2 An Account of the Soil [PT. i 



is the business ol" plant physiologists to ascertain 

 plant requirements, and we must therefore start out 

 with the information they have provided which, how- 

 ever, we must test for ourselves before we finally 

 accept it. 



Six conditions or factors are known to be necessary 

 before the plant will make good growth: the soil must 

 supply a suitable amount of: (1) food, (2) water, and 

 (3) air; (4) it must be at a proper temperature; (5) there 

 must be enough of it to afford adequate root room; 

 (6) it must be free from injurious substances or pests. 

 What is exactly a suitable amount cannot be stated 

 beforehand but can only be found out by trial; be- 

 cause different plants, and even different varieties of 

 the same plant, have different requirements. Thus an 

 azalea needs all the six conditions and so does a barley 

 plant, but the suitable amount is very different in the 

 two cases. It is unfortunate that no one has yet dis- 

 covered any way of finding out the suitable amounts 

 simpler than actual trial because this particular method, 

 though it looks straightforward, is really very cumber- 

 some and liable to give misleading results, as we shall 

 see later on. 



All these six conditions are necessary and no one of 

 them can take the place of any other. If a plant is dying 

 for lack of water it will not recover by receiving more 

 food or more air. A proper supply of all the factors 

 must be maintained, and if any one is insufficient the 

 plant suffers. This proposition looks simple enough but 

 the student must fix it carefully in his mind because it 

 really lies at the foundation of all our work. It is 

 convenient to use a special name for the condition 

 the insufficiency of which is preventing the plant from 



