CHAPTER V 



THE HEALIHY HOST AND ITS EEQUIREMENTS ~ 



We have seen that soil is the meditim in which 

 plant life is made possible. We have also seen 

 that to produce good 5delds in crops it is essen- 

 tial to have a healthy soil— a condition directly 

 dependent upon the work of friendly organisms. 

 When these perform their work imperfectly, or 

 when the soil is overrun by parasitic fungi or 

 by pestiferous animal hfe, the soil is considered 

 sick. 



Let us now consider the plant itself, since 

 practiqally and economically it is the crop 

 that concerns us most. We are interested in 

 the soil only in so far as it is capable of main- 

 taining economic crops. The general needs of 

 plant hfe are the same to a striking extent for 

 higher plants and for the lower microorganisms 

 of the soil. 



Need of Air 



Plants must breathe, since air is indispensable 



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