EOTATION OF CKOPS. 



13 



This, aldiough apparently a general 

 rule, is not without its exceptions. If a 

 pea and a grain of wheat are planted in 

 the same soil, and placed in the same 

 circumstances, it wlU be found that the 

 latter will absorb silex from the soil, 

 the former none; and this shows pretty 

 clearly the power of selection. Dr Dau- 

 beny has shown that certain plants will 

 not absorb strontian; and Saussure that 

 the spiral-rooted polygonum, while it 

 took up common salt (muriate of soda) 

 freely, refused to absorb a solution of 

 acetate of lime. " It is a curious fact," 

 Dr Lindley remarks, "that the poisonous 

 substances which are fatal to man are 

 equally so to plants, and in nearly the 

 same way; so that by presenting opium 

 or arsenic, or any metallic or alkaline 

 poison, to its roots, a tree may be destroyed 

 as readily as a human being." 



Cropping the same ground year after 

 year with the same species of plant, or 

 replanting a forest with the same spe- 

 cies of trees, or indeed planting a young 

 fruit-tree in the same situation and soil 

 from which another had been removed, 

 is attended with failure, for, in addi- 

 tion to the cause assigned above, the soil 

 becomes exhausted of those parts it ori- 

 ginally contained and has given out 

 for the support of the previous crop, 

 though it may still retain material of 

 a different character, sufficient to enter 

 into the constitution of a crop of a dis- 

 similar kind. "As the inorganic mate- 

 rials which enter into the composition 

 of plants vary much in their nature 

 and relative proportions, it is evident 

 that a soil may contain those necessary 

 for the growth of certain species, while 

 it may be deficient in those required 

 by others. It is on this principle," says 

 Professor Balfour, "that the rotation of 

 crops proceeds — those plants succeeding 

 each other in rotation which require dif- 

 ferent inorganic compounds for their 

 growth. In ordinary cases, except in the 

 case of very fertile virgin soil, a crop, by 

 being constantly grown in successive years 

 in the same field, will deteriorate in a 

 marked degree. Dr Daubeny has put 

 this to the test of experiment, by causing 

 plants to grow on the same and on 

 different plots in successive years, and 

 noting the results : — 



VOL. II. 



Potatoes, 



Flax, 



Beans, 



Barley, 



Turnips, 



Oats, 



Average of 5 years. 

 ; in the same plot, 72.9 lb. tubers, 

 in different plots, 92.8 „ ,, 



same, . . 15.0 „ 



different, . 19.9 „ 



same, . . 32.8 „ 



different, . 34.8 „ 



same, . . 30.0 „ 



different, . 46.S „ 



same, . . 104.0 „ 



different, . 17B.0 „ 



same, . . 28.0 „ 



different, . 32.4 „ 



"This shows a manifest advantage in 

 shifting crops, varying from 1 to 75 per 

 cent, the deficiency of inorganic matter 

 being the chief cause of difference." — 

 Manual of Botany. 



" The prevaiUng opinion," Loudon re- 

 marks, " has long been that plants exhaust 

 the soil generally of vegetable food, parti- 

 cularly of that kind of food which is pe- 

 culiar to the crops 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 a good crop of potatoes, and the onion 

 of something which is necessary for the 

 reproduction of a large crop of onions. 

 According to the theory of De CandoUe, 

 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 spe- 

 cies, 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." — 

 Siib. Hort., p. 436. Both these theories 

 have been disputed, and this by practical 

 reasoners, who naturally enough ask, How 

 do they apply to plants long confined to the 

 same soil — an orange tree, for example — 

 which has luxuriated, without being either 

 poisoned or starved, within the hmits of 

 a three-feet square box for a score of years? 

 and how do perennial plants exist in the 

 same soil for as gi-eat a length of time 1 

 The advocates of the one theory say the 

 annual dropping and decay of the foliage 

 supply at once general and particular 

 nourishment. This does not, we suspect, 

 apply to the orange tree we have taken as 

 an example, because the leaves, if they 

 even fell on the surface of the soil in the 



