Application and Persistence 199 



Destruction. The third major factor in persistence of auxins in 

 soils is the destruction of the auxin by microorganisms. The earliest 

 workers on auxin persistence in soils concluded that the destruction 

 was due to microorganism activity, for autoclaving prevented destruc- 

 tion, and moisture and temperature conditions suitable for micro- 

 organism growth were found to be necessary for auxin destruction 

 (DeRose and Newman, 1947; Brown and Mitchell, 1948). An ingenious 

 extension of this point has been made by Audus (1951) who found 

 that respiratory poisons were likewise effective in preventing destruc- 

 tion. This same worker (1950) isolated the first microorganism which 

 was found to be capable of destroying 2,4-D, and it turned out to be a 

 gram negative bacterium of the Bacterium globiforme type. Subse- 

 quently Jensen and Peterson (1952) have isolated another micro- 

 organism capable of destroying 2,4-D, Flavobacterium aquatile. 



The time course in the destruction of some auxins in the soil is 

 shown in figure 83. Audus (1949, 1951) has shown that the rate of 

 destruction varies greatly with the auxin used, 2,4-D being the most 

 rapidly destroyed auxin which was tested and 2,4,5-T being the most 

 persistent. Audus sometimes observed a small, immediate initial loss 

 of auxin activity. He suggested that such an initial loss may be a 

 result of adsorption of the auxin by the soil. The slow rates of micro- 

 bial destruction in the early sections of each of the curves in figure 



^ 20- 



0) 



o 



UJ 



o 



z 



UJ 



I- 

 (^ 



(/) 

 tt: 



UJ 

 CL 



10 20 



CONC. OF 



50 MG/L 



AUXIN 



Fig. 84. The effect of various concentrations of auxin (2,4-D) upon the time of 

 persistence of the auxin in the soil (Newman and Thomas, 1949). 



