200 Auxins in Agriculture 



83 is attributed to a latent period required for the adaptation of the 



microorganism to the new substrate. 



The factors influencing the microbial destruction of auxin in- 

 clude the nature and quantity of the auxin, bacterial activity, and 

 environmental factors of moisture and temperature, plus of course the 

 degree of adaptation of the microflora to auxin destruction. Audus 

 (1951) has shown that a given culture of his microorganisms which had 

 become adapted to the destruction of 2,4-D could readily attack 

 MCPA, but required a long adaptation period before it could attack 

 2,4,5-T. The scant evidence available indicates that the larger the 

 dose of auxin, the longer the period recpiired for its destruction. Some 

 data on this point are shown in figure 84 from which it may be 

 deduced that, in the concentration range used, the bacterial activity 

 or the capacity of the bacteria to attack the 2,4-D were limiting the 

 destruction. 



One of the factors which influence bacterial activity and conse- 

 quently influence 2,4-D destruction is the soil type (Kries, 1946). Ogle 

 (1953) has shown that under identical conditions and after identical 

 treatments with 2,4-D, different soils may take from as little as 10 days 

 to as much as 10 weeks to destroy the auxin. The addition of leaf 



TEMPERATURE 



Fig. 85. The effects of different temperatures upon the persistence of naphthyl 

 phthalamic acid in a fine sand soil. The herbicide was quantitatively estimated by 

 the inhibition of crabgrass seedlings after 42 days (Ogle, 1953). 



