436 ROTATION OF CROPS. 



this object may be effectually obtained, there are certain principles which 

 ought to be adopted as guides. The chief of these is to be derived from a 

 knowledge of what specific benefit or injury every culinary plant does to the 

 soil, with reference to any other culinary plant. It ought to be known 

 whether particular plants injure the soil by exhausting it of particular 

 principles ; or whether, as has been lately conjectured by De Candolle, and 

 as some think proved, the soil is rendered unfit for the growth of the same 

 or any allied 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 that is necessary 

 to insure the reproduction of good crops of potatoes ; and the onion of some- 

 thing which is necessary for the reproduction 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 particular kind of crop : but this injury, 

 according to his hypothesis, is not effected by depriving the soil of the 

 particular kind of nutriment necessary 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 

 herbaceous 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 are said to supply at once general nourishment and particular nourish- 

 ment ; 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 parti- 

 cular excretions. A wood of the pine or fir tribe standing so thick that 

 their roots will form a net-work under the surface, will not poison each 

 other ; but remove these trees, and place a new plantation on the same soil, 

 and they will not thrive ; owing, as we think, to the principles most condu- 

 cive to the growth of coniferous trees being exhausted, as is explained 

 chemically by Liebig. The practical inference from either theory is much 

 the same that is, a change of crops; which is also in conformity with 

 the experience and observation of those who believe in the old theory. 

 The rules adopted by the best gardeners are as follow : 



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

 natural order and tribe most nearly allied to them, should not follow each 

 other. Thus, turnips should not follow any of the cabbage tribe, sea-kale, 

 or horseradish ; nor peas, beans. 



2. 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. 



3. Plants which draw a great deal of nourishment from the soil should 

 succeed, or be succeeded by, plants which draw less nourishment. Hence a 

 crop grown for its fruit, such as the pea ; or for its roots or bulbs, such as 



