78 AGRICULTURE. 



water (which usually contains organic acids), or 

 which may be rendered soluble by the acid reac- 

 tion of the roots. 



IV. It Tends to Retain and Regulate the Heat 



of the sun, and transform it into energy which 

 plants can use. 



V. It Serves as a Habitation for Soil Bacteria, 



which transforms the unavailable free nitrogen of 

 the air into nitrates available for the use of plants. 



^._CONSTITUENTS OF PLANTS. 



I. Chemical Analysis of Plants. 



Many analyses of the tissues of different 

 plants have been made (though by no means of 

 all plants), and through these analyses it has 

 been ascertained that one plant may contain 

 certain compounds — or particular combinations 

 of these elements — which do not exist in some 

 other plants. These analyses show that all 

 plants are essentially made up of fourteen ele- 

 ments, or about that number. 



II. Sources of Plant-food. 



Four of these elements — carbon, oxygen, hy- 

 drogen, and nitrogen — are obtained directly or 

 indirectly from the air, while the soil must sup- 

 ply the remaining ten elements : iron, calcium, 

 silicon, chlorine, sulphur, phosphorus, potas- 

 sium, sodium, magnesium, and manganese. The 

 food elements obtained from the soil are the 



