CAUSES OF ITS BENEFICIAL INFLUENCE. \7 \ 



turnips, potatoes, &c. ; the wheat may be again 

 sowed with advantage after the fourth year ; for, 

 during the interval of three years, the soil will, by 

 the action of the atmosphere, be rendered capable 

 of again yielding silicate of potash in sufficient 

 quantity for the young plants. 



The same precautions must be observed with 

 regard to the other inorganic constituents, when it 

 is desired to grow different plants in succession on 

 the same soil ; for a successive growth of plants 

 which extract the same component parts, must 

 gradually render it incapable of producing them. 

 Each of these plants, during its growth, returns to 

 the soil a certain quantity of substances containing 

 carbon, which are gradually converted into humus, 

 and are for the most part equivalent to as much 

 carbon as the plants had formerly extracted from 

 the soil in the state of carbonic acid. But although 

 this is sufficient to bring many plants to maturity, 

 it is not enough to furnish their different organs 

 with the greatest possible supply of nourishment. 

 Now the object of agriculture is to produce either 

 articles of commerce, or food for man and animals, 

 but a maximum of produce in plants is always in 

 proportion to the quantity of nutriment supplied to 

 them in the first stage of their development. 



The nutriment of young plants consists of car- 

 bonic acid, contained in the soil in the form of 

 humus, and of nitrogen in the form of ammonia, 

 both of which must be supplied to the plants if the 



