36 



CHAPTER m. 



SOURCES OF THE FOOD OF PLANTS. 



Organic food; derived from the soil in forms of combination 

 chiefly. Carbonic acid gas: proportion present in the atmo- 

 sphere : absorbed through pores in the leaves: decomposed in 

 the leaf, and oxygen given off during da3'light. Organic acids 

 in soils. Sources of hydrogen and oxygen ; of nitrogen. 

 Ammonia. Nitric acid. 



SECTION I. ORGANIC FOOD OF PLANTS. 



Having named and described the various substances 

 from which the ash of plants is made up, and which 

 may therefore be considered their inorganic food, we 

 must now see what are the sources of their inorganic 

 as well as of their organic food. 



An organic and an inorganic part being absolutely 

 essential to the existence of every perfect plant, it 

 becomes necessary that the farmer should know where 

 the different bodies come from, that are to make up 

 these parts. This knowledge is of advantage, as en- 

 abling him to increase natural sources of supply, or to 

 devise artificial means of furnishing what is deficient. 



It is quite clear that a plant which is to grow 

 rapidly, must have a constant supply of the two classes 

 of food, and, moreover, that this supply must be pre- 

 sented in a shape immediately available. It is of no 

 use to the crops of the present season, to say to them 

 that there is an abundant supply of manure in the 

 barnyard : they want it near their roots, and will not 

 flourish without it there. So of all other things re- 

 •quired in the soil, they must not only be present, but 



