242 ELEMENTS OF PLANTS. 



II. Calcareous eartli enables the soilj or the plants growing 

 tliereoiij to draw much more nutriment from the atmosphere. 



Every plant, after being completely burned, leaves a small pro- 

 portion of its previous quantity in ashes. This portion, inde- 

 structible by burning, is distinguished by chemists (not with much 

 accuracy of signification) as the inorganic parts of plants ; and 

 these are found to consist of different salts, or chemical compounds 

 of different acids with alkalies proper, and alkaline or other earths, 

 and also some oxides of metals. All these matters, making the 

 whole residue in ashes, in any one plant, or part or product of any 

 plant, rarely amount to as much as one-tenth of the original dry 

 weight ; and in more of other cases fall below the one-hundredth 

 part.* 



The other and much larger portion of all vegetable matters, 

 called by chemists the organic, or that which is destructible by 

 burning, is composed either of three or most generally all four of 

 these elements, carbon, hydrogen, oxygen, and nitrogen or azote. 

 The like division of products, destructible or indestructible by 

 burning, applies to all animal matters, and also the general consti- 

 tution of their different parts ; but in very different proportions. 

 Excepting the solid bony or shelly parts of animal matters, the 

 portion indestructible by burning is extremely small. Of the des- 

 tructible parts of animal matter, azote (or nitrogen), always forms 

 a considerable proportion ; while in most vegetable products it is 

 in very small proportion, and in others entirely wanting. It is, 

 however, always present either in some part, or element, or pro- 

 duct of every plant. It is the proportion of azote, small as it is, 

 which mainly determines the degree of richness and nutritive value 

 of any substance, whether as food for animals, or as manure for 

 growing plants. And according to the quantity of azote contained, 

 is the tendency of either vegetable or animal matter to putrescence, 

 and to give out offensive odours while putrefying. Thus, in a rough 

 way, common observation and experience, and the sense of smell, 

 may afford tolerably accurate tests of the amount of azotic prin- 

 ciples in materials of manures for plants, or food for animals. 



It follows from the consideration of the questions of which the 

 general results only are here stated, that whatever serves to furnish 

 most azote to the soil, in manure, is most conducive to its immediate 

 fertility ; and whatever abstracts mot azote from the soil, without 

 return, is the most exhausting of its immediate productive powers. 

 Having presented these general propositions (which seem to be 

 received by all authorities), let us proceed to inquire as to the 

 sources of the mode of supply of azote, and of the other much 

 more abundant constituents of plants. 



* See tables of proportions for ashes from many vegetable products re- 

 ported by Boucsingault, p. 53, 4, Rural Economy, Am. Ed. of Eug. Trans. 



