ALTERNATION OF C 



to lie uncultivated for a year. During this 

 interval of rest, the soil, in a great measure, 

 regains its original fertility. 



It has been further observed, that certain 

 plants, such as peas, clover, and flax, thrive 

 on the same soil only after a lapse of years ; 

 whilst others, such as hemp, tobacco, hclian- 

 thus tuberosus, rye, and oats may be culti- 

 vated in close succession when proper ma- 

 nure is used. It has also been found, that se- 

 veral of these plants improve the soil, whilst 

 others, and these are the most numerous, 

 impoverish or exhaust it. Fallow turnips, 

 cabbage, beet, spelt, summer and winter 

 barley, rye and oats, are considered to be- 

 long to the class which impoverish a soil ; 

 whilst by wheat, hops, madder, late turnips, 

 hemp, poppies, teasel, flax, weld, and lico- 

 rice, it is supposed to be entirely exhausted. 



The excrements of man and animals have 

 been employed from the earliest times for 

 the purpose of increasing the fertility of 

 soils j and it is completely established by all 

 experience, that they restore certain consti- 

 tuents to the soil, which are removed with 

 the roots, fruit or grain, or entire plants 

 grown upon it. 



But it has been observed that the crops are 

 not always abundant in proportion to the 

 quantity of manure employed, even al- 

 though it may have been of the most power- 

 ful kind ; that the produce of many plants, 

 for example, diminishes, in spite of the ap- 

 parent replacement by manure of the sub- 

 stances removed from the soil, when they 

 are cultivated on the same field for several 

 years in succession. 



On the other hand it has been remarked, 

 that a field which has become unfitted for a 

 certain kind of plants was not on that ac- 

 count unsuited for another; and upon this 

 observation, a system of agriculture has 

 been gradually founded, the principal ob- 

 ject of which is to obtain the greatest possi- 

 ble produce with the least expense of ma- 

 nure. 



Now it was deduced from all the foregoing 

 facts that plants require for their growth 

 different constituents of soil, and it was 

 very soon perceived, that an alternation of 

 the plants cultivated maintained the fertility 

 of a soil quite as well as leaving it at rest or 

 fallow. It was evident that all plants must 

 give back to the soil in which they grow 

 different proportions of certain substances, 

 which are capable of being used as food by 

 a succeeding generation. 



But agriculture has hitherto never sought 

 aid from chemical principles, based on the 

 knowledge of those substances which plants 

 extract from the soil on which they grow, 

 and of those restored to the soil by means of 

 manure. The discovery of such principles 

 will be the task of a future generation, for 

 what can be expected from the present, 

 which recoils with seeming distrust and 

 aversion from all the means of assistance 

 offered it by chemistry, and which does not 

 understand the art of making a rational ap- 



plication of chemical discoveries? A futura 

 generation, However, will derive incalcula- 

 ble advantage from these means of help. 



Of all the views which have been adopted 

 regarding the cause of the favourable effects 

 of the alternations of crops, that proposed 

 by M. Decandolle alone deserves to be men- 

 tioned as resting on a firm basis. 



Decandolle supposes that the roots of 

 plants imbibe soluble matter of every kind 

 from the soil, and thus necessarily absc.rb a 

 number of substances which are not adapted 

 to the purposes of nutrition, and must sub- 

 sequently be expelled by the roots, and re- 

 turned to the soil as excrements. Now, as 

 excrements cannot be assimilated by the 

 plant v/hich ejected them, the more of these 

 matters which the soil contains, the more 

 unfertile must it be for the plants of the 

 same species. These excrementitious mat- 

 ters may, however, still be capable of assi- 

 milation by another kind of plants, which 

 would thus remove them from the soil, and 

 render it again fertile for the first. And if 

 the plants last grown also expel substances 

 from their roots, which can be appropriated 

 as food by the former, they will improve the 

 soil in two ways. 



Now a great number of facts appear at 

 first sight to give a high degree of probabi- 

 lity to this view. Every gardener knows 

 thai a fruit-tree cannot be made to grow on 

 the same spot where another of the same 

 species has stood ; at least not until after a 

 lapse of several years. Before new vine- 

 stocks are planted in a vineyard from which 

 the old have been rooted out, other plants 

 are cultivated on the soil for several years. 

 In connexion with this it has been observed, 

 that several plants thrive best when growing 

 beside one another; and on the contrary, 

 that others mutually prevent each other's 

 developement. Whence it was concluded, 

 that the beneficial influence in the former 

 case depended on a mutual interchange of 

 nutriment between the plants, and the in- 

 jurious one in the latter on a poisonous 

 j action of the excrements of each on the 

 I other respectively. 



A series of experiments by Macaire- 

 Princep gave great weight to this theory. 

 He proved beyond all doubt that many 

 plants are capable of emitting extractive 

 matter from their roots. He found that the 

 excretions were greater during the night 

 than by day (?), and that the water in 

 which plants of the family of the Legumir- 

 nosce grew acquired a brown colour. Plants 

 of the same species placed in water im- 

 pregnated with these excrements were im- 

 peded in their growth, and faded prema- 

 | turely, whilst, on the contrary, corn-plants 

 I grew vigorously in it, and the colour of the 

 j water diminished sensibly ; so that it ap- 

 i peared as if a certain quantity of the excre- 

 I ments of the Leguminosce had really been 

 j absorbed by the corn-plants. These ex- 

 periments afforded, as their main result, 

 i that the characters and properties oi the ex 



