44 



It is important to realize that the explana- 

 tion of these various effects, injurious and beneficial, 

 by reference to changes of aeration is entirely hypo- 

 thetical, Too much water in the soi}., a texture which 

 is too compact, and the like are injurious to plants. 

 They also decrease the ease and degree of soil aeration. 

 It is pure assumption to say that the injury is due to 

 the decreased aeration. This assumption is probably 

 true but it cannot be accepted on the basis of evidence 

 which is to entirely inferential. The soil conditions which 

 control plant health are complex. Deficient drainage or 

 excessive compacting will alter not only the air- supply 

 of the soil but also its water supply, the nature and 

 number of the microflora and fauna, the chemical re- 

 actions occurring (as, e.g., through adsorption or 

 catalysis), the mechanical ease or difficulty of the 

 penetration of roots, and possibly still other factors. 

 Any or all of these may affect the plant growth or be- 

 havior quite as importantly as the supply of air to the 

 roots. 



The published cultural experiments on soil 

 aeration are marred by similar failure to take into 

 account the real complexity of the conditions. Thus 



