SOIL AS A MEDIUM FOR PLANT GROWTH 3 



some are derived from the air and some from water, but 

 the larger number must be obtained from soil. They may 

 be classified thus : 



Substances obtained from air or water : 



Carbon Hydrogen 



Oxygen Nitrogen 



Substances obtained from soil : 

 Nitrogen 



Phosphoric acid [phosphorus] * 

 Potash [potassium] 

 Lime [calcium] 

 Magnesia [magnesium] 

 Iron 

 Sulfur 



All these substances are essential to the normal devel- 

 opment of farm crops. Carbon, oxygen and nitrogen are 

 found in air. Hydrogen and oxygen are in water. Plants 

 obtain their carbon from the air; their oxygen from 

 both air and water; their hydrogen from water; their ni- 

 trogen, in the case of certain plants only, from the air. The 

 other substances are found in all arable soils, from which 

 plants obtain them after they have become dissolved in the 

 soil water. While arable soils contain all these substances, 

 the fact that they must be in solution before plants can use 



1 This list of plant-food materials gives the names commonly used. 

 Thus the terms phosphoric acid, potash, and lime are the ones used in con- 

 nection with fertilizers. Nitrogen is sometimes spoken of as ammonia by 

 fertilizer manufacturers, but the most general term is nitrogen. The words 

 in brackets following the unbracketed words indicate other names some- 

 times found, but not used in this book. 



All the substances in this list are capable of uniting with certain other 

 substances to form various combinations. When present in the soil they are 

 not likely to be in the same combinations as when present in plants. When, 

 therefore, phosphoric acid in soil or in a plant is spoken of, nothing is implied 

 regarding the form in which it exists. 



