- 
lowed out in a subsequent paragraph, on the 
sixth and last of the general laws which explain 
the system of rotations. 
Plants of different families and different ha- 
bits do not exhaust the soil in the same manner 
of pasturing within it, or of exploring and di- 
minishing its magazines of nourishment. “The 
roots of plants of the same family,” observes 
Chaptal, “extend themselves in a similar manner, 
penetrating to the same depth, growing of the 
same length, and exhausting all that portion of 
the soil which they encompass, or with which 
they come in contact. Such roots as run nearest 
the surface, and occupy the least space, are the 
most divided. Perpendicular roots that pene- 
trate far into the earth, throw out but few radi- 
cles near the surface, and draw their nourish- 
ment from a considerable depth. If in trans- 
planting a turnip or beet, the point of the tap- 
root be cut off, so that it can no longer run down 
into the soil for nourishment, it will send out 
small fibrous roots from all parts of its surface, 
which extending to a certain distance, absorb 
the nutritive principles from such portions of 
the soil as they are able to reach; and in this 
case the form of the root will be roundish instead 
of being tapering and long. It would appear, 
then, that the roots of plants exhaust only such 
portions of the soil as are in contact with them ; 
and thus long perpendicular roots may be en- 
abled to obtain from a soil abundant nourish- 
ment, even when its upper portion has been 
completely exhausted by such as naturally run 
near the surface. The roots of plants of the same 
species, and of such also as are of analogous cha- 
racter, uniformly extend themselves in a similar 
direction, whenever the character of the soil 
prevents nvé-their free development ; so that 
| 
they all occupy the same stratum of earth, and | 
are nourished by it. Hence it is that trees 
rarely flourish when, in the same ground, they 
are made to succeed others of the same species ; 
and especially when sufficient time has not in- 
tervened for the roots of the first to decay, so as 
to afford to the stratum of soil occupied by them 
a fresh supply of manure.” 
All plants do not return to the soil either the 
same quantity or the same kind of manure. 
“ All more or less exhaust the soil of its nutri- 
tive juices; but, on the other hand, they all 
likewise restore to it, in the remains they leave 
behind them, something in a degree to repair 
this loss. The cereal and oleaginous plants are 
the most exhausting of all, and at the same time 
return the least to the soil. When plants, in 
being gathered, are pulled up with their roots, 
they do not leave anything to repay for the 
nourishment which they have received. It is 
true that other kinds of plants consume, in 
seeding, a large proportion of the manure which 
is contained in the soil; but then the roots of a 
part of these penetrate far into the soil, separate 
its parts, and render it loose and open to a great 
depth; while others, again, cover its surface 
with their numerous leaves, and these being de- 
tached from their stalks during the progress of 
vegetation, compensate in part for what has 
been taken from it. There are some plants, 
also, the stalks and roots of which still retain 
their succulence even after their fruit is ma- 
tured; and which, therefore, in their decompo- 
sition, restore to the soil a portion of the juices 
of which they had deprived it; certain of the 
legumes are of this character.” The residuary 
matter after harvesting, whether in the form of 
roots, stubble, radical leaves, fallen stems or 
shed seeds, is of still greater importance. 
is a point of interest,” remarks Boussingault, 
“to ascertain what quantity of elementary mat- 
ter is left in the soil after each kind of crop 
in the rotation. Precise knowledge of this de- 
scription may even be important in calculating | 
rotations; for it is obvious that the remains of 
the crop now on the ground must influence that 
which is to follow, and in the course of a rota- 
tion the sum of the residuary matters must be 
regarded as a supplement or addition to the 
manure put into the ground at its commence- 
ment. In the systems of rotation very generally 
followed at the present time, the influence of 
these residuary matters is manifest, and it is 
partly by their means that we can explain how 
a quantity of manure, frequently very moderate, 
should suffice for the whole of the crops in a 
productive rotation. All the world acknowledge 
that the residues of crops compensate in greater 
or less degree for what is carried away in the 
shape of harvest, and that in some cases they 
even add to the fertility of the soil; for in grow- 
ing crops that leave a large quantity of residue, : 
it is precisely as if a smaller quantity were taken 
“Tt | 
