EFFECTS OF PLANTS UPON THE SOIL. 97 



There are some salts which enter naturally into the com- 

 position of certain plants ; the pellitory and nettle, for 

 instance, which grow upon the borders of the sea, contain 

 muriate or sulphate of soda ; these vegetables, transported 

 into other soils, afford no vestige of these salts, and their 

 growth is less vigorous. M. le Marquis de Bullion has 

 proved that the turnsol, raised in earth containing no nitre, 

 does not, upon analysis, afford a vestige of any ; but that 

 plants of the same kind, raised in the same soil, but wa- 

 tered with a solution of nitrate of potash, are charged with 

 that salt. 



Generally speaking, a superabundance of salts, especially 

 if they be of kinds very soluble in water, injures vegeta- 

 tion : this is particularly the case when the salts are not 

 such as enter naturally into the plants, amongst the num- 

 ber of their constituent principles. Salts of foreign na- 

 tures cannot be useful, excepting as they may serve, in 

 very small quantities, to excite and stimulate the organs of 

 plants. The great value of sulphate of lime as a manure, 

 is owing to its insolubility, which allows water to contain 

 but a very small portion of it at once; so that it passes 

 into plants very gradually, and thus its effects are pro- 

 longed for several years ; till, as I have before observed, 

 the soil is exhausted of it. 



The quantity and quality of the salts contained in plants 

 may be ascertained by an analysis of the ashes arising 

 from burning them in a dry state. It may not be useless 

 to mention here some facts which may throw light upon 

 this subject. 



Kirwan and Ruckers have proved, that an equal weight 

 of herbaceous plants furnishes more ashes than of ligne- 

 ous plants. M. Pertuis has found, that the trunks of trees 

 afford less ashes than the branches, and these last less than 

 the leaves. Evergreens yield less ashes than trees and 

 shrubs that shed their leaves in autumn. On the other 

 hand. Hales and Bonnet have observed, that the perspira- 

 tion of herbaceous is greater than that of ligneous plants, 

 and that that of evergreens is less than that of plants 

 which shed their foliage. These circumstances may ex- 

 plain why some plants afford more ashes than others. The 

 water which is evaporated by transpiration deposits in the 

 cells of the plant the salts which it had held in solution, 

 and is replaced by a new quantity, which is in its turn 

 thrown out, leaving behind it an additional portion of 

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