478 Systems of cropping Kitchen- Gar dens. 



the soil is rendered unfit for the growth of the same or any alh'ed 

 species, by excretions from the roots of plants ; while the same 

 excretions, acting in the way of manure, add to the fitness of 

 the soil for the production of other species. The prevailing 

 opinion, as every one knows, has long been, that plants exhaust 

 the soil, generally, of vegetable food ; particularly of that kind of 

 food which is peculiar to the species growing on it for the time 

 being. For example, both potatoes and onions exhaust the soil 

 generally ; while the potato deprives it of something which is 

 necessary to insure the reproduction of good crops of potatoes ; 

 and the onion of something which is necessary for the reproduc- 

 tion of large crops of onions. According to the theory of De 

 Candolle, both crops exhaust the soil generally, and both render 

 it unfit for the repetition of the particular kind of crop : but this 

 injury, according to his hypothesis, is not effected by depriving 

 the soil of the particular kind of nutriment requisite for the 

 particular kind of species; but by excreting into it substances 

 peculiar to the species with which it has been cropped, which 

 substances render it unfit for having these crops repeated. Both 

 these theories, or rather perhaps hypotheses, are attended with 

 some difficulty in the case of plants which remain a great many 

 years on the same soil; as, for example, perennial- rooted herba- 

 ceous plants and trees. The difficulty, however, is got over in 

 both systems : by the first, or old, theory, the annual dropping 

 and decay of the foliage is said to supply at once general nou- 

 rishment and particular nourishment; and by the second, ornew, 

 theory, the same dropping of the leaves, by the general nourish- 

 ment which it supplies, is said to neutralise the particular ex- 

 cretions. It must be confessed, that it is not very obvious how 

 general nourishment, dropped on the surface of the soil, can 

 neutralise 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 

 hypotheses 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 gar- 

 deners, are as follows : — 



Crops of plants belonging to the same natural order or tribe, 



