188 THE FARMER OF TO-MORROW 



came to the conclusion that nitrogen, potash 

 and phosphorus were the only elements which 

 were important, as being limited in supply. 



To make use of any of its elements of food, 

 a plant must find it present in the soil in 

 available form. Upon the meaning of this 

 word "available" hangs the whole of the Liebig 

 hypothesis. It was believed by him, and is 

 taught to-day, that although potash or phos- 

 phorus may be present in the soil in great 

 quantities, it is useless to plants unless it is 

 soluble, either through the agency of water 

 alone, or by means of the carbonic acid secreted 

 by the roots of growing plants. The question 

 of immediate soil fertility, then, depended not 

 on how much of a given element was present 

 in the soil, but on whether or not it existed in 

 soluble form. 



The natural process of weathering, that is, 

 the action of the natural agencies of heat, cold, 

 sunlight, etc., gradually effected chemical com- 

 binations of these soil minerals with bases that 

 rendered them soluble; as do cultivation and 

 the action of decaying organic manures. 



In other words, the available mineral ele- 

 ments in the soil are being liberated auto- 

 matically by natural forces aided by man. As 



