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both systems : by the first, or old, theory, the annual dropping 
and decay of the foliage is said to supply at once general nour- 
ishment and particular nourishment; and by the second, or 
new, theory, the same dropping of the leaves by the general 
nourishment which it supplies, is said to neutralize the particu- 
lar excretions. It must be confessed, that it is not very obvi- 
ous how general nourishment, dropped on the surface of the soil, 
can neutralize the excrementitious matter, deposited many feet 
beneath the surface ; as in the case of long-rooted herbaceous 
plants, like the saintfoin, lucern, &c.; and deep-rooting trees, 
such as the oak, &c. Nevertheless, we find that these plants 
will remain a longer period on the same soil than others, the 
roots of which never go to any great depth beneath the surface; 
such as the fibrous rooted grasses, the strawberry, &c., and the 
pine and fir tribe. We mention these things to show, that though 
it is not yet determined which is the true theory, yet that the fact 
of plants injuring, or diminishing the fertility of the soil, both 
generally and particularly, does not admit of a doubt. In 
the absence of principles founded on whichever of these hypoth- 
eses may be true, recourse is obliged to be had to rules drawn 
from the experience and observation of those who believe in the 
old theory. These rules, as adopted by the best gardeners, are 
as follows : — Crops of plants belonging to the same natural 
order or tribe, or to the natural order and tribe most nearly al- 
lied to them, should not follow each other. Thus, turnips should 
not follow any of the cabbage tribe, sea-kale, or horseradish; nor 
peas, beans. Plants which draw their nourishment chiefly 
from the surface of the soil should not follow each other, but 
should alternate with those which draw their nourishment in 
great part from the subsoil. Hence carrots and beets should 
not follow each other; nor onions and potatoes. Plants 
which draw a great deal of nourishment from the soil should 
succeed, or be succeeded by, plants which draw less nourish- 
ment. Hence, a crop grown for its fruit, such as the pea ; or 
for its roots or bulbs, such as the potato or the onion ; should 
be followed by such as are grown solely for their leaves, such 
as the common borecole, the cellery, the lettuce, &c. Plants 
which remain for several years on the soil, such as strawberries, 
asparagus, &c., should not be succeeded by other plants which 
remain a long time on the soil, but by crops of short duration; 
