ELEMENTS OF PLANT-FOOD 3 



which they may have undergone in their transition to 

 the present combinations in the soil, as well as the fact 

 that the physical properties of the soil are primarily 

 determined by its derivation, render their study of 

 fundamental concern in order to understand the soil' 

 as a medium for plant-growth. The classification and 

 detailed study of the soil is inseparably linked with its 

 derivation, because determined by it. On one side, it 

 supplies certain elements of food whose relative abun- 

 dance is determined by their distribution in the original 

 rocks and their concentration or dissipation through 

 geological changes, and, on the other side, it affords 

 the physical medium for the development of the plant. 



I. THE ELEMENTS OF PLANT-FOOD 



The plant must have certain food elements for its 

 growth and development. These elements are affected 

 by the changes to which the rock is subjected, and in 

 the end will reflect the character of these changes. 



1. Elements essential to plant-growth. The. essen- 

 tial elements of plant-food are ten in number, to which 

 may be added three others which seem to be useful 

 under certain conditions. The essential elements may be 

 divided into two groups, on the basis of their origin: 

 (1) The elements derived entirely and only from the 

 solid portion of the soil. These are calcium, magnesium, 

 potassium, phosphorus, iron and sulfur. (2) The ele- 

 ments derived either directly or indirectly from air 

 and water. These are carbon, hydrogen, oxygen and 

 nitrogen. 



