53 



since numerous organisms are facultative anaerobes and 

 can respire sither way. The existence of special organs 

 for root respiration is strong evidence in favor of the 

 need of aerobic respiration on the part of the roots of 

 species which possess such organs, but means nothing a3 

 to the nature of root respiration in the much more numer- 

 ous species in which special aerating organs are lacicing. 

 The evidence as to soil aeration is essentially 

 that most plants appear to grow better under conditions of 

 moderate water content in the soil and moderate oppenness 

 of soil texture, which conditions are also favorable, or 

 are believed to be favorable, to soil aeration. The con- 

 clusion of the cultural benefits of soil aeration is prob- 

 ably true but this particular argument in favor of it is of 

 little weight. I.lany other factors besides soil aeration 

 differ in the so-called "well-aerated" and "poorly- 

 aerated" soils. To ascribe, a priori , the cultural re- 

 sults to soil aeration only, is to ignore more than halsf 

 of the problem. It may be agreed, however, that the 

 growth of anaerobic organisms and the accumulation of in- 

 termediate respiration products, which result, immediately, 

 from bad aeration and, ultimately, from excess of water, 

 produce, in themselves, unfavorable ("toxic") soil con- 

 ditions. It appears, however, that plants vary greatly 

 in their power to withstand these toxic conditions and 

 that soils vary similarly in their power of neutralizing 

 or removing the toxins, even in the absence of oxygen. 



