188 OF MANURE. 



and with salts of phosphoric acid, prepared in 

 chemical manufactories, exactly as at present medi- 

 cines are given for fever and goitre. 



There are some plants which require humus and 

 do not restore it to the soil by their excrements ; 

 whilst others can do without it altogether, and 

 add humus to a soil which contains it in small 

 quantity. Hence, a rational system of agricul- 

 ture would employ all the humus at command for 

 the supply of the former, and not expend any of 

 it for the latter ; and would in fact make use of 

 them for supplying the others with humus. 



We have now considered all that is requisite in a 

 soil, in order to furnish its plants with the materials 

 necessary for the formation of the woody fibre, the 

 grain, the roots, and the stem, and now proceed to the 

 consideration of the most important object of agri- 

 culture, viz. the production of nitrogen in a form 

 capable of assimilation the production, therefore, 

 of substances containing this element. The leaves, 

 which nourish the woody matter, the roots, from 

 which the leaves are formed, and which prepare the 

 substances for entering into the composition of the 

 fruit, and, in short, every part of the organism of a 

 plant, contain azotised matter in very varying pro- 

 portions, but the seeds and roots are always 

 particularly rich in them. 



Let us now examine in what manner the greatest 

 possible production of substances containing nitro- 

 gen can be effected. Nature, by means of the 



