Application and Persistence 201 



mold (Kries, 1946) or manure (Brown and Mitchell, 1948) to the soil 

 have been observed to increase destruction. The previous application 

 of auxin has been likewise shown to increase auxin destruction as a 

 consequence of the enrichment of the soil with microorganisms adapted 

 to auxin destruction (Audus, 1951). 



Among environmental factors, moisture has been shown to be 

 important in the destruction of 2,4-D. Excessively dry conditions clearly 

 retard destruction as seed germination tests of Mitchell and Marth 

 (1946) indicated, and increasing moisture residts in more rapid break- 

 down. Very low temperatures also retard auxin destruction and lead 

 to prolonged persistence (Jorgensen and Hamner, 1948). The respon- 

 siveness of the destruction of naphthyl phthalamic acid to various 

 temperatures is shown graphically in figure 85. The temperature 

 response again depends upon the soil type, being much more rapid 

 in soils of high organic matter content (Ogle, 1953). 



With respect to auxin persistence in general, not a great deal is 

 known about the fate of auxins applied to plants, and the general 

 means by which auxins are destroyed in the soil have only recently 

 been outlined. Auxins applied to plants seem to disappear from 

 effective loci in most plants by some quite rapid means. Some are 

 undoubtedly metabolized into ineffective end products, but some seem 

 to persist (Dhillon and Lucas, 1950) though perhaps not in a readily 

 extractable form (Bonner and Thurlow, 1949). Auxins applied to 

 soils may be lost by adsorption onto colloidal material, by leaching, 

 or microbiological destruction. Consequently such factors as tempera- 

 ture, moisture, constitution of the soil and of the auxin may all alter 

 auxin persistence in any one place. From the evidence at hand it would 

 seem very unlikely that the common herbicidal auxins would persist 

 in soils for more than one growing season, except under dry, sterile 

 or very cold conditions. 



